This is an exemplary video, even by Mark's high standards. The approach to trouble shooting using the schematic and the process of elimination is logical and very clearly explained and the video and explanation of the testing steps are excellent. Little details like the focus on the tube pins being tested and the clarity of the schematic are exceptional and appreciated. Thanks Mark, this was very useful.
I'm an engineering student just getting into vintage audio work, I can't overstate how helpful this was! Large schematics definitely feel less impossible to work with. Thank you for the help!
Amazingly helpful and clear. Every one of these that I watch makes this whole process a little less mysterious and scary. I understand so much more about amps just from watching your videos!
Hey Mark, hello from Canada! (Re)discovered vacuum tube amplifiers about six months ago and your site shortly thereafter. Love what you’re doing and looking forward to what you have planned for 2021. Cheers, Cole
Such a great video thank you so much for sharing that and showing us the method or approach you used to solve that problem. That's a joke you should have titled the video how to replace 120 pico farad capacitor!
A good review of classic troubleshooting techniques, thanks Mark. - cut the problem in half, evaluate and *repeat* - DC voltage checks! - same resistance, different voltage drop -> biasing and current inequalities -> look for the cause - don't always assume a resistor is a resistor or a capacitor is a capacitor, capacitors can turn into resistors or even short out. Keep an open mind. Cheers, - Eddy
I sell electric repair units and electric repair unit accessories. Something not right with that tube bias... Hah, such a clear and well thought out video. Thanks!
Thanks for the video Mark. You taught how to break down sections which is very helpful with issues like this amp had. I would like to see a video on how to set the bias on these units!!! Thanks!
Best and most succinct lesson ever. I'm into a 2-cabinet 1958 Magnavox w/ stereo record changer and mono receiver (i guess that year was the cusp of stereo?). Can't seem to find a clear schematic because I'm not so adept at digging yet. Power amp 175-67 is its chassis # I need to pull the receiver/preamp? chassis out again to get its #, but really, no need. this thing is so simple, right? This video is the new grail, thanks.
*have I commented similarly before on your posts? -forgive the spam. I watch all of you guys and trying to get myself to do this machine w/o frying it, or worse, myself. :^) thanks again, stay hungry.
Very useful, especially as I have a similar issue with my WAD KT88 amp at the moment. It's on the "bench" (my computer desk!) and I'm about to try replacing all the valves to start with. One channel is very quiet and not working clearly, I'll have to get the multi-meter out if the replacement valves don't work (no oscilloscope). I have the schematic and even details of what points in the amp to start testing voltages.
Not really liking the new mic. It seems to be picking up more background noise and adds some hollowness to your voice. Old mic seemed to only pick up your voice and gave it a warmer tone. Unless you can fiddle with sensitivity and tonal mix, I'd stick with the old one.
One of your best videos! Real time signal tracing with the schematic and logically working thru it. Does it matter what function the signal generator input is used, ie, aux, tape, phono?
Again you make it look so easy and simple. Great tips on troubleshooting and narrowing the problems down. Up in the air over the mic, the one sounds like it is in stereo and can tell when you moved around, Wondering if you got it so when you play an amp it will reproduce the sound somewhat better. Thanks a lot again and hope you don't get too much snow there
If you can't get a schematic for an integrated like this or other amps, you really need to know your circuits, and I am guessing that most viewers here are not in that camp. Having (and of course, knowing how to read) a schematic makes it easy to puzzle out and get to the trouble spot quickly, otherwise you need to know how tubes work and in what stages to expect, etc. I think that being able to read and use schematics greatly helps in understanding how equipment works and helps educate one in electronics. As far as the test gear, pretty much spot on, signal source, scope, dvm, voltage AND current monitoring variac, tools and soldering gear and monitoring speakers and/or load at a minimum. Not cheap to buy, so maybe this will give some people an idea why techs charge actual money to fix electronics, a bench like Mark's is not cheap!
Question for you. Coming out of that 6u8 tube the plate and cathode each feed a 7591 tube. My levels are great coming out of the 6u8 tube but output is low out of power tubes. Both sides same. Voltages all ok, tested o/p transformers all test good..anything ideas?
I am saving to get a single ended tube amp . I have very very easy speaker to drive which were made in Italy, Casta B Speakers. I had Scott mono blocks that to my ears just were too soft and slow. Wish to buy one A class single ended tube amp maybe 4 amps
im having a issue with my tube preamp, its oscillating really really slowly on the output, you cant hear it in the speakers but i put it on my oscilloscope just to see whats up with a little hum i was getting from the power amp coming from this little preamp when my power amp is turned up all the way. anyway what im seeing is a random super slow DC+ to DC- on the putout side, it seems to kinda walk around up and then down randomly....then farther down then back up just in the plus or negative 200mv or so range. also its on both left and right channels and they move up and down at the same time together. any idea ? i can send you a pic or short vid of what it looks like. oh yeah and its a (Little DOT MKii) thats doing this.
So one channel was not louder but the other channel was quieter. It is sometimes hidden in the details, as at the early moments of this video, there was a slight phase shift. There was the smoking gun?
Thinner sound from the new mic, along with higher background noise. My humble opinion only. Everything else excellent as usual. Between Blueglow, Uncle Doug and Mr. Carlson, every angle is covered.
How do you get a clean scope signal without the ground lead? I know it may work do to the power ground of the scope or a lead from the scope chassis to the amplifier, but with or without and isolation transformer the scopes chassis ground needs to be common with the amplifier chassis.
I’m also not understanding this. What is the return path for the oscilloscope signal. Is it plugged into the isolation transformer also. Otherwise an excellent explanation. Many thanks.
why do you need an isolation tranny IF the unit already contains an isolated power transformer. what is the difference between: wall - > iso + variac + iso [power transformer] and wall - > variac + iso [power transformer] ?
In the early days of TV/Radio/electronics many times one side of the power cord was simply tied to the chassis. In those days they did not have the polarized two prong plugs. An isolation transformer was a MUST. It is NOT necessary with units that have a properly wired power transformer, but for the techs, using the ISO transformer was the easiest way to avoid really bad problems.
This is an exemplary video, even by Mark's high standards. The approach to trouble shooting using the schematic and the process of elimination is logical and very clearly explained and the video and explanation of the testing steps are excellent. Little details like the focus on the tube pins being tested and the clarity of the schematic are exceptional and appreciated. Thanks Mark, this was very useful.
I'm an engineering student just getting into vintage audio work, I can't overstate how helpful this was! Large schematics definitely feel less impossible to work with. Thank you for the help!
Great walk-through. Very instructive
No "schematic (link below)" but it was easy enough to find. Nice work finding the problem.
Amazingly helpful and clear. Every one of these that I watch makes this whole process a little less mysterious and scary. I understand so much more about amps just from watching your videos!
Hey Mark, hello from Canada! (Re)discovered vacuum tube amplifiers about six months ago and your site shortly thereafter. Love what you’re doing and looking forward to what you have planned for 2021.
Cheers,
Cole
Excellent review on how to systematically evaluate and repair. Really enjoyed it.
Such a great video thank you so much for sharing that and showing us the method or approach you used to solve that problem. That's a joke you should have titled the video how to replace 120 pico farad capacitor!
A good review of classic troubleshooting techniques, thanks Mark.
- cut the problem in half, evaluate and *repeat*
- DC voltage checks!
- same resistance, different voltage drop -> biasing and current inequalities -> look for the cause
- don't always assume a resistor is a resistor or a capacitor is a capacitor, capacitors can turn into resistors or even short out. Keep an open mind.
Cheers,
- Eddy
I sell electric repair units and electric repair unit accessories. Something not right with that tube bias... Hah, such a clear and well thought out video. Thanks!
Thanks for the video Mark. You taught how to break down sections which is very helpful with issues like this amp had. I would like to see a video on how to set the bias on these units!!! Thanks!
Great video!
Your instruction on how to approach the troubleshooting methods is very informative. The kind of information I'm watching for.
Best and most succinct lesson ever. I'm into a 2-cabinet 1958 Magnavox w/ stereo record changer and mono receiver (i guess that year was the cusp of stereo?). Can't seem to find a clear schematic because I'm not so adept at digging yet. Power amp 175-67 is its chassis # I need to pull the receiver/preamp? chassis out again to get its #, but really, no need. this thing is so simple, right? This video is the new grail, thanks.
*have I commented similarly before on your posts? -forgive the spam. I watch all of you guys and trying to get myself to do this machine w/o frying it, or worse, myself. :^) thanks again, stay hungry.
Great video. Very helpful. Thank you
Very good job. I would also mention setting your scope to AC coupling.
Great explanation how to follow an squematic. I always learn something new with your videos. Thanks for sharing
Very useful, especially as I have a similar issue with my WAD KT88 amp at the moment. It's on the "bench" (my computer desk!) and I'm about to try replacing all the valves to start with. One channel is very quiet and not working clearly, I'll have to get the multi-meter out if the replacement valves don't work (no oscilloscope). I have the schematic and even details of what points in the amp to start testing voltages.
This is a great video ! Thank you for sharing your knowledge of how to put away a whole elephant with just a knife and fork. Cheers !
Thanks, Mark, another great offering. Some clear and logical troubleshooting tips indeed.
Not really liking the new mic. It seems to be picking up more background noise and adds some hollowness to your voice. Old mic seemed to only pick up your voice and gave it a warmer tone. Unless you can fiddle with sensitivity and tonal mix, I'd stick with the old one.
One of your best videos! Real time signal tracing with the schematic and logically working thru it. Does it matter what function the signal generator input is used, ie, aux, tape, phono?
Again you make it look so easy and simple. Great tips on troubleshooting and narrowing the problems down. Up in the air over the mic, the one sounds like it is in stereo and can tell when you moved around, Wondering if you got it so when you play an amp it will reproduce the sound somewhat better. Thanks a lot again and hope you don't get too much snow there
Exactly! Its supposed to be a full range stereo mic but for $160, not sure its the best sounding. Still in search...
I quit reverse engineering after my last job for Allcom. Couldn't handle being chased by various agencies with the fate of the world in my hands. 😁
If you can't get a schematic for an integrated like this or other amps, you really need to know your circuits, and I am guessing that most viewers here are not in that camp. Having (and of course, knowing how to read) a schematic makes it easy to puzzle out and get to the trouble spot quickly, otherwise you need to know how tubes work and in what stages to expect, etc. I think that being able to read and use schematics greatly helps in understanding how equipment works and helps educate one in electronics. As far as the test gear, pretty much spot on, signal source, scope, dvm, voltage AND current monitoring variac, tools and soldering gear and monitoring speakers and/or load at a minimum. Not cheap to buy, so maybe this will give some people an idea why techs charge actual money to fix electronics, a bench like Mark's is not cheap!
How do you keep your shop so neat and clean? that's amazing...
Question for you. Coming out of that 6u8 tube the plate and cathode each feed a 7591 tube. My levels are great coming out of the 6u8 tube but output is low out of power tubes. Both sides same. Voltages all ok, tested o/p transformers all test good..anything ideas?
I am saving to get a single ended tube amp . I have very very easy speaker to drive which were made in Italy, Casta B Speakers. I had Scott mono blocks that to my ears just were too soft and slow. Wish to buy one A class single ended tube amp maybe 4 amps
Hi Mark, I am using a pair of Goldnote S6-XL, 90DB @ 6 ohms and was wondering how the Unison S6 tube amp would perform. It’s rated at 35 amps, thanks
im having a issue with my tube preamp, its oscillating really really slowly on the output, you cant hear it in the speakers but i put it on my oscilloscope just to see whats up with a little hum i was getting from the power amp coming from this little preamp when my power amp is turned up all the way.
anyway what im seeing is a random super slow DC+ to DC- on the putout side, it seems to kinda walk around up and then down randomly....then farther down then back up just in the plus or negative 200mv or so range.
also its on both left and right channels and they move up and down at the same time together. any idea ? i can send you a pic or short vid of what it looks like.
oh yeah and its a (Little DOT MKii) thats doing this.
So one channel was not louder but the other channel was quieter. It is sometimes hidden in the details, as at the early moments of this video, there was a slight phase shift. There was the smoking gun?
Thinner sound from the new mic, along with higher background noise. My humble opinion only. Everything else excellent as usual. Between Blueglow, Uncle Doug and Mr. Carlson, every angle is covered.
Are scope leads grounded to the chassis of the Scott? And is the Scott plugged into an iso transformer?
variac is always plugged into an isolation transformer. no scope ground leads attached. easy stuff
How do you get a clean scope signal without the ground lead? I know it may work do to the power ground of the scope or a lead from the scope chassis to the amplifier, but with or without and isolation transformer the scopes chassis ground needs to be common with the amplifier chassis.
I’m also not understanding this. What is the return path for the oscilloscope signal. Is it plugged into the isolation transformer also. Otherwise an excellent explanation. Many thanks.
why do you need an isolation tranny IF the unit already contains an isolated power transformer.
what is the difference between:
wall - > iso + variac + iso [power transformer] and
wall - > variac + iso [power transformer] ?
In the early days of TV/Radio/electronics many times one side of the power cord was simply tied to the chassis. In those days they did not have the polarized two prong plugs. An isolation transformer was a MUST. It is NOT necessary with units that have a properly wired power transformer, but for the techs, using the ISO transformer was the easiest way to avoid really bad problems.
I agree with the others, the old microphone did a better job with your voice
New mic is horrible. I liked the old one much better, it sounded professional. This one keeps favoring my left ear, and really tinny.
Yum!
Gouging my eyes out with a solder tip.
I woulda just told him to use the balance knob :P
Caps. Every dang time lol
new mic is awful.
I agree with the others, the old microphone did a better job with your voice