No way! He’s back! GREAT to see you, Mark! Glad you found a little time to play…. you look good.

Mark, I concur with others. Great to see you back in the video saddle. Your videos really helped me to (safely!) refurbish two HH Scott amplifiers. Also, like you, I’m recovering from a heart attack. Working on amps lowers ones stress level. Just don’t let your cardiologist know that you’re messing around with 450 VDC!

Mark, it is great to see your back again. Thank you for the video and troubleshooting techniques.

I bought a used MarkHill tube stereo Amp that was blowing fuses two of the 4 el34 tubes had the centering key broke off and one of them was clocked into the wrong position.. Easy fix great score

I would put it on the dim bulb tester and then troubleshoot it using normal procedures. Now you aren’t wasting fuses bust more importantly you get to see the results of whatever you do.

YOOOOOOOOO! Great to see you brother.

I found your channel since the last clip and it's great to see you publishing new content. I really appreciate the topics and the friendly, no-nonsense way you present them. Hope you'll have the time to keep posting once in a while!

Great to see you back Mark! Your content, clarity and style have been missed.

Excellent video. I have a Marshall DSL40C (40w, 2 x EL34) that has the same problem. and I actually noticed that the Pentode/Triode switch was in between. Hope I will get it fixed using your instructions.

Nice to see you back !

Whaaaaaaat? You’re back. Glad to see you again.

Glad you're back Mark. Your videos are very helpful and informative.

Hey Mark welcome back! A real treat. Thanks

Great to see another fine video by you. Thanks!..

Glad to have you back!!!

Great see you back bro! Hope all is well.

Hello sir Thank you for these help troubleshooting videos You are the only one doing these Love it

So excited to get notified of a new video-

It's alive!

welcome back Mark-Van-Winkle. Been a while, must have over slept. I sure the time lag was not from doing things like living life or anything like that. Good to see a video {smile emoji here).

Our buddy is finally out of hiding👍👍😁😁

Nice to see you around Mark, never got to build this amp but I’m still playing the SE 807 amp. You ever finish the 6sn7 preamp?

I Had a Cary SLI-80 that had the UL switch. I switched them with power on all the time. Never had an issue.

In my experience: First check the tube(s). In a push pull design you may find one flat tube, if you do - then replace the control grid coupling cap(s) with a good HV Mylar cap - change both tubes and test. The coupling caps are notorious and may not be shorted. BUT even a few uA of leakage will blow the output. It is the output that is capable of conducting a bunch of current. Second the 5AR4 may be toast. I got caught when buying a Tungsol new manufacture 5AR4. These pups may not sustain a sufficient reverse voltage and they die spectacularly. Some how I doubt that switching from UL to Triode caused a failure. But things happen. CAREFUL when turning on an amp without the power tubes. Depending on the design you can easily over voltage the remaining components. Use a variac and check the input current while advancing the line voltage. STOP if the B+ starts to exceed the bypass cap wvdc rating.

Thanks Mark! How do you determine the value for the screen grid resistor and what is its function/purpose?

I’m going through some of my own troubleshooting myself with an old late 80s PCB amp. Recently fixed a terrible wall of crackle coming from my Fender Super 112 by replacing the standby switch as it was arcing once actuated, cleaned it right up. I also gave the amp a huge refresh meaning all electrolytics and a multitude of resistors. I’m having an issue with it now where it makes my tubscreamer and really any OD pedal for that matter sound harsh and fuzzy. The largest tell-tale sign is my crybaby wah creates distortion in the toe position when going into the clean channel. Just to clarify, these issues do not happen with my other amps so I know it is the amp in question, not the pedals. I already checked the speaker by jumping the speaker leads to my other amp’s speaker and the problem remained, tomorrow I will try isolating whether the issue is in the preamp or power amp, any ideas what this could be? My bets on a failing coupling cap in the preamp stages but I’m not getting any crackling in any of the pots, only hiss in the clean channel’s bright mode.

🎉👍

Can you do this with a different amp that isn’t your particular amp? Edit: oh I saw the end and you said you can try it with other tube amps. Great!

I liked your video. I have one question. What is the purpose of the 200K Ohm resistor between the plate of the 6AQ8 and the plate of the KT88. I try to understand circuits when I see them. I'm not sure what the resistor does. Thank you.

I have question on my AA 151 Heathkit Amp I am rebuilding the fuse blows after powering up I followed your suggestion of pulling all the tubes off and powering up the, fuse remain intact but when I replace the GZ34 the fuse blow again. I tested the GZ34 and it tested good on the tube tester what could cause the fuse to blow on a tube that tested good

Hey Mark my son turned my 120 watt Randall amp on without it being hooked into the speaker cab and it blew a fuse I got a new fuse for it and as soon as I turn it on it's on but when I hit the standby to turn on it blows that fuse do you know what might be the trouble with it

Say the power supply had an issue. One where a typical ohm meter simply said, yea theres a resistance.. but at hv there was a break down of insulation. Could a meg-ohm meter be used to troubleshoot?

I know y'all a positive vibe type chan and forum but cmon. To whomever bothered him so much that his 1st video in awhile ,looking 20-30 pounds lighter was about this? Yo, LMGTFY : Methodical troubleshooting of vacuum tube amplifier circuits. There's probably at least 4 on this chan those being POWER, SOURCE, OUTPUT, FX and dont forget the forums have a search function and your not the first person to have this problem.

So... should one use a make-before-break UL switch?

when you switch the kt88 grid to 455volts the 450volts anode up to 1kv like a voltage buster thats why you blow your fuses

Mark - what's your thoughts if I am blowing fuses approximately every 2 weeks? Does your analysis still stand? Thanks!

Hello Mark I send you that email. I think I must buy a new GZ34 tube when I put this in the fuse blows. I let you know what the outcome is in a few days. Thanx for this video. Greetings from Dennis

hi Mark my question is can i use 3k5 OPT instead 5K for KT88 build?

My money is on a "shorted screen grid to cathode" in the KT88 tube. Place your bets boys.

Where have you been >

Not good practice to switch ....

earth conected resistancw and resistance close to 3way switch resistance is to low also resistancr on ecc85 is to low value that is connected to the ground all capacitors rating voltages are wrong connected for 450 to 500 volts need a 1.6kv capacitor conected and not a 400 volt all schematic circuit is wrong connected

If that is blowing fuses just use a non blowing up fuse 🤣🤣🤪