silver faced champ help

[dwhutchens]

I have a silver faced champ that had a shorted 6v6 that let some smoke out of the OT. I have repladed the OT, tubes, filter cap and coupling caps. I'm getting very little clean volume and grainy distortion starts at around 2 to 3 on the volume pot.

I looked at the signal with a scope and signal generator and have a nice clean sine wave comeing off the first half of the 12ax7 but when I put the scope on the wiper of the treble pot and turn up the volume one half of the sine waves remains clean while the other gets clipped.

I replaced the cap and resistor on pin 8 and the resistor on pin 6 of the 12ax7with no improvement. Any ideas where to go next?

Thanks,
David

[drewl]

What's the plate voltage on pin 6 of the 12AX7?
If the OT smoked a bit, it could be damaged loading down the B+ power supply.

[Enzo]

Is there screen voltage on pin 4 of the 6V6?

[dwhutchens]

Hey drewl, thanks for the responce.

The OT was replaced. I have the amp plugged into a variac dialed down to 108 vac to get 360v at the first filter cap.

The 12ax7 readings are as follows:
pin 1 = 233
pin 3 = 1.56
pin 6 = 231
pin 8 = 1.49

The 6v6 readings are:

pin 3 = 349
pin 4 = 355
pin 8 = 20

Thanks again,
David

[dwhutchens]

Yea, I got 355 on pin 4. The diagram calls for 350 on the plate and the screen. Looks like my screen is about 5 or 6 volts higher than the plate. Is the 1k power resistor not dropping enough voltage? I posted all my readings in a reply to drewl's post.
Thanks,
David

[Euthymia]

You're getting 20V on the cathode of the 6V6, so the cathode resistor is not open....

Try replacing the 22uF capacitor (may be marked "25," but 22uF is fine) on pin 8 of the 6V6.

If that doesn't help, pull the output tube and with the power off, measure from the grid pin of the output tube socket to ground. Should be 220K.

[Enzo]

ANd that 108 volts on the mains may dial down your B+, but it also drops your heaters to 5.6 volts. COuld be a factor.

That clipped signal at the treble wiper - how does its amplitude compare to a guitar signal at that point? The first stage will amplify a very large signal cleanly. You could simply be overdriving that second triode. Adust the signal so it is a large as it wil get at the treble wiper before it clips. Now look at the signal at the 6V6 grid at pin 5. Does it look the same? Just because the first stage will kick out 30 volts of signal doesn't mean the following grid will respond well. That tube in only biased at a volt and a half, so much more signal than that will clip it.

Did we ever determine if you played the amp through different speakers?

Try disconnecting the feedback wire. and try disconnecting the OT secondary wires and connecting them directly to a speaker. Just in case tehre might be a solder bridge or mechanical failure in the speakre jack. If the output is shorted you might have to turn the amp up way into distortion just to be heard. COuld that be the case?