According to me, a hum balance pot is a good way to go. Inside my amp I placed a 250 Ohms 4W potentiometer for the purpose and it works great! But I think the voltage divider method worth the try.
On the lower diagram of the 6.3V secondary winding, is the line connecting what could be a true center tap of the secondary with a point between the the two 100R resistors actually meant to be there or is it a mistake in the diagram that was not corrected? That is, at 4:15 Mark refers to a virtual center tap between the two 100R resistors but an actual CT is depicted in the diagram.
I just built a nice stereo HiFi 6v6 P-P Ultralinear. Sounds fantastic but has a very low level buzz/hum. It is independent of volume, source or no load on source. B+ is stable but a bit low i have a weak rectifier i need to replace. Phase inverters and pe-amp are 6SN7. Pulling preamp lowers the hum very very slightly, pulling PI tubes eliminates it so it is not coming from PA stage. Filaments are AC with no CT on transformer. I have tried the virtual CT and potentiometer method in the past but have not had success with it. This is the first i have learned about floating the filaments above ground, did some research and apparently it is also common to use the cathode bias voltage of a PA stage to float the filaments. My PA stages are fixed biased around 60mA resulting in an almost perfect 20v bias. Would it be reasonable to use the bias voltage of one channel to float the filaments at 20VDC to eliminate some of the buzz from the filament supply? Sources I have read say it should put next to no load on the bias circuit since its not actually drawing current just creating a reference. Yes i realize i could supply the filaments with regulated DC, but my goal on this amp is to use zero modern technology.
the other heater centre tap is to take the centre tap of the heater supply to the cathodes of the output tubes in class A/B 1 or class A1 to elevate the ground
I have a BIG, funky looking 60 watt Webster Chicago PA amp (unknown working condition), cathode-biased, with 4) 6L6GB'S that I want to convert for guitar use. Couldn't find a schematic so I've been drawing one by hand; come to find it apparently floats the 6L6 cathodes by means of the tail-end of the main B+ supply (after all the preamp tubes are powered), fed thru a BIG 5K power resistor and connected to the 6L6 cathodes! Or, to look at it another way, perhaps the cathodes of the 6L6's are floating the tail end of the power supply? I found a somewhat similar Rauland amp schematic in my Sam's collections that does pretty much the same thing; that schem shows the 6L6 cathodes at 25 volts above ground, though that doesn't seem particularly unusual for a cathode biased amp (one shared 100 ohm power resistor to bias all four 6L6's). I expect it'll be quite a project to figure out how this beast works and try to bring it back to life. It has three mic inputs, each with its own 5879 pentode (mounted on shock-isolating rubber grommets), a 6SJ7 after the input pre's, a 6SN7 for the bass and treble controls (Jones or Baxandall? It has small inductors wired to the pots), and a 6SL7 phase invertor.
Great video. Thanks Mark. At 3 minutes into the video (3:00/7.:53) ... lower schematic ... are the two 100R resistors (as would be used to create an artificial center tap) still required when there's a real Center Tap? What would they do, other than just dissipate power? cheers, Simon
Hi! Can i use elevated heater with fixed bias AB poweramp or its just only for cathode bias poweramp? If i have center tap on the heater and the heater is elevated, how should i wire the ground, if i would like to use DC for preamp tubes(hi-gain amp)?
So if you were to lower the the two 100 Ohm resistors to say two 47 Ohm 2 watt resistors, would that offer more, less, or the same cancelling effect in filtering out hum and noise? I'm wondering why 100 Ohms? Thanks Great Vids!
To reduce load on the transformer winding while still remaining effective. If you have the head room you can go lower if you want, but waste more power.
So each of those 100K Ohm resisters with 3.15V AC across it will always draw 0.0315 mA. They will dissipate 0.09819 mW of power. Do I have that right??
Those resistors need to be a low value of 100 Ohms or 220 Ohms to shunt leakage currents from the transformer primary. 100 Ohm 1/2W or 220 Ohm 1/4 are usually used.
Tons of debate on this one. Seems these filaments were designed to work on AC and while they work on DC, many purist say not as well. Otherwise you would see a bunch of small LM317 or similar voltage regulator in tube amps, which you don't... I'm not sure there is a right answer, just an opinion.
the datasheet of a 12ax7 for example says Heater Voltage Ac OR Dc , 12.6 series or 6.3 parallel. From my experience If you can have a real good ripple free DC voltage source it would be better for a single heater tube (especially if it is in the preamp chain or at any high gain stage).
mamoynas I just tried this yesterday. I hooked up my dc power supply to all of the heater circuits in my amp, expecting it to eliminate all hum, pretty low anyways, however: it made absolutely no difference at all. 60hz hum stayed exactly the same. The music sounded the same. I just put it back the way it was, that was pointless, my hum must be from the transformers being so close to the output tubes.
@@michaelhelgeland4588 get a piece of Ferris (not aluminum) sheet metal and ground it with an alligator jumper to your star grounding point then hold and move it between tubes, tubes and transformers etc. as a makeshift test shield, if you cant find it topside then its probably wires too close or crossed leaking RF etc. If the amp is already built, and you more interested in output than glow you find that it indeed is a tube too close to transformer there are good looking polished tube shields cheap aluminum to very nice Stainless?
@@doncalloway4125 Thanks Don, I was actually experimenting with this the other night. My amp is a Nytone SA-2000 so it's not the best tube amp ever built but I've restored it. I actually discovered that the hum is 10 times worse in the Phono stage even though the phono inputs are shielded. I got to messing around and made an aluminum foil shield around all of my input wires, I also made extra shields for the capacitors. I abandoned the phono preamp on my amp and turned on the one to the turntable and just hooked it up to FM which is identical to AUX. This made most of the audible hum disappear at least at useable volumes if I crank it up past 3 I can't be in the same room, and thats's where the hum is most noticeable now, but with no signal I can turn it up to 10 and it will still hum at a good 30-40 db at 60 HZ there. I added a choke and more filter caps that did nothing but maybe improve bass response. Also any 12ax7 I would put in the front end would whistle at my unusable volume levels, I converted it to use 6n2p-EV and it is much quieter no whistle and no microphonic effect when tapped on not even at full volume.
It would not as long as there is no ground reference in the circuit. AC is a value of fluctuating voltage based on a reference, usually that reference is ground (0V) by referencing a higher voltage you will still have 6.3VAC with respect to that reference. The heaters will still only see 6.3v.
According to me, a hum balance pot is a good way to go. Inside my amp I placed a 250 Ohms 4W potentiometer for the purpose and it works great! But I think the voltage divider method worth the try.
Thanks for explaining this in more detail.
On the lower diagram of the 6.3V secondary winding, is the line connecting what could be a true center tap of the secondary with a point between the the two 100R resistors actually meant to be there or is it a mistake in the diagram that was not corrected? That is, at 4:15 Mark refers to a virtual center tap between the two 100R resistors but an actual CT is depicted in the diagram.
I just built a nice stereo HiFi 6v6 P-P Ultralinear. Sounds fantastic but has a very low level buzz/hum. It is independent of volume, source or no load on source. B+ is stable but a bit low i have a weak rectifier i need to replace. Phase inverters and pe-amp are 6SN7. Pulling preamp lowers the hum very very slightly, pulling PI tubes eliminates it so it is not coming from PA stage. Filaments are AC with no CT on transformer. I have tried the virtual CT and potentiometer method in the past but have not had success with it. This is the first i have learned about floating the filaments above ground, did some research and apparently it is also common to use the cathode bias voltage of a PA stage to float the filaments. My PA stages are fixed biased around 60mA resulting in an almost perfect 20v bias. Would it be reasonable to use the bias voltage of one channel to float the filaments at 20VDC to eliminate some of the buzz from the filament supply? Sources I have read say it should put next to no load on the bias circuit since its not actually drawing current just creating a reference. Yes i realize i could supply the filaments with regulated DC, but my goal on this amp is to use zero modern technology.
the other heater centre tap is to take the centre tap of the heater supply to the cathodes of the output tubes in class A/B 1 or class A1 to elevate the ground
I have a BIG, funky looking 60 watt Webster Chicago PA amp (unknown working condition), cathode-biased, with 4) 6L6GB'S that I want to convert for guitar use. Couldn't find a schematic so I've been drawing one by hand; come to find it apparently floats the 6L6 cathodes by means of the tail-end of the main B+ supply (after all the preamp tubes are powered), fed thru a BIG 5K power resistor and connected to the 6L6 cathodes! Or, to look at it another way, perhaps the cathodes of the 6L6's are floating the tail end of the power supply? I found a somewhat similar Rauland amp schematic in my Sam's collections that does pretty much the same thing; that schem shows the 6L6 cathodes at 25 volts above ground, though that doesn't seem particularly unusual for a cathode biased amp (one shared 100 ohm power resistor to bias all four 6L6's). I expect it'll be quite a project to figure out how this beast works and try to bring it back to life. It has three mic inputs, each with its own 5879 pentode (mounted on shock-isolating rubber grommets), a 6SJ7 after the input pre's, a 6SN7 for the bass and treble controls (Jones or Baxandall? It has small inductors wired to the pots), and a 6SL7 phase invertor.
Great video. Thanks Mark. At 3 minutes into the video (3:00/7.:53) ... lower schematic ... are the two 100R resistors (as would be used to create an artificial center tap) still required when there's a real Center Tap? What would they do, other than just dissipate power?
cheers, Simon
Hi! Can i use elevated heater with fixed bias AB poweramp or its just only for cathode bias poweramp? If i have center tap on the heater and the heater is elevated, how should i wire the ground, if i would like to use DC for preamp tubes(hi-gain amp)?
Good info here!
With regard to the centre tap application, what voltage handling capacity is need for the resistors?
So if you were to lower the the two 100 Ohm resistors to say two 47 Ohm 2 watt resistors, would that offer more, less, or the same cancelling effect in filtering out hum and noise? I'm wondering why 100 Ohms? Thanks Great Vids!
To reduce load on the transformer winding while still remaining effective. If you have the head room you can go lower if you want, but waste more power.
Hi can you write to me the reference of the japanese site cause you tube give me a weird traduction ,i speak french .thanks a lot
Does it cause any problems using rectified dc filament voltage?
So each of those 100K Ohm resisters with 3.15V AC across it will always draw 0.0315 mA.
They will dissipate 0.09819 mW of power.
Do I have that right??
Those resistors need to be a low value of 100 Ohms or 220 Ohms to shunt leakage currents from the transformer primary. 100 Ohm 1/2W or 220 Ohm 1/4 are usually used.
Why not just send DC to the filaments by way of a full wave rectifier and capacitor to reduce 60 Hz hum ?
Tons of debate on this one. Seems these filaments were designed to work on AC and while they work on DC, many purist say not as well. Otherwise you would see a bunch of small LM317 or similar voltage regulator in tube amps, which you don't... I'm not sure there is a right answer, just an opinion.
the datasheet of a 12ax7 for example says Heater Voltage Ac OR Dc , 12.6 series or 6.3 parallel. From my experience If you can have a real good ripple free DC voltage source it would be better for a single heater tube (especially if it is in the preamp chain or at any high gain stage).
mamoynas I just tried this yesterday. I hooked up my dc power supply to all of the heater circuits in my amp, expecting it to eliminate all hum, pretty low anyways, however: it made absolutely no difference at all. 60hz hum stayed exactly the same. The music sounded the same. I just put it back the way it was, that was pointless, my hum must be from the transformers being so close to the output tubes.
@@michaelhelgeland4588 get a piece of Ferris (not aluminum) sheet metal and ground it with an alligator jumper to your star grounding point then hold and move it between tubes, tubes and transformers etc. as a makeshift test shield, if you cant find it topside then its probably wires too close or crossed leaking RF etc. If the amp is already built, and you more interested in output than glow you find that it indeed is a tube too close to transformer there are good looking polished tube shields cheap aluminum to very nice Stainless?
@@doncalloway4125 Thanks Don, I was actually experimenting with this the other night. My amp is a Nytone SA-2000 so it's not the best tube amp ever built but I've restored it. I actually discovered that the hum is 10 times worse in the Phono stage even though the phono inputs are shielded. I got to messing around and made an aluminum foil shield around all of my input wires, I also made extra shields for the capacitors. I abandoned the phono preamp on my amp and turned on the one to the turntable and just hooked it up to FM which is identical to AUX. This made most of the audible hum disappear at least at useable volumes if I crank it up past 3 I can't be in the same room, and thats's where the hum is most noticeable now, but with no signal I can turn it up to 10 and it will still hum at a good 30-40 db at 60 HZ there. I added a choke and more filter caps that did nothing but maybe improve bass response. Also any 12ax7 I would put in the front end would whistle at my unusable volume levels, I converted it to use 6n2p-EV and it is much quieter no whistle and no microphonic effect when tapped on not even at full volume.
does it make the heater voltages higher than 6.3? how not?
It would not as long as there is no ground reference in the circuit. AC is a value of fluctuating voltage based on a reference, usually that reference is ground (0V) by referencing a higher voltage you will still have 6.3VAC with respect to that reference. The heaters will still only see 6.3v.
Does this apply to '70s solid-state as well?
Solid state doesn't have heaters :)
Why not just put in a bridge and run DC Hum problem solved