Fender Silverface Champ tone stack bypass and dropping of signal at high volume

[Delta362]

I'm working on a Fender Silverface Champ (mid 70's) that came in for a recap and some mods. I installed all new filter caps and power supply dropping resistors matching the original values in this amp (40/20/20uF, 1kohm, 10kohm), I changed the power tube cathode bypass resistor to 680ohm 5W and the bypass cap to a 25uF 63VDC.

I also added a push pull pot to lift the 15k mid resistor to bypass the tonestack (raw mod on RobRob site).

New rectifier (TAD 5Y3GT Premium) and power tube (TAD 6V6GT-STR Red base). Preamp tube is original.

All works well when lifting the tone stack until I get up to 7 and above on the master volume. Then the sound begins to cut in and out, almost fart-like. I swapped in a JJ 5Y3 as that dropped all HV by about 25VDC compared to the TAD 5Y3. Still had the same issue. I then swapped in a JJ 6V6 and the issue goes away. I have a second champ in here getting the same set of mods, It also only has the issue with the TAD 6V6.

I'm guessing my signal is too hot for it. Suggestions on how to address the circuit to allow for using either power tube with the volume dimed? Grid stopper on power tube? Screen grid resistor? Other?

Thanks!

[mozz]

What's wrong with the original tubes? Why the 680?

[Delta362]

The amp was a recent purchase, cheap, and the power tube was whitecapped.

I went with the 680 over the stock 470 to cool the tube slightly.

[Helmholtz]

What "master volume"?

What is the 6V6 cathode voltage with the "bad" tube?

What is B+?

Do you have a scope + sig gen?

[Delta362]

Sorry, there's only one volume pot. No master volume per se. My mistake on the lingo.
One other mod I did was the heaters are now twisted with the 100 ohm per side to do the humdinger circuit. Working very well.

Voltages as follows:

Voltages based on which power tube was installed at the time

Filter cap 40/20/20uF
TAD JJ
450V 448
446V 444
419V 417

Tube sockets
TAD JJ
Pin 3 437 436
Pin 4 446 443
Cathode 31.3 30.2

Yes to the scope and signal generator.

[Helmholtz]

I suspect the amp breaking into oscillation above some signal level. The TAD tube tube might have more gain (higher gm) causing instability.
For a test disconnect the NFB resistor.

Is the OT original?
Does your signal generator provide square wave output? What is the max signal level available?

[Delta362]

With the tone stack still in, and the NFB removed, it exhibits the signal farting out with volume on 10. With the tone stack bypassed, it's even worse.

OT is original.

Yes, my signal generator can produce a square wave. It can get up to 20V signal as well.

[Helmholtz]

Ok, as a first test turn all controls fully up and feed a 400Hz sine to the vol pot wiper (V1B grid pin 7) with NFB connected and 8R dummy load.
Scope the output with increasing signal level.

Any signs of instability?

Any difference in output level between tubes before clipping?

[Delta362]

No instability noticed during the testing.

I turned up all pots to maximum. Injected a 400Hz sine wave at pin 7 of V1. Turned the level of it up until the onset of clipping. Then turned it up higher to get a comparison as well.

Both tubes were clean up to 600mV. Onset of lower half of waveform started around 700mV. Then took images of both with a 1V signal on the grid for comparison of a hot signal.

I also took additional waveforms up to 5V input as I didn't notice much difference between the tubes. I'll break up the messages so that this one is all JJ images.

[Delta362]

And now waveforms of the TAD

[Delta362]

I notice this farting out sound on the TAD when I rapidly strum an A chord. No problem when doing that with the JJ in there.

[Helmholtz]

Ok. no significant difference between tubes here.
But max clean output is low at 3.5W. Should be around 6W.
Is the scope calibrated and the dummy load 8R?

[Delta362]

I've not had the scope calibrated so I can't say how far out of calibration it may be. I'm guessing I need to look into that, though I don't suspect it's wildly off.

The dummy load is a TE HSC100 8 ohm 5% 100W resistor.

[mozz]

Champs are usually 4 ohm.

[Delta362]

Oh that's right! I've been running the chassis into my speaker cabinet or my 8 ohm dummy load.

I don't have a 4 ohm dummy load. I suspect my issues are not issues with the amp, but rather with how I set it up. I'll have to run it thru its own speaker cabinet and see if it clears up the issue. Big brain fart on my part! Thank you for bringing my oversight to light!