Amp repair where do I start

[LoveChet]

Off of pin 1 plate there is only 1 coupling cap going to next stage. The 150 mv output Is on the upstream side of that cap. To get the signal right at pin 1 plate there is 170v dc. Can I put my scope right on pin 1 to see the signal in AC position and not damage scope ? It's a Tektronics 2221a

[Helmholtz]

You probably mean the 250p treble cap of the tonestack.
Yes you can connect your scope to pin 1 but set scope input to AC coupling first.

[LoveChet]

I will check the signal right on pin 1 in AC position. Results to follow. What do you suspect as the issue ?

[LoveChet]

This is my first try at scopes and debugging but I'm guessing the tube is shorted based on the same signal being on both grid and plate

[Helmholtz]

Don't jump to conclusions.
The plate voltage is good and a shorted tube wouldn't pass a signal.
Let's see the plate signal.
Could as well be a bad cap.
Apart from that a new V2 should be easy to try.
But could also be something else(where).

[Greg Robinson]

1kHZ test signal is too low for much to pass through the treble cap. Most will pass through the mid and bass caps. I bet the stage and tonestack are fine, and you'll have a healthy output at the treble pot wiper.

[LoveChet]

4 volt signal right on plate pin 1

[LoveChet]

So like you said the 250pf cap didn't allow 1000 cycle thru it but the signal would get over to the bass and treble pots via the other two paths ?

[LoveChet]

For ha ha I guess i could ramp up the frequency and see that it passes thru the 250 pf cap. I did check for dc on the upstream side of that cap and there was none and that's good I assume. So is that stage good so far ?

[Greg Robinson]

Just check for signal at the output of the tonestack.

[Helmholtz]

4 volt signal right on plate pin 1

With 150mV at the grid this means gain of 4/0.15 = 27.
Should be about twice that number. Check the cathode cap.
EDIT: As the cathode cap is only 2.2nF, a gain of 27 looks good.

Make sure not to mix up RMS and peak-to-peak voltages.

The 250p has an impedance around 600k at1kHz. With a standard Fender tonestack wiring it should pass more than 50% of the 1kHz signal.
If you really have only 150mV after the cap, it might be open.

[LoveChet]

The 4 v was p-p. should I talk in terms of rms in future ? You said gain s/b twice 27. I use a 12AY7 which has lower gain than a stock 12AX7 per the tone guys mods. Is that the gain issue ? The treble wiper gets 1-2v based on the fat sw setting. Also the treble wiper gets 1-2 v based on the treble pot setting. The bass pot has no effect at all regardless of setting ! How can I check the 250pf cap. And how can I check the cathode cap ?

[Helmholtz]

The 4 v was p-p. should I talk in terms of rms in future ? You said gain s/b twice 27. I use a 12AY7 which has lower gain than a stock 12AX7 per the tone guys mods. Is that the gain issue ? The treble wiper gets 1-2v based on the fat sw setting. Also the treble wiper gets 1-2 v based on the treble pot setting. The bass pot has no effect at all regardless of setting ! How can I check the 250pf cap. And how can I check the cathode cap ?

General convention is that AC voltages are given as RMS values unless otherwise indicated. Peak-to-peak is shown as Vpp. Vpp = 2.82 Vrms with a sine.
You can use either to determine gain, just don't mix them up.
I just saw that the cathode cap is only 2.2nF.This results in a corner frequency of 48kHz(!).
Means that the cathode is essentially unbypassed at all audio frequencies and your gain of 27 looks right.

Did you recheck the signal directly after the 250p cap?

[LoveChet]

Since a 12AX7 had gain of 100 and a 12AY7 a gain of 40 I guess the gain im experiencing of 27 would be 67 if I put in a 12AX7. With the other cap issues you suggest withstanding, is the low 27 gain off your radar as an issue with the V2A circuit including tone stack. I do wonder why the Bass pot has zero effect !

[LoveChet]

At the 150mv 1k signal I'm using, yes the reading upstream of the 250pf cap is only 150mv. I can recheck