First Revision Ampeg VT-22 Low and Distorted Output

[Helmholtz]

On the schematic you will find that some cathode voltages can not be true if plate voltage is true (based on supply nodes).
.

Anything worse than cathode or plate voltage being off by 20%?

[HNKTNK]

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.

You're right, I have 10v on pin 8 of V4.

[HNKTNK]

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.

So I experimented with isolating the preamp and power amp. I gotta say, I didn't really notice a huge difference either way. Running the preamp out sounded a little less squishy and distorted, but then again, I was running into the power amp of a 300w SS bass head. Part of me wonders if I fixed the issue by replacing the 12DW7 and R207 and maybe I just remember the amp sounding different than it really does. Still seems off that I'm getting breakup well before noon and I can crank this thing without blowing my ear drums.

[Helmholtz]

Scoping and measuring output power would tell.

[HNKTNK]

Scoping and measuring output power would tell.

Pardon my ignorance, but I'm not totally sure what I'd be looking for. I'm not really sure what a, say, sine wave is really supposed to look like on the outputs at any given volume. I'd assume the majority of guitar amps would show clipping of some sort, even if it sounds "clean". Any input would be appreciated. I can borrow one from work this week.

[Helmholtz]

You'll need a sine signal source, say 50mV /400Hz and an 8R dummy load that can take the full power of the amp.
Amp controls are adjusted for best sine reproduction.

[g1]

With the scope and load, you can see where the power amp starts to clip full and measure it to see if it is putting out proper power.

Have you checked the DC volts at the bias filter cap C23 and at pin5 of the power tubes?

[g1]

Anything worse than cathode or plate voltage being off by 20%?

Just for example, try to reconcile voltages at V1 pins 1 and 3.

[HNKTNK]

With the scope and load, you can see where the power amp starts to clip full and measure it to see if it is putting out proper power.

Have you checked the DC volts at the bias filter cap C23 and at pin5 of the power tubes?

Gotcha, I’ll grab a signal generator and an oscope from work this week and give that a try.

I haven’t done any measurements in either place. I’ll check that out after work sometime this week.

[HNKTNK]

I decided to go ahead and do it now before I leave the house. -61v and c23 and pin 5 of all power tubes.

[g1]

What is the line voltage at your wall AC plug? Also check the heater voltage (AC volts between pins 2 and 7 of any power tube socket).

[HNKTNK]

Voltage at the outlet is 121vac and my heater supply is good,

[Helmholtz]

Just for example, try to reconcile voltages at V1 pins 1 and 3.

Look a little closer: The plate resistor is shared by 2 triodes (doubling the current), making this a common plate mixer.

I use to do some plausibility checks before I comment on measured voltages.

[g1]

Look a little closer: The plate resistor is shared by 2 triodes (doubling the current), making this a common plate mixer.

I use to do some plausibility checks before I comment on measured voltages.

Ok, my mistake from being in a hurry. I'm fairly sure there are errors that I've come across before, just never documented. Here's another example. If supply node A, C, and D voltages are all true, then V202 pin 6 and 8 voltages must be wrong (need around 33mA current there).
Maybe it's just some of the node voltages that are off, but if they are, then so will some of the tube voltages.

[Helmholtz]

Here's another example. If supply node A, C, and D voltages are all true, then V202 pin 6 and 8 voltages must be wrong (need around 33mA current there).

Oh well, seems you're trying to keep me busy.

V202b draws about 17mA and cathode and plate voltages indicate a point C voltage of 440V, that's just 10% above schematic.
10% to 20% deviations can be caused by resistor tolerance.
Voltage errors caused by 10% resistor tolerance may or may not add (error propagation).

That's why I said >20% and relevant, particularly meaning to the OP's problem.

No doubt there are (more severe) errors in Ampeg, Fender, Marshall, etc. schematics. I spotted quite a few myself.