First Revision Ampeg VT-22 Low and Distorted Output

[Helmholtz]

What schematic are you using?
Where are V202 and V203?

[HNKTNK]

v4vt22.pdfMy bad. This one.

[Helmholtz]

Tube voltages don't look too bad.

That 200V at V3 belongs to pin 3 not pin 2.
Grid voltages are typically not specified as they cannot be measured correctly with a DMM (readings always too low).
But if grid voltage was off, the cathode voltage would be off too.

Is that 0V at V4 pin 8 or 8.0V?

[HNKTNK]

Ah, gotcha on V3. I'll measure pin 2 on Saturday. I have 0vdc on pin 8 of V4.

[HNKTNK]

Is the difference in voltage on pin 5 of V201 not a big deal?

[HNKTNK]

*measure pin 3

[Helmholtz]

Ah, gotcha on V3. I'll measure pin 2 on Saturday. I have 0vdc on pin 8 of V4.

As said, no sense measuring pin 2 as it's a grid. That voltage is not required as long as cathode voltage (pin 3) is good.

0V at pin 8 of V4 would be a problem but I don't trust the measurement as plate voltage is good.

Generally tube voltage being off by +/- 20% is normal.

[HNKTNK]

I'll measure pin 8 of V4 again on Saturday. Do you have any pointers for my issue if all of my tube voltages look ok? I have also tested all of my pots and rockers. No problems there.

[g1]

Normally, B+ in these amps is higher than schematic because of modern line voltage. 522V at power tube plates makes me think power tubes may be running real hot and pulling down the B+.

You never did get back to the results of using the 'ext.amp' jack to isolate the problem to either the preamp or power amp.

[HNKTNK]

Hey g1, It took me awhile to get around to getting a new 12DW7. I tried the other day, but it was kind of hard to tell with sound coming from the speakers of both amp cabs. I got a dummy load, so I'll try that again on Saturday. I'll check the bias as well.

[Helmholtz]

Also spray a little Deoxit D5 into all of the selector switches (from both ends) and operate them a few times.

[HNKTNK]

Also spray a little Deoxit D5 into all of the selector switches (from both ends) and operate them a few times.

Will do

[g1]

I should also mention not to take all the voltages on the schematic as gospel. There are some errors.

[Helmholtz]

I think a totally wrong but relevant circuit voltage wouldn't go unnoticed

[g1]

I think a totally wrong but relevant circuit voltage wouldn't go unnoticed

Do you mean in the amp or on the schematic? On the schematic you will find that some cathode voltages can not be true if plate voltage is true (based on supply nodes).
In the amp I agree, anything way off should be noticeable.
I only mentioned because sometimes people start chasing schematic voltages in this model, where those voltages can not be possible.