There are amps out there like Mesa boogie simulclass where they have a pair of each EL34's & 6l6's. But these are pairs of tubes and the amp is designed to allow this arrangement. No amp that I know of uses one of each different tube as a pair. It just doesn't work in a balanced way. People spend all day trying to create a matched pair of tubes for their amp circuits. EL34 tubes require a power transformer that can handle the current demand which is more than a 6L6 demands on a circuit.

Thanks Jazz.   

Quote from: jbarrie88@sbcglobal.net on May 23, 2024, 05:44:09 PMPardon my inexperience. How would I do that without providing a URL?

When you are in the 'quick reply' area click on the 'preview' button.  There will be an option to directly attach your files here.

I recently purchased a TSA30H head for a small price. According to the seller the volume goes way down after a little while. After reading rave reviews on this amp and figuring the problem sounds like just some bad tubes I went ahead with the deal.
Then looking for a schematic I stumbled on this thread and started thinking that my small price wasn't small enough.
Anyway, plugged it in today, hooked to a V30 cab, and I'm quite happy with the sound. Yes, there's a slight hum on ridiculous volume, but not more than on many an old Fender. After a few minutes I got the opposite of what I was expecting. The volume shot way up giving me a slight heart attack and some angry looks from the missus. Played for another 10 minutes or so without further issues. Put the guitar down and left the amp to warm up for half an hour. Sure enough, after that the volume was extremely low.
So I pulled the tubes for visual inspection and much to my surprise it had one 6L6 and one EL34 inside. Now I understand that you should not put EL34's in an amp set up for 6L6, but could that be the whole reason for the soft-loud-soft sequence?
I'm trying to get my head around it, but I just don't understand tube behavior well enough.
I also understand that asking this on a solid state forum is sort of blasphemous, but this seems to be the only thread on this amp in the whole world wide web.
One way or the other, I ordered a pair of 6L6's for proof and pudding but that will take a few days.
I was also wondering if using an EL34 can do damage to the rest of the circuit and if so, what I should be looking for?

Here are the files.

Pardon my inexperience. How would I do that without providing a URL?

Bad caps should not make the supply voltages read high.
Getting the correct power supply voltages should be your first step.  You might be lucky enough that it solves all your problems.

I can't access dropbox files.  Could someone please attach the power amp and power supply schematic from the first post?

DrGonz78: If I play soft there is no sound at all, have to strum hard to get any sound, and then after 15 seconds or so of the amp being switched on, it starts to make a screeching/crackling sound and gets louder and higher over time.  Also, if I strum hard after the noise starts, you can still hear the guitar, but it is still very distorted and cannot be manipulated by the volume knobs.  Reverb seems to work though.  And switching from the clean to distortion channels with the footswitch seems to function, but they are still both distorted and very midrange-e, flat, no power.

Tassieviking:  When I plug the guitar into the return jack (Line-in) on the back, I get the same result.  I do not have another amp with a line-in/line-out.  Also, when I disconnect (unplug) the preamp board from the power amp board, the screeching noise basically stays the same.  Thinking problem is in the power amp board?

If I recall correctly, I think I was getting higher voltages (like 54V) at the +-37V and (approx. 24V) at the +-15V.  I suppose I could try and replace the caps?  Or could it be the RCA 3792 and/or 3716 power transistors?  Also, I wonder if I should be checking all solder joints on the preamp board?

One other thought, when last working (a long time ago) the amp speaker used to thump when I turned it off,  Now it just makes a weak cracking-like noise from the speaker, (no speaker thump like it used to do).  Even when connected to the KMD speaker cabinet, it does the same thing.  Not sure where to go next....

JMF Spectra 125 Amp and KMD 4x10 Cabinet

JMF Spectra 125 Amp Back

JMF Spectra 125 Amp Back Closeup

The reverb level is at the output of the first amplifier after the reverb output.
Pull out the reverb tank inputs and see if the hum exists.
If no, the drive to the tank is the culprit.
If yes, pull out the output from reverb tank and see if the hum exists.
If yes, the first amp after the tank output need to be checked.
If no, see if there are any other transformer nearby.
The tank will make hum if kept nearer to other transformers.

Regards.

Do you have access to another Amp?
If you do try connecting the send to the other amps return to see if the pre-amp is good, and then connect the other amps send into this amps return to see if the problem is in the power amp.
You could try the guitar into the return jack but it would be very low output, that would test the power amp by itself.
The best place to start checking might be the power rails, the +-15V and the +-37V.
Sometimes the main filter capacitors can dry out when not used for a long time.

Just a quick reply but perhaps study up on crossover distortion in a solid state amp. The symptoms you describe are similar. Play soft and it sounds fine. A hard attack signal will reveal the crossover distortion. It might be biased cold but that might not be what is going on. Just something to consider.