Mr. Carlson has such a wonderful and clear didactic style. My Dad was a Seargant Tech at WWII end with a radio operator's license He built his own current-limited power supply, lights and all, just as Paul demonstrates elsewhere. He used to fix all kinds of fifties-sixties-era HiFi stuff for his friends while I played in the basement while he worked. Some of it 'rubbed off' on me. Pertinent to this video: When I was a teen I used to fix guitar tube and discrete transistor amplifiers, organs, etc. I recall that the VOX AC100(four EL34 push-pull output the original Beatle amp) had a open wirewound resistor in a frame below a bimetallic leaf switch with contacts that closed when the wirewound heated it up - the current limit resistor was in parallel with this bimetal, mounted insulated from the wirewound heater. Clearly, it was engineered with filament flash in mind - a clever automatic solution - and this video made me finally realize just why it was designed. I should have kept up interest in electronics and done a degree in it but life interrupted and I ended up sidetracked before learning the theory underpinning all the practical repairs I had learned to do. Sigh.
Using a constant current source is very gentle on the heaters and is easy using an LM317. The heater supply AC is rectified to DC that gives the CCS the voltage compliance headroom it needs to work properly. Another technique is a negative temperature coefficient thermistor - it does require a bit of experimentation and does drop a bit of voltage even warmed up. With newish tubes with good "emission" running at just below the rated voltage doesn't seem to hurt performance. I was told that the "flashy" heater tubes were meant for series connection off the mains voltage using a ballast resistor that drops the excess voltage and limits the inrush current. This was popular with cheap radios and TV sets where a penny saved equaled more profit! I completely agree with Mr Carlson that gentle treatment of the heaters helps prolong service life. The one possible exception might be directly heated triodes where the flash helps blast off cathode poisoning (there were several tube "rejuvenators" that worked on this principle).
Great tips. I once had a 1950's Allen 3-manual church organ which used 12AUX7 tube oscillators, one for each note. They were built into 6-note chassis and the chassis were built on to a large floor rack. There were three racks so you can imagine the number of tubes involved. I never counted them but when I switched the organ on the lights in the house dimmed for a second. It took about as much current as a clothes dryer. But it sure shook the house when I "pulled all the stops".
Good stuff, and yes, even back 30 odd years ago, (when tubes were supposedly cheap ....), I did this. On my Hallicrafters Rx and Tx, I installed a big wire-wound resistor across the power switch (each rig is supplied by an individually switched power board back in my rack). So when the main switch for that particular piece of equipment is turned on, there is a soft start for all the heaters/filaments, before the power switch on the actual equipment is turned on to bridge the resistor and bring full power to the tubes. Good also for re-forming the capacitors - in that case, the HT switch is also turned on some minutes later with the power switch still 'off', so there is half LV and something like half HT in that mode. Not as good as running through a Variac, but a quick precautionary tactic.
Love all your videos, so informative and helpful. It always amazes me how much you actually know and can remember. :) Sometimes I forget what day it is.
Once again something new you have bestowed upon me. Never had heard of filament flash, thanks for the education. I could listen to you for hours, you are very knowledgable.
I did something similar with my old tube gear. I used a relay (with a simple timing circuit) to short out a current limiting resistor that I installed in series with the filament supply.
Just for info: I have always used a "soft start" set up for all of my vacuum tube circuits. Some have been manual, with a switch and resistor, and others have been automatic with a time delay relay and resistor. Both types have always been successful in eliminating any filament or heater flash and in saving all of my favorite (and rather expensive) tubes used in my audio circuits. I also use the same type of soft delay in the B+ power supply circuits for all of my pre-amps and power amps. Good luck and happy listening to everyone!
Very interesting. I have seen this in the past and always wondered what was going on, it seems the tubes would eventually fail or have a shorter life but never knowing the actual reason ... just buy another tube I guess. Thanks for the insight, very informative and helpful !
A Temperature stable Vacuum Tube. A paradigm shifter in light of many preconceived ideas dating back to the age of drifting oscillators. Realy enjoied this little demonstration.
Some of the RCA CTC series tube chassis used a thermistor in the filament chain to provide a soft start. When we got a call for a dead TV, the first check was the thermistor if it was open, very common, we'd remove the body of the resistor and twist the leads together. That way, we didn't have to remove the chassis to solder in a new part. There was another thermistor in the degaussing circuit that could fail, but it rarely happened.
Great demonstration. Within amateur radio RF power amplifiers, many have a Soft Start circuit, which eliminates the in-rush current to the electrolytic capacitors and power tube filaments.
Mr Carlson, Another flash of brilliance :). Really enjoy your tech tips. Great using that end cap as a base for the test fixture. Another useful PVC enclosure is an electrical entrance el. I have used them in photoelectric projects. Thanks again for all your great videos.
My hat's off to Mr Carlson for spotting something I have surely missed for many decades. Seeing the 'flash' was something I associated with european-made versions of the twin triodes, or american-production parts marked with the european-standard "ECC83" type number. In all those decades I never once looked closely enough to observe just one (only) of the two heaters exhibit this trait. He's right, that can't be correct. But I am at a loss to explain it. Just goes to show that even after nearly 50 years of experience with a technology, there's always something more to learn. Probably what's kept my interest alive for so long. Thanks again for your very-observant work.
That was awesome Mr.Carlson. Wish I was twenty five years younger I'd go back to the electronics course I'd dropped of from and complete it successfully with so much inspiration that's coming from a person such as you. Your videos rekindle the passion for this art and science. Thank you so much and best wishes to all those aspirants out there. Good luck, cheers! 😊👍
@@MichiganPeatMoss I became a patron just out of curiosity because of that amazing capacitor tester - best $2 a month I've *ever* spent. What a great teacher.
You can solve the problem by placing an inrush current limiting thermister in series with the power cord, which actually drops in resistance as it heats. This also protects solid state diode circuits, though only when turned on cold. It will always protect the tubes, though, because they only flash when cold. My 11 tube RCA Color Television dot/color bar/crosshatch generator uses one of these, and always turns on gently. Amazingly this piece of equipment has ALL of its original components, which test and work perfectly.
I have a Swan MK I H-F amplifier. How I save my expensive 3-400Z tubes is to use a Variac rotating the control on the Variac slowly upwards taking about two minutes to reach full line voltage. This way you do not shock the tube's filaments. I've had the amp for over 25 years, the tubes have not gone soft or lost power at 15 or 10 meters. I do have a matched pair of 3-400Z's stored in a cool, dry place, but they may never get used unless I get rowdy and drive the amp to 3.5KW with 100W drive from my Drake Twins. Your way for smaller tubes is great, I will have to remember this.
Cycle your tubes once a year. You don't want them to go gassey! Gas is a real problem in large transmitting tubes like 3-500, 3-400 and 4-400 tubes. It a really RISKY buying NOS (new old stock) transmitting tubes from the likes of eBay. They can be gassey and no good even though they have never been used. 73, Jim W7RY
I recall from my teenage years in the 60's dismembering old radios and TVs (In those days, TV repair shops would happily give away scrap sets to hobbyists). I would frequently see an arrangement of series connected heaters with a thermistor to "soft-start" them.
Great info, I was not aware of this issue. I am getting back into vacuum tubes after 25 years in modern solid state electronics. For that fix with the MOSFET a Triac could be used for an AC system. The current will jump when the triac fires but should still eliminate the heater flash.
Hi Mr. Carlson I have seen this often in valves - firstly, when I was little, I noticed the flash in one of the valves in our B&W telly. This was a transformerless design with all the filaments tapped across the mains via heavy duty resistor to absorb the difference. The telly ran for approx nine years like this. When I git a bit older, I read in the 1950's hobby magazine that it was known about & "didn't seem to hurt the valve". In 2014 I built the Mullard 10+10 amplifier using two EF 86's & four ECL86's. Three out of the four ECL86's flash on startup so it will be interesting to note how long these filaments last. I have always assumed that the flash was normal - partly 'cos of our telly & partly because of the article. Thank you very much for posting this - it's the first time I've ever heard of it being a problem:)
When I repaired vacuum tube equipment in the 1970s era, I noticed some older equipment had ntc resistors in series with the filament chain to prevent inrush currents to extend the life span of the tubes. There were also vacuum tube TVs and amplifiers that had the “instant on” feature with the simple switch/resistor in the filament chain as Mr. Carlson described. The switch was integrated with the on/off-volume control. Excellent presentation as always sir.
Yes indeed, thanks for the thorough and well-paced explanations! I'm a tube/amp newbie, and when I was rebuilding some preamps about a week ago (the paint-by-numbers way :P ) I came across a tube that did this. I think it was a 12AU7, NEC branded, and original to the amp. A quick swap took care of my concern, and now I know what happened! Cheers!
Ah so now I know what this little phenomenon is! Ultimately I was suspicious of 2 Eico-branded 12au7's (Mullard) and just replaced them both (VTVM and a homebrew tube amp). I kinda figured they were not quite made right, but I had no idea this was so widespread, or has a fix for that matter. Thanks for sharing. Love your videos. I've learned a ton from them.
I've been working with tubes (TV and Hi-fi, later T&M) since the early '70s, and before that watching my father and uncle (hobbyists) as a child. I'd seen cases of heater flash but never gave it much thought or knew what to call it. In my years working in repair shops I think I saw some American brands that had flash. Of course there were many brands of tubes back then that are now gone and forgotten (Philco anyone?) Thanks for teaching me something new about tubes! It's been a long time.
Thanks Paul for touching on the main problem I encounter when buying old radios. I cringe when I hear a seller saying he plugged in his Grandfather's old radio and it didn't do anything. Cold filaments & discharged leaky capacitors put a tremendous surge load on an already marginal power supply. Better if they would sell that radio completely untested and let me bring it up on a variac. I have gotten some with exploded electrolytics that popped metal foil ribbon under the chassis in the high voltage area.
Outstanding video. This issue has always reminded me of the Phoebus cartel back in the 1920s . That was suppose to have gone away, but instead I think the practice of these sorts of shenanigans multiplied.
I've noticed that in series filament (rectified filament with 60 or more volts across 5 preamp tubes) preamps, that this problem is multiplied, and that with 12AX7A's the chinese rubiy tubes seem to be the best . Not only do they draw less current when on, but the cold resistance is not such a small fraction of the warm resistance. Also they seem to be very consistent from one tube to the next.
What I have been doing for years is using an older style time delay relay like was used in commercial/industrial equipment back before PLC's became the norm. The kind with a octal tube socket base and a timing adjustment knob on top. Most are rated with 10 amp relay contacts so they handle a pretty good load. I ended up with hundreds of them years ago at a auction of a local relay manufacturing plant that was moving operations over seas. Went there to buy a Bridgeport milling machine and ended up coming home with the Bridgeport milling machine and hundreds of these little time delay relays along with a bunch of other goodies. Took me a while to find a use for them but I found this filament flash problem a great place to use them. The ones that have a time adjustment range of 1-60 seconds is perfect for this application. Just mount a octal socket, break the filament feed line and run it through the relay and install the dropping resistor across the relay contacts.
A very handy tip,Thanks - I've been thinking about this problem very recently and came up with the idea of a 50% stand by preheat through switching the filaments in to series pairs when the amp is switched off(a stand by if you like) and back into parallel at turn on (I'm sure not an original idea) but yes- soft start seems like a much better cure for this problem- again Thanks, All the best Mr Carlson.
...the earlier vacuum tube TV sets with instant on, used a DIODE in series with the heater string- that allowed exactly HALF of the AC power to flow through the vacuum tube heaters while the TV set was turned off- and when the TV set was turned ON, the diode was bypassed, and the vacuum tubes received full power. ...on a side note: I remember when these particular TV sets were OUTLAWED in New York City, because they were considered to be a waste of electricity!!!
walking thru your tips and learning so much, i had no interest in electronics back in UnderGrad then got a job in Medical R&D and was really handicapped by the knowledge gap. i was stuck working ergonomic, man machine interface, beam guidance and materials science for Data clarity. this basic stuff should get its way into the STEM program here in the US or Scouts program for Technology. Great basics that can be used to understand circuits and know what and why of so much stuff in everyday use. Modern Day Mr. Wizard stuff that I used to watch as a kid. Thank You for providing this ground level 101 material, extremely well presented and most understandable you little test kits are great too. Now as a Patreon I want to explore all your material and get to where i can solve some on my hobby issues in electronics as well as setup all my test gear for these old radios. very encouraging and energizing for this old brain.
You could also use a thermal time delay relay to short out the current limiting resistor. It has the simplicity of your switch solution and the autonomy of your mosfet solution.
+Eric Wasatonic LOL, You know, just outside the shot to your left at the test fixture scene, I had a 115NO20T on the bench.... But I figured, who is going to go get one of these old thermal delay relays. Maybe like you mentioned, I should have shown it. Thanks for your input Eric!
+Mr Carlson's Lab, I use thermal relay tubes all the time, lol. I have never used them to ramp up heaters, so that's next on the list. Brilliant idea - thank you!
I just bought some 115N010s, 115NO15s, 115N020s, 115N030s, and 115N060s. The prices on those have really skyrocketed recently. It's crazy. If you have opened up that Lambda PSU on your bench, you know it has a 115 volt thermal time delay inside it, too. I think mine has a 60 second inside it. I like the way the old octal thermal time delays will let me short cycle something off and back on without waiting a minute for the time delay to it's run full cycle again.
Thanks for pointing this effect out, as most viewers will not know about it This is why heater filament windings on transformers were made "soft", high inductance or resistance. To limit the inrush current old commercial designs also user uranium dioxide resistors (NTC characteristics) in filament circuits, or on older power amps I own from the 30's they have a preheat winding (2.5V) on the filament transformer winding, which is operated by turning the main switch. AFAIK this was basicly to prevent unloaded DC buildup when applying power, due to direct heated rectifier tubes vs indirectly heated power and pre amplifier valves, which took longer to conduct. This also added highly to the reliability, which a commercial PA system was designed for. For non commercial, mass production, the inrush effect did not matter, as you stated "as long as it reaches the stated useable time" or warranty time (6 month from sale to the end customer), the mfg was fine. And, like Philips/ Valvo (Mullard/ Amperex, ...) being one of the biggest tube makers in the world, you needed the replacement purchase business to keep the factories going.
Many years ago, we used a diode in series with the 6.3VAC output of the transformer and used a delayed relay (about 10 sec) to short it. Just another way to protect AC filaments. For DC filaments you can use same setup, but with a string of 4 or more diodes in series, to drop the voltage enough to avoid flash and then the delayed relay shorts them all.
Excellent explanation. I had a tube doing that in my Drake TR4CW and didn't really understand why. I swapped in a new tube and no problem. When using my vacuum tube receivers (R4B) I usually just leave them on all day if I'm going to be using them later in the day. That's to limit the # of filament shocks. I think I need to build a 'step start' for the whole radio.... if I knew how😄
Thanks for the excellent video! Over many years I've seen the flash phenomena in tubes and just thought it was the nature of the beast... It did not occur that this could be a defect that would shorten tube life; very interesting!
Remember the instant on TV's in the 60's and 70's in which the filaments were run at a reduced voltage when the set was plugged in and turned off? Did that extend or shorten their lifespan? I occasionally pick one of these up at a garage sale etc. to tinker with. Thank you
Always a good learning experience at Mr. Carlsons lab! I thought for sure he would use a 555 timer for the delay on the second circuit. The tube was more “period” though. Nice to know 💯
Back in the 1960s, before solid-state TVs, some manufacturers had an "Instant-On" feature. They would keep a few volts on the tube filaments, including the CRT, all the time. Of course, this increased your electric bill. Some TVs had a switch on the back that allowed you to turn off the instant feature. It was probably around 3-4 Volts for the 5-Volt tubes and 7-8 Volts for the 12-Volt tubes.
The better vacuum tube equipment had means of limiting turn on surge with either thermistors or “surgistors”. The surgistor was a wirewound resistor attached next to a normally open bi-metallic contact across it. This was connected in series with either the power line or the tube heaters. When the power to the equipment was turned on the resistor would limit the initial surge while heating the bi-metallic contact. After a few seconds the contact would close removing the current limiting of the resistor, and the contact would stay closed due to the slight heating of the current through the bi-metallic contact arm. I was very surprised that nobody seemed to be aware of this common solution to this old problem.
I use something similar for old series set radios. The 60 watt light bulb with a switch across it built on a 4 inch steel electrical box. After about 15 seconds warm up, I throw throw the switch for full power. Had the flash problem with the 35W4's blowing the filament prematurely and this cured the problem.
This idea would have saved at least half of the tetrodes which I was forced to replace in many of the CCA and Continental AM (broadcast) transmitters over the years! (mostly Raytheon were affected by premature filament failure) Now, Svetlana is the only available vendor for many of them! Many thanks, am sending this link to many of my friends!
I've ben arguing about this for years . I knew it was a build problem . So many people look for the flash as somthing cool , when its like having a tiny ark welder on the filiment .. Great idea , defintitly adding it to my tube testers.
I agree. He looks very young compared to his level of knowlege. I am in my mid 60s, a Ham by hobby and somewhat experienced with older gear, yet his knowlege and logic surpasses my expectations exceedingly. Almost seems suspicious ... hybrid human perhaps, maybe a time traveler, or maybe just really smart, who can say, but he makes excellent videos full of useful information at any rate.
Very informative video. I've watched a bunch of your videos, and learned a lot, but this is my first comment. How would a hobbyist go about finding out the component specifications in that mosfet circuit? Let's say for a 60's vintage tube radio with 5 tubes with 6,3v heated filaments. I get a lot of the physics and the math, I'm just not sure what I should be referring to or where to start. Thanks a lot for your videos! I also really appreciate your thoroughness in your electronic work.
...I know I'm late- but first off- the circuits in this video, used DC for the heater voltage!! A "5 tube radio" uses AC for the heater voltage, and the FET circuit would NOT work with AC- but the "resistor and switch" circuit might work.
Thank you! Fantastic film as ever Mr. Carlson. Please could you tell us what value resistor would be best in the simple 'switch and resistor' circuit for an ECC83 in RIAA position V2 of a Zhili (Douk) EAR834 phono pre?
Excellent video! You are very knowledgeable on electronics. It seems just about anything that uses electricity has issues with high current draw when powered up. Capacitors, power transformers, filament light bulbs, motors, and the list goes on. Some devices NEED full current to kick em into gear, but most electronics would be happier if that "inrush" current could be suppressed a bit, like your flashing tube example. Items that use modern cheap switching power supplies should be left alone though. They arn't too fond of a slow voltage rise when powered up. But, old skool iron transformer equipment driving tubes would be a good candidate for some kind of ramp up to full voltage instead of slamming it w/full mains power IMO. An example: I have, yes have an old Heathkit solid state stereo amp that has no speaker protection circuit or relay like most modern amps to. When I powered it up, the speakers would thump something terrible. Ya, I learned if I switched the speakers off, waited about a minute, then switched them on again, no thump. But I had to remember to do that everytime. So, I built up a simple delay timer circuit and using a hi wattage lower ohm resister in series with the main lead from the transformer to the rectifier driving the B+ to the output transistors. When the amp was turned on, the relay would be open, forcing power through the resistor slowly charging the B+ capacitors for a few seconds. After the time out, the relay would close thus providing full power from the transformer! NO more thump! Amp is pushing 50 years old now! I made the mod around 20 years ago and amp has never had a failure!
good video. I keep thinking that those really fine leads are okay as long as they kept them short enough so that the pin keep the temperature down until the rest of the element warmed up and lowered the current.
I can see that the filament that has the flash has a loop in it with no coating before it connects to the pin. The one with no flash has the coating brought right up to where the attachment is made.
Good info! Thanks. Now, what about using a high wattage resistor in series with the filament transformer? I would install a "warm up" mode switch in a convenient location to bypass the resistor. Or maybe incorporate a time delay circuit with a relay to bypass the resistor. More like the speaker time delay for transistorized amplifiers to eliminate the "thump". Thanks again for the great video.
Interesting topic. I wonder whether in many pieces of equipment the over current on start up is softened by the heater transformer voltage sagging at switch on, then recovering as the heater filament warms. This is likely as the transformer will be rated for "hot" operational current. Another option for hi-fi applications is to use a dc constant current heater supply instead of a constant voltage supply.
ive been meaning to ask where you learned all that you know. vacuum tube technology has been out of favor for a lot of years, so i thought, "hes not old enough to get it on the job, so he must be a serious tinkerer. but just now you mentioned making a test tool to deal w pallets full of vacuum tubes that had heater flash so. that either means im way older than i wanna be or you absobed a lot of good info in your formative years!
I'm not sure weather you are talking about modern made valves (tubes) or older pre 1970. I started work in Marconi Instruments (English Electric) in the late 60's started on mainly valve test equipment. I do not remember any case of heater (filament) flash. As I remember most valves were mainly English Electric (EE) or Mullard but there were others I cannot recall. I'm sure if we had this problem especially EE would have felt with it. Thanks for anothe great, well described video
Very interesting trick. Seems like the incandescent lamps also always burn out on ignition. I'm building a 5e3 tube guitar amplifier, and I wanted to replace the classic 'standby' switch (which I'm not convinced is doing any good) with your 'pre-heat' switch. Which resistor value would you recommend for a double 6v6, 12ax7 and 12ay7 setup (it also has a pilot 6.3v lamp)?
Best reason to engineer DC filament supplies with a soft start feature, that ramps up the voltage over a period of time. I usually choose about 45 seconds, and do not start the high voltage supply up until the filaments have been on full for about 2 minutes. Also observe the heater to cathode voltage limits in the tube manual. In some cases you may have to create several filament supplies and operate them electrostaticly floated at a specific positive or negative voltage in order maintain the specification for any given tube. In a typical circuit of mine a differential cascode triode set has a pair of cathodes near ground, and another pair at about +125. In this case, a high impedance voltage divider is created to bias the filament at about +67 volts. this puts the cathodes of all four elements at far less than the +-100 volts specified for the tubes used. The long tail current source for the differential set above also consists of a series stack of parallel triodes similar to the tubes used in the diff amp above it. Again, the design centers of where the cathodes are below ground, and a single filament supply biased to a point midway between the cathodes is used. If the volage between the cathodes was near or exceeds the heater to cathode specification, additional filament supply will be needed. In some amplifiers, I need 3 floated and biased filament supplies in the front end circuit, and 2 separate supplies for the power tubes, each bias point driven by a buffer to 'follow' the instantaneous voltage at the cathodes of the low impedance power high current tubes in the Circlotron topology balanced bridge output stage.
Just run the filaments all the time IN standby hot but not all the way.... I have seen this in highbred television sets... right before they went all solid-state..... Old-timer here... Good video, Sir
Наука
Mr. Carlson has such a wonderful and clear didactic style. My Dad was a Seargant Tech at WWII end with a radio operator's license He built his own current-limited power supply, lights and all, just as Paul demonstrates elsewhere. He used to fix all kinds of fifties-sixties-era HiFi stuff for his friends while I played in the basement while he worked. Some of it 'rubbed off' on me.
Pertinent to this video:
When I was a teen I used to fix guitar tube and discrete transistor amplifiers, organs, etc.
I recall that the VOX AC100(four EL34 push-pull output the original Beatle amp) had a open wirewound resistor in a frame below a bimetallic leaf switch with contacts that closed when the wirewound heated it up - the current limit resistor was in parallel with this bimetal, mounted insulated from the wirewound heater. Clearly, it was engineered with filament flash in mind - a clever automatic solution - and this video made me finally realize just why it was designed. I should have kept up interest in electronics and done a degree in it but life interrupted and I ended up sidetracked before learning the theory underpinning all the practical repairs I had learned to do. Sigh.
Indeed.
"This resistor is there... because I can!"
*LOVE IT!*
Using a constant current source is very gentle on the heaters and is easy using an LM317. The heater supply AC is rectified to DC that gives the CCS the voltage compliance headroom it needs to work properly. Another technique is a negative temperature coefficient thermistor - it does require a bit of experimentation and does drop a bit of voltage even warmed up. With newish tubes with good "emission" running at just below the rated voltage doesn't seem to hurt performance. I was told that the "flashy" heater tubes were meant for series connection off the mains voltage using a ballast resistor that drops the excess voltage and limits the inrush current. This was popular with cheap radios and TV sets where a penny saved equaled more profit! I completely agree with Mr Carlson that gentle treatment of the heaters helps prolong service life. The one possible exception might be directly heated triodes where the flash helps blast off cathode poisoning (there were several tube "rejuvenators" that worked on this principle).
Great tips. I once had a 1950's Allen 3-manual church organ which used 12AUX7 tube oscillators, one for each note. They were built into 6-note chassis and the chassis were built on to a large floor rack. There were three racks so you can imagine the number of tubes involved. I never counted them but when I switched the organ on the lights in the house dimmed for a second. It took about as much current as a clothes dryer. But it sure shook the house when I "pulled all the stops".
Good stuff, and yes, even back 30 odd years ago, (when tubes were supposedly cheap ....), I did this. On my Hallicrafters Rx and Tx, I installed a big wire-wound resistor across the power switch (each rig is supplied by an individually switched power board back in my rack). So when the main switch for that particular piece of equipment is turned on, there is a soft start for all the heaters/filaments, before the power switch on the actual equipment is turned on to bridge the resistor and bring full power to the tubes. Good also for re-forming the capacitors - in that case, the HT switch is also turned on some minutes later with the power switch still 'off', so there is half LV and something like half HT in that mode. Not as good as running through a Variac, but a quick precautionary tactic.
Awesome video. This is why I soft start all my tube RXs with a variac but I love this analysis and demo with automatic circuitry. Love it!!!
Love all your videos, so informative and helpful. It always amazes me how much you actually know and can remember. :) Sometimes I forget what day it is.
I'd love to see "Tech Tips Tuesday" come back sometime. These were really great short info videos :)
Big thumbs up for all the vids!👍 the close-ups were just awesome! I never knew about those flashes!
Ingenuity in simplicity! Thank you for a great video Mr. Carlson
+Audiophile Vintage
Thanks! Glad your enjoying.
Once again something new you have bestowed upon me. Never had heard of filament flash, thanks for the education. I could listen to you for hours, you are very knowledgable.
+Tech Chuck Legg
Thanks for the kind words Chuck!
I did something similar with my old tube gear. I used a relay (with a simple timing circuit) to short out a current limiting resistor that I installed in series with the filament supply.
Just for info: I have always used a "soft start" set up for all of my vacuum tube circuits. Some have been manual, with a switch and resistor, and others have been automatic with a time delay relay and resistor. Both types have always been successful in eliminating any filament or heater flash and in saving all of my favorite (and rather expensive) tubes used in my audio circuits. I also use the same type of soft delay in the B+ power supply circuits for all of my pre-amps and power amps. Good luck and happy listening to everyone!
Soft start HT = Valve rectifier
@@PrinceWesterburg...THE HEATER CURRENT DOESN'T GO THROUGH A VACUUM TUBE RECTIFIER- ONLY THE B+ DOES.
Very interesting. I have seen this in the past and always wondered what was going on, it seems the tubes would eventually fail or have a shorter life but never knowing the actual reason ... just buy another tube I guess. Thanks for the insight, very informative and helpful !
Fantastic has always,Mr Carlson ! Thank you for your time and hard work.
All my best.
Bobby
+Bobby Tectalabyss
Thanks Bobby!
A Temperature stable Vacuum Tube. A paradigm shifter in light of many preconceived ideas dating back to the age of drifting oscillators. Realy enjoied this little demonstration.
Thanks so much for sharing this, this is one of those things that isn't easily explained, but obviously needs it :)
+Majes Alum
Your Welcome!
Some of the RCA CTC series tube chassis used a thermistor in the filament chain to provide a soft start. When we got a call for a dead TV, the first check was the thermistor if it was open, very common, we'd remove the body of the resistor and twist the leads together. That way, we didn't have to remove the chassis to solder in a new part. There was another thermistor in the degaussing circuit that could fail, but it rarely happened.
Great demonstration. Within amateur radio RF power amplifiers, many have a Soft Start circuit, which eliminates the in-rush current to the electrolytic capacitors and power tube filaments.
Mr Carlson, Another flash of brilliance :). Really enjoy your tech tips. Great using that end cap as a base for the test fixture. Another useful PVC enclosure is an electrical entrance el. I have used them in photoelectric projects. Thanks again for all your great videos.
+Todd Anonymous
Thanks for the kind words Todd!
My hat's off to Mr Carlson for spotting something I have surely missed for many decades. Seeing the 'flash' was something I associated with european-made versions of the twin triodes, or american-production parts marked with the european-standard "ECC83" type number. In all those decades I never once looked closely enough to observe just one (only) of the two heaters exhibit this trait.
He's right, that can't be correct. But I am at a loss to explain it.
Just goes to show that even after nearly 50 years of experience with a technology, there's always something more to learn.
Probably what's kept my interest alive for so long.
Thanks again for your very-observant work.
Your very welcome!
Excellent info and explained very well! I remember once in a while seeing a tube do that and thinking it wasn't long for this world
That was awesome Mr.Carlson. Wish I was twenty five years younger I'd go back to the electronics course I'd dropped of from and complete it successfully with so much inspiration that's coming from a person such as you. Your videos rekindle the passion for this art and science. Thank you so much and best wishes to all those aspirants out there. Good luck, cheers! 😊👍
It's not too late. Restart here. "become a patron". :)
@@MichiganPeatMoss I became a patron just out of curiosity because of that amazing capacitor tester - best $2 a month I've *ever* spent. What a great teacher.
You can solve the problem by placing an inrush current limiting thermister in series with the power cord, which actually drops in resistance as it heats. This also protects solid state diode circuits, though only when turned on cold. It will always protect the tubes, though, because they only flash when cold. My 11 tube RCA Color Television dot/color bar/crosshatch generator uses one of these, and always turns on gently. Amazingly this piece of equipment has ALL of its original components, which test and work perfectly.
Love this series. So glad I discovered it !
I have a Swan MK I H-F amplifier. How I save my expensive 3-400Z tubes is to use a Variac rotating the control on the Variac slowly upwards taking about two minutes to reach full line voltage. This way you do not shock the tube's filaments. I've had the amp for over 25 years, the tubes have not gone soft or lost power at 15 or 10 meters. I do have a matched pair of 3-400Z's stored in a cool, dry place, but they may never get used unless I get rowdy and drive the amp to 3.5KW with 100W drive from my Drake Twins. Your way for smaller tubes is great, I will have to remember this.
Cycle your tubes once a year. You don't want them to go gassey! Gas is a real problem in large transmitting tubes like 3-500, 3-400 and 4-400 tubes. It a really RISKY buying NOS (new old stock) transmitting tubes from the likes of eBay. They can be gassey and no good even though they have never been used. 73, Jim W7RY
OK You got me totally addicted watching your repairs of everything, some of which I now have. Cannot find a Jackson 591
I recall from my teenage years in the 60's dismembering old radios and TVs (In those days, TV repair shops would happily give away scrap sets to hobbyists). I would frequently see an arrangement of series connected heaters with a thermistor to "soft-start" them.
Great info, I was not aware of this issue. I am getting back into vacuum tubes after 25 years in modern solid state electronics. For that fix with the MOSFET a Triac could be used for an AC system. The current will jump when the triac fires but should still eliminate the heater flash.
I don't use tubes but enjoyed the demonstration, explanation and solution... thumbs up!
Hi Mr. Carlson I have seen this often in valves - firstly, when I was little, I noticed the flash in one of the valves in our B&W telly. This was a transformerless design with all the filaments tapped across the mains via heavy duty resistor to absorb the difference. The telly ran for approx nine years like this. When I git a bit older, I read in the 1950's hobby magazine that it was known about & "didn't seem to hurt the valve". In 2014 I built the Mullard 10+10 amplifier using two EF 86's & four ECL86's. Three out of the four ECL86's flash on startup so it will be interesting to note how long these filaments last. I have always assumed that the flash was normal - partly 'cos of our telly & partly because of the article. Thank you very much for posting this - it's the first time I've ever heard of it being a problem:)
You're Welcome, glad you enjoyed!
When I repaired vacuum tube equipment in the 1970s era, I noticed some older equipment had ntc resistors in series with the filament chain to prevent inrush currents to extend the life span of the tubes. There were also vacuum tube TVs and amplifiers that had the “instant on” feature with the simple switch/resistor in the filament chain as Mr. Carlson described. The switch was integrated with the on/off-volume control.
Excellent presentation as always sir.
You’re a mensch, Mr. Carlson. Thank you for all that you do 👌🙏🤙
Thanks Paul. Another great Tech Tuesday. I'm enjoying them. Seems like Planned Obsolescence to me.
+Robert Calk Jr.
I think your right!
Thank you for a very interesting and educational presentation. I learned a lot today.
Excellent video here, as always Paul. Have a quite similar protection in my amp as well :-) Very good and valuable tech tip. Take care
+TRXBench
Thanks Peter!
+TRXBench Agreed I learned alot in this video.
Yes indeed, thanks for the thorough and well-paced explanations! I'm a tube/amp newbie, and when I was rebuilding some preamps about a week ago (the paint-by-numbers way :P ) I came across a tube that did this. I think it was a 12AU7, NEC branded, and original to the amp. A quick swap took care of my concern, and now I know what happened!
Cheers!
+alecjahn
Glad you enjoyed the video!
The soft start filament circuit in various forms is very common in both commercial and homebrew high powered, amateur radio vacuum tube amplifiers.
Ah so now I know what this little phenomenon is! Ultimately I was suspicious of 2 Eico-branded 12au7's (Mullard) and just replaced them both (VTVM and a homebrew tube amp). I kinda figured they were not quite made right, but I had no idea this was so widespread, or has a fix for that matter. Thanks for sharing. Love your videos. I've learned a ton from them.
Not a problem, glad you enjoyed!
I Love This Guy. I have learned so much in the short time I've been a subscriber.
I've been working with tubes (TV and Hi-fi, later T&M) since the early '70s, and before that watching my father and uncle (hobbyists) as a child.
I'd seen cases of heater flash but never gave it much thought or knew what to call it.
In my years working in repair shops I think I saw some American brands that had flash.
Of course there were many brands of tubes back then that are now gone and forgotten (Philco anyone?)
Thanks for teaching me something new about tubes! It's been a long time.
Thanks Paul for touching on the main problem I encounter when buying old radios. I cringe when I hear a seller saying he plugged in his Grandfather's old radio and it didn't do anything. Cold filaments & discharged leaky capacitors put a tremendous surge load on an already marginal power supply. Better if they would sell that radio completely untested and let me bring it up on a variac. I have gotten some with exploded electrolytics that popped metal foil ribbon under the chassis in the high voltage area.
Outstanding video. This issue has always reminded me of the Phoebus cartel back in the 1920s . That was suppose to have gone away, but instead I think the practice of these sorts of shenanigans multiplied.
+cny02253
Ya, I agree. Many technologies suffer from this sort of action. Thanks for your comment!
I've noticed that in series filament (rectified filament with 60 or more volts across 5 preamp tubes) preamps, that this problem is multiplied, and that with 12AX7A's the chinese rubiy tubes seem to be the best . Not only do they draw less current when on, but the cold resistance is not such a small fraction of the warm resistance. Also they seem to be very consistent from one tube to the next.
The same circuit, but with a time relay, i use on my halogen lighting. After that the lifetime of the used bulbs were longer.
+Bernd B.
Thanks for your comment!
What I have been doing for years is using an older style time delay relay like was used in commercial/industrial equipment back before PLC's became the norm. The kind with a octal tube socket base and a timing adjustment knob on top. Most are rated with 10 amp relay contacts so they handle a pretty good load. I ended up with hundreds of them years ago at a auction of a local relay manufacturing plant that was moving operations over seas. Went there to buy a Bridgeport milling machine and ended up coming home with the Bridgeport milling machine and hundreds of these little time delay relays along with a bunch of other goodies. Took me a while to find a use for them but I found this filament flash problem a great place to use them. The ones that have a time adjustment range of 1-60 seconds is perfect for this application. Just mount a octal socket, break the filament feed line and run it through the relay and install the dropping resistor across the relay contacts.
+MikesRadioRepair .CBRadio
Sounds like those relays would be great. Thanks for your comment Mike!
This is great! I have a tube in my amp that does this. Now I know what to do. Thanks very much!!
Glad to help!
A very handy tip,Thanks - I've been thinking about this problem very recently and came up with the idea of a 50% stand by preheat through switching the filaments in to series pairs when the amp is switched off(a stand by if you like) and back into parallel at turn on (I'm sure not an original idea) but yes- soft start seems like a much better cure for this problem- again Thanks, All the best Mr Carlson.
...the earlier vacuum tube TV sets with instant on, used a DIODE in series with the heater string- that allowed exactly HALF of the AC power to flow through the vacuum tube heaters while the TV set was turned off- and when the TV set was turned ON, the diode was bypassed, and the vacuum tubes received full power.
...on a side note: I remember when these particular TV sets were OUTLAWED in New York City, because they were considered to be a waste of electricity!!!
walking thru your tips and learning so much, i had no interest in electronics back in UnderGrad then got a job in Medical R&D and was really handicapped by the knowledge gap. i was stuck working ergonomic, man machine interface, beam guidance and materials science for Data clarity. this basic stuff should get its way into the STEM program here in the US or Scouts program for Technology. Great basics that can be used to understand circuits and know what and why of so much stuff in everyday use. Modern Day Mr. Wizard stuff that I used to watch as a kid. Thank You for providing this ground level 101 material, extremely well presented and most understandable you little test kits are great too. Now as a Patreon I want to explore all your material and get to where i can solve some on my hobby issues in electronics as well as setup all my test gear for these old radios. very encouraging and energizing for this old brain.
I'm glad to assist you in your journey Dennis!
Mr. Carlson ROCKS!!!! Love your vids!!!
You could also use a thermal time delay relay to short out the current limiting resistor. It has the simplicity of your switch solution and the autonomy of your mosfet solution.
+Eric Wasatonic
LOL, You know, just outside the shot to your left at the test fixture scene, I had a 115NO20T on the bench.... But I figured, who is going to go get one of these old thermal delay relays. Maybe like you mentioned, I should have shown it. Thanks for your input Eric!
+Mr Carlson's Lab, I use thermal relay tubes all the time, lol. I have never used them to ramp up heaters, so that's next on the list. Brilliant idea - thank you!
Glad you enjoyed!
I just bought some 115N010s, 115NO15s, 115N020s, 115N030s, and 115N060s. The prices on those have really skyrocketed recently. It's crazy. If you have opened up that Lambda PSU on your bench, you know it has a 115 volt thermal time delay inside it, too. I think mine has a 60 second inside it. I like the way the old octal thermal time delays will let me short cycle something off and back on without waiting a minute for the time delay to it's run full cycle again.
@@Satchmoeddie...it's a LOT cheaper to use a 555 timer and a relay-(!)
Thanks Paul! Your video's are just the greatest!
+Thema inproblem
Thanks for your kind words!
Very well explained, well done, thank you
Thanks for pointing this effect out, as most viewers will not know about it
This is why heater filament windings on transformers were made "soft", high inductance or resistance. To limit the inrush current old commercial designs also user uranium dioxide resistors (NTC characteristics) in filament circuits, or on older power amps I own from the 30's they have a preheat winding (2.5V) on the filament transformer winding, which is operated by turning the main switch. AFAIK this was basicly to prevent unloaded DC buildup when applying power, due to direct heated rectifier tubes vs indirectly heated power and pre amplifier valves, which took longer to conduct. This also added highly to the reliability, which a commercial PA system was designed for.
For non commercial, mass production, the inrush effect did not matter, as you stated "as long as it reaches the stated useable time" or warranty time (6 month from sale to the end customer), the mfg was fine.
And, like Philips/ Valvo (Mullard/ Amperex, ...) being one of the biggest tube makers in the world, you needed the replacement purchase business to keep the factories going.
I'm a new subscriber...What a wonderful and informative channel.
Thank you for your time and sharing your knowledge.
+Crazy Uncle Duke
Your welcome. Glad to have you aboard!
Many years ago, we used a diode in series with the 6.3VAC output of the transformer and used a delayed relay (about 10 sec) to short it. Just another way to protect AC filaments.
For DC filaments you can use same setup, but with a string of 4 or more diodes in series, to drop the voltage enough to avoid flash and then the delayed relay shorts them all.
Very informative. Thank you Mr. Carlson.
Your welcome!
Excellent explanation. I had a tube doing that in my Drake TR4CW and didn't really understand why. I swapped in a new tube and no problem.
When using my vacuum tube receivers (R4B) I usually just leave them on all day if I'm going to be using them later in the day.
That's to limit the # of filament shocks. I think I need to build a 'step start' for the whole radio....
if I knew how😄
Great topic. I am looking at my guitar amps for heater flash in the tubes right now. Thanks for the great simple tip.
Your welcome Jim!
Thanks for the excellent video! Over many years I've seen the flash phenomena in tubes and just thought it was the nature of the beast... It did not occur that this could be a defect that would shorten tube life; very interesting!
+vonzigle
Your welcome!
Tremendously enjoy your videos. Thank You.
+plantopguy5
Great! Thanks for your comment!
Remember the instant on TV's in the 60's and 70's in which the filaments were run at a reduced voltage when the set was plugged in and turned off? Did that extend or shorten their lifespan? I occasionally pick one of these up at a garage sale etc. to tinker with. Thank you
Always a good learning experience at Mr. Carlsons lab! I thought for sure he would use a 555 timer for the delay on the second circuit. The tube was more “period” though. Nice to know 💯
Back in the 1960s, before solid-state TVs, some manufacturers had an "Instant-On" feature. They would keep a few volts on the tube filaments, including the CRT, all the time. Of course, this increased your electric bill. Some TVs had a switch on the back that allowed you to turn off the instant feature. It was probably around 3-4 Volts for the 5-Volt tubes and 7-8 Volts for the 12-Volt tubes.
I loved this episode. Thank you sir.
+YankeeIngenuity
Your Welcome!
Good stuff as always. Thanks for the videos.
Carl
+skycarl
Thanks Carl!
Classic Carlson simplicity. Thank you.
Seems like a perfect application for a standby switch. Is the defect due to the heater leads having that extra thin lead up to the thicker tungsten?
The better vacuum tube equipment had means of limiting turn on surge with either thermistors or “surgistors”. The surgistor was a wirewound resistor attached next to a normally open bi-metallic contact across it. This was connected in series with either the power line or the tube heaters. When the power to the equipment was turned on the resistor would limit the initial surge while heating the bi-metallic contact. After a few seconds the contact would close removing the current limiting of the resistor, and the contact would stay closed due to the slight heating of the current through the bi-metallic contact arm. I was very surprised that nobody seemed to be aware of this common solution to this old problem.
I use something similar for old series set radios. The 60 watt light bulb with a switch across it built on a 4 inch steel electrical box. After about 15 seconds warm up, I throw throw the switch for full power. Had the flash problem with the 35W4's blowing the filament prematurely and this cured the problem.
+ClockWatcher
Great solution! Glad to managed to save those 35W4's. Thanks for your story!
This idea would have saved at least half of the tetrodes which I was forced to replace in many of the CCA and Continental AM (broadcast) transmitters over the years! (mostly Raytheon were affected by premature filament failure) Now, Svetlana is the only available vendor for many of them! Many thanks, am sending this link to many of my friends!
+AMStationEngineer
Thanks for spreading the news! Glad your enjoying the video's!
I've ben arguing about this for years . I knew it was a build problem . So many people look for the flash as somthing cool , when its like having a tiny ark welder on the filiment .. Great idea , defintitly adding it to my tube testers.
Back in the day? What are you, about 34?😂 Seriously, when are you going to show us your tube based time machine? Love your channel sir!
I agree. He looks very young compared to his level of knowlege. I am in my mid 60s, a Ham by hobby and somewhat experienced with older gear, yet his knowlege and logic surpasses my expectations exceedingly. Almost seems suspicious ... hybrid human perhaps, maybe a time traveler, or maybe just really smart, who can say, but he makes excellent videos full of useful information at any rate.
CharlesM1957
Too funny!...Mr. Carlson is a combination of Spock 🖖 and Tesla in the body of a mild mannered nerd/technician (courteously proposed 😊).
Paul was the lost timelord
No time machine. It's all just about slowing down the ageing process with an external resistor.
I worked for a large, name valve hifi company and now people say I talk with the knowledge of someone in their 80s!
Very informative video. I've watched a bunch of your videos, and learned a lot, but this is my first comment.
How would a hobbyist go about finding out the component specifications in that mosfet circuit? Let's say for a 60's vintage tube radio with 5 tubes with 6,3v heated filaments. I get a lot of the physics and the math, I'm just not sure what I should be referring to or where to start.
Thanks a lot for your videos! I also really appreciate your thoroughness in your electronic work.
...I know I'm late- but first off- the circuits in this video, used DC for the heater voltage!! A "5 tube radio" uses AC for the heater voltage, and the FET circuit would NOT work with AC- but the "resistor and switch" circuit might work.
Thank you! Fantastic film as ever Mr. Carlson. Please could you tell us what value resistor would be best in the simple 'switch and resistor' circuit for an ECC83 in RIAA position V2 of a Zhili (Douk) EAR834 phono pre?
Fascinating. I'm off now to try and impress someone with my new found knowledge on heater flash :)
Excellent video! You are very knowledgeable on electronics. It seems just about anything that uses electricity has issues with high current draw when powered up. Capacitors, power transformers, filament light bulbs, motors, and the list goes on. Some devices NEED full current to kick em into gear, but most electronics would be happier if that "inrush" current could be suppressed a bit, like your flashing tube example. Items that use modern cheap switching power supplies should be left alone though. They arn't too fond of a slow voltage rise when powered up. But, old skool iron transformer equipment driving tubes would be a good candidate for some kind of ramp up to full voltage instead of slamming it w/full mains power IMO. An example: I have, yes have an old Heathkit solid state stereo amp that has no speaker protection circuit or relay like most modern amps to. When I powered it up, the speakers would thump something terrible. Ya, I learned if I switched the speakers off, waited about a minute, then switched them on again, no thump. But I had to remember to do that everytime. So, I built up a simple delay timer circuit and using a hi wattage lower ohm resister in series with the main lead from the transformer to the rectifier driving the B+ to the output transistors. When the amp was turned on, the relay would be open, forcing power through the resistor slowly charging the B+ capacitors for a few seconds. After the time out, the relay would close thus providing full power from the transformer! NO more thump! Amp is pushing 50 years old now! I made the mod around 20 years ago and amp has never had a failure!
good video. I keep thinking that those really fine leads are okay as long as they kept them short enough so that the pin keep the temperature down until the rest of the element warmed up and lowered the current.
I have done it by placing a light bulb in series with ac input with a switch to bypass once tubes start to warm up.
I love light bulb limiters!
Awesome video and explanation. Thanks as always.
+John Fine
Thanks for stopping by John!
I can see that the filament that has the flash has a loop in it with no coating before it connects to the pin. The one with no flash has the coating brought right up to where the attachment is made.
Interesting stuff. Glad I stumbled onto you're channel.
+notionSunday
Glad your enjoying the video's!
Damn, now I have to replace one of the tubes in my (German made) headphone amp. A little knowledge, etc. Thanks for the great video.
Cheers, John
+Zone1242
Glad you enjoyed John!
Good info! Thanks. Now, what about using a high wattage resistor in series with the filament transformer? I would install a "warm up" mode switch in a convenient location to bypass the resistor. Or maybe incorporate a time delay circuit with a relay to bypass the resistor. More like the speaker time delay for transistorized amplifiers to eliminate the "thump". Thanks again for the great video.
+Greenfield 54
Sounds reasonable! Thanks for your comment!
Brilliant! This explains so much. Thanks
You're Welcome Ronald!
Fantastic video! I've always wondered about this. Many EL84 guitar amps I've worked on suffer from this problem.
+Ryan Blecher
Thanks Ryan!
Very nice explanation and solutions.
+David Gillooly
Thanks David!
Interesting topic. I wonder whether in many pieces of equipment the over current on start up is softened by the heater transformer voltage sagging at switch on, then recovering as the heater filament warms. This is likely as the transformer will be rated for "hot" operational current. Another option for hi-fi applications is to use a dc constant current heater supply instead of a constant voltage supply.
You are the best elec teacher i know :)
Happy New Year lad, good video.
+David Okanagan
Thanks David!
Awesome episode thank you!
You're welcome Dave!
ive been meaning to ask where you learned all that you know. vacuum tube technology has been out of favor for a lot of years, so i thought, "hes not old enough to get it on the job, so he must be a serious tinkerer. but just now you mentioned making a test tool to deal w pallets full of vacuum tubes that had heater flash so. that either means im way older than i wanna be or you absobed a lot of good info in your formative years!
I'm not sure weather you are talking about modern made valves (tubes) or older pre 1970.
I started work in Marconi Instruments (English Electric) in the late 60's started on mainly valve test equipment.
I do not remember any case of heater (filament) flash.
As I remember most valves were mainly English Electric (EE) or Mullard but there were others I cannot recall.
I'm sure if we had this problem especially EE would have felt with it.
Thanks for anothe great, well described video
That's a great idea I've seen that in my 35 year old guitar amp and thought sure be nice to stop that really good info thanks for sharing.
+wade hicks
Glad you enjoyed Wade!
Very interesting trick. Seems like the incandescent lamps also always burn out on ignition.
I'm building a 5e3 tube guitar amplifier, and I wanted to replace the classic 'standby' switch (which I'm not convinced is doing any good) with your 'pre-heat' switch.
Which resistor value would you recommend for a double 6v6, 12ax7 and 12ay7 setup (it also has a pilot 6.3v lamp)?
Best reason to engineer DC filament supplies with a soft start feature, that ramps up the voltage over a period of time. I usually choose about 45 seconds, and do not start the high voltage supply up until the filaments have been on full for about 2 minutes. Also observe the heater to cathode voltage limits in the tube manual. In some cases you may have to create several filament supplies and operate them electrostaticly floated at a specific positive or negative voltage in order maintain the specification for any given tube. In a typical circuit of mine a differential cascode triode set has a pair of cathodes near ground, and another pair at about +125. In this case, a high impedance voltage divider is created to bias the filament at about +67 volts. this puts the cathodes of all four elements at far less than the +-100 volts specified for the tubes used. The long tail current source for the differential set above also consists of a series stack of parallel triodes similar to the tubes used in the diff amp above it. Again, the design centers of where the cathodes are below ground, and a single filament supply biased to a point midway between the cathodes is used. If the volage between the cathodes was near or exceeds the heater to cathode specification, additional filament supply will be needed. In some amplifiers, I need 3 floated and biased filament supplies in the front end circuit, and 2 separate supplies for the power tubes, each bias point driven by a buffer to 'follow' the instantaneous voltage at the cathodes of the low impedance power high current tubes in the Circlotron topology balanced bridge output stage.
...SOUNDS COMPLICATED-(!)
Just run the filaments all the time IN standby hot but not all the way.... I have seen this in highbred television sets... right before they went all solid-state..... Old-timer here... Good video, Sir
4:04 saw an Allen-Bradley Panelview CRT doing that once, years ago
Excellent. Many thanks.
+MrShobar
Your Welcome!
Very nice video. Thank you very much.
+esmaeil nam
Your welcome.