VHT Pitbull Hundred/CL - Preamp issue after changing power tubes

[Wintercept]

Hi all. My friend brought a 2000 VHT Pitbull Hundred/CL with EL34's to me with some kind of volume fluctuation that I have not been able to reproduce. I did, however, find that it was red-plating a bit. The amp has a "dual class" mode via a switch on the back where one tube on each side of the power amp is run with cathode bias instead of fix. I found that the cathode biased tubes red-plated in dual-class mode, but not in the full fixed bias mode. The fixed bias was solid at ~13 watts per tube, and closely matched between the pairs. I am struggling to grasp how dissipation is measured in cathode bias, but clearly it's two hot so the cathode resistors (two in parallel it seems) need to be changed for different values. The old EL34's definitely have some life left but they looked hot and worn so I decided to throw in a new set just to see. It went downhill from there.

I brought the amp up on my current limiter, seemed good so I bypassed the limiter and let it warm up fully. A few seconds after I took it out of standby there was a little pop through the speaker (like tapping a microphone) and no sound with a guitar plugged in. If I hit the strings hard I get a farty sound. A sine wave from my frequency gen comes through, but it's way too quiet. Oh, and it still red-plates in dual class mode , so the following troubleshooting was done in fixed bias mode exclusively:

The new power tubes are biased OK, a touch colder than the old ones. I plugged my guitar into the effects return and it sounds perfect and loud, which verifies that the output section is functioning and I suppose the phase inverter as well. I put in all new preamp tubes and the issue persists. Now, I've done some probing with an oscilloscope and verified that the very first triode stage is working. Tomorrow I plan on finding where the signal goes from there with the scope and ensuring that each triode has plate voltage.

It is quite a complex amp and I have no hopes at getting a schematic, but I am confident I can figure it out if i take my time. To me it sounds like some part of the preamp is not biased or something. I know triodes are generally cathode biased for gain stages, and a lot of tweakability in more modern amps comes from switching cathode resistors and bypass caps, so perhaps the issue lies there. The amp has a few vactrols but I'm not exactly sure how to test those in circuit, as well as several small relays.

I will report back after I dig deeper, but I'm not expert by any stretch, so any ideas or advice is much appreciated!

[R.G.]

Cathode biased outputs ride with their grids at a different DC level than fixed bias tubes. Is it possible that the capacitors separating the sets of grids are leaking?

A fully cathode biased output tube runs with its grid at 0vdc; a hybrid cathode + fixed bias tube runs with its grid at between 0Vdc and some negative voltage from the fixed bias voltage part; a fixed bias tube runs with its grid at just the fixed bias voltage.

[Stevem]

Sounds to me like there's a good chance the output transformer is bad.

Put a 150 mv signal into the amp .

Put all controls wide open and if it has a master volume set it to 3/4.

Set your meter for ac volts .

Hook it up across pin 5 of the output tubes, one pin 5 on each side of the of the OT primary ( not pin 5 to ground) and you should read a easy 18 volts of drive signal there.

If you do then you should be hearing good clean output from the speaker .

If you do not then it's time for a new OT.

[Wintercept]

Cathode biased outputs ride with their grids at a different DC level than fixed bias tubes. Is it possible that the capacitors separating the sets of grids are leaking?

Thank you for the response R.G. I am actually using your "quick and less-dirty" oscillator as my test signal! I will look into this once I sort out the preamp.

Sounds to me like there's a good chance the output transformer is bad.

Thanks Steve! (I am also a Steve). I believe the output section from the input of the phase inverter to the speaker is working properly. When I plug my guitar into the effects return it sounds perfectly clean and can get very loud. Patching the effects send to the return makes no difference.

The signal is definitely getting lost in the preamp somewhere. I verified that all the triode stages have proper voltage on their plates. Both channels have 3 gain stages with an optional 4th (although it comes before the 3rd stage, this is just how the manual describes it) when set to "HI GAIN," and they both use the same triodes for stages 1, 3, and the optional 4. The stages also share a triode configured as a cathode follower before their respective tone stacks. So each channel has its own triode for gain stage 2.

Starting my trace on the green channel with all controls at noon and ~795Hz sine-ish wave at ~100mV on the input, I verified that the input (stage 1) and gain stage 2 (both in V1) were working as they should. I skipped forward to the input of stage 3 (V3B) and found that the signal was low and asymmetrically clipped. I then went backwards from there and ended up at this resistor R27:

Image_20250612_111617_465.jpeg

On the south side of this resistor coming from C18 off the anode of the green channel's 2nd stage, I get my perfect enough sine wave at 6.4V RMS up to ~20V RMS when the GAIN is maxed. But on the north side I get the same asymmetrical clipped wave form at about 1.4V RMS I got on the grid of the stage 3 triode (V3B). The wire marked X1 goes to the HI GAIN switch for the green channel. Keeping in mind that both channels have the same issue, I think I've narrowed it down to something in between the output of each channels unique triode stage and where they are summed into stage 3. There is quite a bit of circuitry here so I have my work cut out for me...

[Wintercept]

Woohoo! Well I figured out the issue in the preamp, the 100k cathode resistor (R36) on the cathode follower stage before the tone controls was open circuit. I found this almost by sheer chance as I was probing the circuit from the area I found that 1) I had just a few mV on the cathode of the follower stage, and 2) when I touched my volt meter lead there the amp sort of came back to life. So I powered down, measured the resistance which at its lowest measured like 20 megohm then ascended to open circuit (makes sense as the amp volume would decay when taken off standby). Then I clipped in a fresh 100k resistor across it and sure enough the amp roared back to life.

This is the little bugger that bit the dust:

Image_20250614_133033_177.jpeg

And while I'm at it, the PCB could use a good cleaning:

Image_20250612_200522_803.jpeg

After that I'll have to sort out the dual class red plating issue then baby's good as new; well, aside from the 25 year old electrolytics...