First Revision Ampeg VT-22 Low and Distorted Output

[HNKTNK]

As the title says, I have a VT-22 with low and distorted output, which happened out of the blue one day. This is the first revision with toggles instead of rockers for the standby/power/polarity. It starts to distort with the volume up about 1/4 of the way on both channels and continues to distort further with no increase in volume as I increase the volume pot. Before this started happening, I was not really able to get the amp to break up at all and the volume sitting half way was unbearably loud and clean. The distorted sound is actually pretty musical and nice. No abnormalities in sound e.g. popping, hissing, crackling, etc. I haven't had much time to troubleshoot yet, but I'm wondering if anyone has experienced anything similar. I made sure my cables, cabs, guitars weren't the cause. I put in 4 6l6GC and did not get any change in sound. I went through and tested all of the resistors in the amp and did not find any to be open, short, or out of tolerance. None of the preamp tubes appear to be microphonic. All of the caps are original and should be replaced, I've just never had one fail and give me this specific result. One thing I'm curious about is that I've always used the EXT. SPEAKER jack because the INT. jack has always been crackly and I've just been too lazy to do anything about it. It has always worked fine that way until now. I have the impedance selector switch sitting at 4ohm to match my cab. Can you see any reason for the EXT.S jack could cause me issues? I'm going to start taking voltages tomorrow after work. I'm just curious to see if anyone has experienced something similar with an old V series.

[g1]

The ext.spkr jack should not work with nothing plugged into the spkr jack. The spkr jack has a shorting contact that shorts the output to ground if you forget to connect a speaker (to protect the amp).
If the shorting contact is dirty or oxidized, it will allow the amp to work with only the ext. spkr jack in use. Maybe that was why your amp worked that way. Or the shorting contact is bent and not doing it's job.
If it was just oxidization allowing you to use the amp that way, it may have partially 'self-cleaned' and is now shorting part of the output to ground.
Try using the spkr jack and see if it works better. If not, first step is probably to check the DC voltages on the power tubes with no signal applied.

[HNKTNK]

Thanks for the reply. I have seen threads on here with a similar explanation of the function of the speaker outs. I did try the int. jack again when I was poking around and was still experiencing the same thing. Crackling, unpleasant sounding signal. I'll clean that jack with a wire brush and some contact cleaner. Could you foresee an issue with the int. jack causing my low/distorted volume while using the ext. jack?

[g1]

I tried to explain how the int.spkr jack could cause the problem when using only the ext.spkr jack. Are you now asking about using both at the same time?

[HNKTNK]

I see what you're saying after reading that again, thanks. No, I'm only running one cab through the ext. jack. Thanks for the reply.

[g1]

The 'ext.amp' jack will function as either a preamp out, or a power amp in. So if you have another amp with loop jacks you can isolate the problem to preamp or power amp (or power supply if problem occurs no matter what).
To test the V4 preamp, run the 'ext.amp' to another amps 'power amp in' (or FX return) jack.
To test the V4 power amp, connect your guitar to the other amp. Run from other amp's 'pre-amp out' (or FX send) to the V4's 'ext.amp' jack.

[HNKTNK]

I’ll give that a try tomorrow after work.

[HNKTNK]

So I tested the int. speaker out jack with my dmm and didn't find any problems with it. I went ahead and cleaned it, too. It's been awhile since I used that jack instead of the EXT speaker jack, so maybe I didn't remember what it was doing exactly, but the output from the int. is super low volume and distorted. Sounds like a little battery powered amp. Using the ext. speaker jack (nothing connected to the int. jack), it's not nearly as bad. I can see that the solder on the int. jack is more fresh than the rest of the solder, so I'm wondering if it was potentially replaced with an incorrect jack at some point. It doesn't look like the jack that fliptops.net sells for it and appears to have a little more going on with it than the schematic shows, although I do see the shunt g1 mentioned. Still seems weird that I'm able to get audio only using the ext. I'll attach a pic.

[g1]

That is a TRS (stereo) jack with a shunt/switch contact on the tip lug. Definitely not stock and the ring lug should not be connected to anything.
Probably not responsible for all your new issues, but should be sorted out anyway.
Did you get a chance to try the 'ext.amp' jack as a send or return?

[HNKTNK]

I haven't yet. I kinda had a one track mind going when the output jack started seeming fishy to me. Do you know which is pre and which is power? Both upper and lower ext.amp jacks are labeled the same. I can give that a try tonight.

[g1]

Both the 'ext.amp' jacks function as either pre-out or power in. Seems weird I know, but you can get away with a lot with tubes.

[HNKTNK]

So I noticed that V3 isn't glowing. Not sure how I'm just now seeing that. All of my voltages on it look good, but it is pretty noisy. I'm going to see if I have anything laying around that will work

[g1]

Move the wire that is currently connected to the ring lug over to the sleeve lug as shown by red arrow.

edit: removed incorrect pic, see post #16

[The Dude]

Some tubes don't glow very brightly. Turn off the lights in the room and have a look. I'm assuming this is the 6CG7? If so, pins 4&5 are the filament. See if you have filament voltage to those pins on the socket. Also check resistance/continuity from pin 4-5 of the tube to see if the filament is open.

[HNKTNK]

It seems like there's something I'm not understanding about the sends and returns. I'm running ext. amp from my v4 to the send of a gvt to test the power amp. I should connect a signal to the input of the gvt, correct? It doesn't seem to affect the sound when I adjust eq on the gvt.