Thanks Mark, I just started restoring my first console amp. I learned a lot from this video. Please keep making them.

Wow. Thanks. I also watch Mr Carlson's Lab videos. They are very technical. I just started watching your videos. They are much better from a "let's get it done" point of view. Everything you are doing and explaining is something I can just start using. I'll keep watching!

Thanks for pumping out the videos lately Mark!

That was a great troubleshooting video Mark, learned something new today "Negative Degeneration" issue.

Really intriguing. I would have guessed the fault incorrectly. When I saw practically no cathode voltage on the "left" tube (left side of the screen, not necessarily left channel) I thought something must be wrong. The cathode voltage on the right side tube made sense to me - there's current flowing in the cathode bias resistor, therefore you expect a voltage drop - exactly the same way a cathode follower works. But lo and behold, it looks like the cathode bypass cap of the right tube was open! More head-scratching to do on my side .... but thanks for the puzzle!

Great walkthrough of troubleshooting. Starting from input and going all the way to out put on DC and AC with scope. I've heard some say if you don't know where to start, start in the middle but going through start to finish makes a lot more sense. Thanks for the great lesson, I always enjoy your videos.

Love the troubleshooting vids. Thanks, Mark!

Good, practical instruction. Very useful...

Well done Mark.. you make it look easy and in reality it is as long as proper safety is observed. Thanks so much

Incredibly useful Mark! Thank you for this video.

Great lesson Mark

think this is the best fault finding video you ever made mark those caps look like some audio fhile trouble makers

Excellent vid, Mark. Thanks!

Excellent video ! Now I'm ready to do this !

thank you Mark. very interesting video and educative too

This was most interesting and informative. Just heard of Totem Pole recently in your video. Looks similar to SRPP or Cascode triode circuit.

Hmm you measured 60V over the bypass capacitor but the caps seem to be 330µF 35V ??? not surprised it did not last, I am surprised it lasted al long as it did, a 100V cap would be more realistic.

Thanks for the great video. I just learned a few things about my power amp.

Nicely done - a time-efficient run-through but still managed to be a thorough fault-finding process and all without a schematic :) I'm assuming you changed out those 27k resistors in the PS and if so, out of interest, how far out were they? I'd also replace the faulty capacitor, with a higher voltage part as 35v seems low; and do the same on the other side assuming it's the same part. (edit) Just saw that Martin Ahlqvist had already mentioned the capacitor's voltage rating being low.

Hmm. Considering it's an 'AC' problem (no clipping or non-linearity), I'd just probe around, comparing the channels with an AC-coupled 'scope! Having not viewed the entire video yet, my guess is a plate or grid resistor that is out-of--value, changing the gain. UPDATE: DUH! I should've realized that if it's an AC problem, to check cathode bypass cap,s. Out-of-value gain resistors WOULD affect DC performance! :)

i would twist all the ac wiring in this amp

Learning allot from you thanks

Late to the show as usual, but did anyone mention that there was about 60 volts across those 35 volt cathode bypass capacitors? No wonder that capacitor was damaged. It will happen again unless they are upgraded in voltage rating.

I have a big mystery. This tube amp, when I plug in both RCA input cables, vocals/voices become recessed/distant/thin. If I plug only the left RCA or only the right RCA, then vocals/voices are back and overall volume is higher. So, it seems the circuit subtract one channel from the other and amplifies what is left. Ever seen anything like that?

Looks like option 2 of the 300 build ?

Well, to quote Dave... don't turn in on, tear it apart :D Another perusal through the P2P circuits of a tube amp, looking forward to it! Cheers,

Thanks for share

Fantastic.....cheers.

Hey man, would you consider doing a video on general safety and how not to get electrocuted when working on vintage gear? I zapped myself BAD trying to replace a resistor on a dynaco last week. I have 70s receiver I want to attempt a recap on and I don’t want to die. There’s not a ton on KZfaq about audio gear and what exactly to do to make sure you don’t get zapped. Thanks a lot, enjoying your videos and learning a lot.

Hi there i have the same problem with the one channel plays lower than the other but the problem is the potentiometer. I use a stereo 1M potentiometer where the one section has a difference of about 10K from the other. I bought 3 different potentiometers and none of them have both sections with exactly the same resistance. also if you do the right left channel test, the music from the right channel is heard low and in the left channel and vice versa. . Do you know what I could do to solve these problems? I've really tried everything

hi ! hey mark i need some help ! i have a 6v6 pp mono amp. here is the problem ! no sound comes out , the sound goes in through a 12ax7 tube and it reaches the output of the 6v6 tubes but it does not come out to the speakers , i changed tubes , no luck , i know the sound reaches pin one of the 6v6 but it does not come out of pin 3 which is the plate of the tube and that is where the taps of the output transformer is connected . another isssues is that the schematic needs 310 v dc on the plates of the tubes according the schematic , but i get 427 vdc , the power tran. is supposed to be 315vca x 2 centre tapped with 6.9 volt filaments . this is the transformer is in the amp , and the proper voltages on the primary side are correct as per schematic , but how come i end up with 427 volts dc on the plates ! is this the reason for no sound out to the speakers ? when i probe with my signal tracer sound is going into the tubes but i get no sound out of the plates of the outputs . can you advise as what is going on ?

Could you make a video showing how to use a chassis punch?

outstanding video! really good practical advice. one question: what kind of Oscilloscope probe did you use? 10:1, 100:1 ??? I am completely paranoid about screwing up my oscilloscope! I do use an isolation transformer on tube radios etc--but its only rated at 300 watts--so not really suited for a amp like this...any tips really appreciated. Thanks!

good logical trouble shooting.If you don't bypass that ac signal, it will offset the dc bias of the tube on the load line and therefore reduce the gain, ie negative feedback to the grid.

recommendations for what to look for on an oscope? looking at cheap digital ones just for troubleshooting like this.

When the bypass capacitor is not in the cathode circuit there is Negative Degeneration, any reasons why there is negative cathode degeneration or negative emitter degeneration which lowers the AC gain of a circuit or stage? What is causing Negative Degeneration?

So the signal we saw on the bad capacitor was being sent to ground instead of getting amplified by the output tube. Do I have this about right?

Is it me or when paralleling capacitors the positive and negative had been reversed?

Top notch

would adding a choke prevent red-plating?

That was fun.

I don't understand how that preamp circuit can work without a plate resistor?

that capacitor doesn't even that old

Bad caps, bad caps, Whatcha gonna do ? Whatcha gonna do when they leak on you ?

What is a rectifier capacitor? You solved that like a real amateur.

You are testing voltages while the amp is not under load , I see a problem with this

I'll bet those cathode caps are getting cooked by the cathode resistors.