Early 70s Ampeg VT40 - Unwanted distortion?

[the_product8]

Hello,
I've been researching a problem for a while now without much luck on finding what's wrong.
I've got an early 70s top-loader VT40 that's in great shape cosmetically, but has some issues internally, and I'd REALLY like to get this
amp solid, since when it works properly it sounds amazing.

Anyway, the problem seems to start after the amp is on, completely (i.e. off standby), and I've been playing through it for probably 10-15mins. plugged into the bright channel.
The signal starts to fizz, or seems to sort of squash a bit maybe, when I play at any volume. It's not an extreme problem, that is, the fizzing or whatever, isn't
obscenely obvious to maybe someone that didn't play these things often. It is noticeable compared to how it sounds in the first 10 or 15mins, however,
after the amp is taken off stand-by.
The fizz starts on the decay of the note played. The attack seems fine, but this it sort of fizzes out as the note decays.
Again, it sounds fantastic until the thing gets good and warm. I don't notice anything else like, power fading out...it seems strong throughout the test and fizzing, and I don't have to continually keep turning it up like its losing power.
As well, the problem goes away if put the amp back on standby for a min or two, and flip it back on; it'll play well for maybe 5mins or so doing this, but then the fizzing starts again.

I've had one guy do some work on it, and he was able to get rid of a nasty hum, and fixed one of the channels where it seemed to volume pot was completely broken (now the other, normal, channel has broken since, but I'm pretty sure its fixable). Both him and I have gone through the tubes on the good (bright) channel, and I've switched them all out (even the 6K11, that is hard to find and pricey) at some point to see if the problem was any of them...and the problem persisted.
I've also replaced all speakers with brand new eminence legends to eliminate that being a problem.
Basically I've dumped in a good bit of money into this thing, and its still not performing up to par; it's close however.
If I can get the normal channel working again and get this fizzing thing fixed it'll be a great amp.

Just wondering what the next step here is? Filter caps?

Thanks a lot!

[Amp Kat]

Sounds like a connection problem more than anything so look for corroded parts,jacks, even possible pots or connections. Caps usually hum when they go bad not fizz and touch up the tube sockets and make sure they are making good contact. A tube pin inside a socket not making good contact can cause this. If you can isolate the preamp from the power like maybe a send or preamp amp out / line out ect.. try it to another amp to see if the problem is coming from the pre section or the power section.

[the_product8]

Amp Kat,
Thanks for the reply - good info.
So if I were to run the line out like you're saying, and the problem persists, we could say that the problem is on the preamp section? and if it doesn't, the power amp section?
Thanks!

[g1]

Not sure what you mean by "top loader". I'm assuming there is only one type of VT40, pretty much the same as a VT22 but with only two power tubes.
If the problem were in the power amp, it would be the same through the other channel. You said you only have the problem in the bright channel, so that rules out the power amp. These amps have the preamp tube sockets mounted to the circuit boards, and their solder joints eventually go bad due to heat and vibration. Resoldering the sockets may cure the problem. That would be my first guess. But before doing any more, make sure to play with the sensitivity switch a bit, in case it is dirty/oxidized.

[the_product8]

Not sure what you mean by "top loader". I'm assuming there is only one type of VT40, pretty much the same as a VT22 but with only two power tubes.
If the problem were in the power amp, it would be the same through the other channel. You said you only have the problem in the bright channel, so that rules out the power amp. These amps have the preamp tube sockets mounted to the circuit boards, and their solder joints eventually go bad due to heat and vibration. Resoldering the sockets may cure the problem. That would be my first guess. But before doing any more, make sure to play with the sensitivity switch a bit, in case it is dirty/oxidized.

Right, two power tubes only on this, and from what I can tell, pretty much the same as the VT22, except the earlier models had the controls and inputs on the top (http://www.guitar-museum.com/uploads...86270119-1.jpg).
The normal channel on mine doesn't really work at all. The volume pot I believe is completely busted and won't even really move without a good bit of effort and noise and not really any good output.
I did run out of the external amp and into the input of a Twin that I have around...hopefully I did this correctly...but I'm pretty sure I didn't hear the problem coming from the twin, but did hear it in the VT40 still. They sounded great together until the Ampeg started kind of doing its fizzing out thing. I haven't noticed anything weird or crackly with the sensitivity rockers or any of the other rockers really, since they were fixed by the first guy that took a look at it; or maybe they're mostly fixed, I dunno. I'll check em out again tomorrow.
So with this being said, would we suspect its something in the power amp side?
Thanks for your help and time!

[g1]

The ext. amp jack can be used as preamp out or power amp in. If you have an amp with a preamp out (or FX send) you could run it into the Ampeg's ext. amp jack to test the Ampeg's power amp section. If you hear the fizz you will know it is from the Ampeg's power amp section.

To disable the Ampeg's power amp while doing the test you performed with the twin, (preamp test) try removing the phase splitter tube (usually the preamp tube closest to the power tubes).
This way you will only hear sound from the twin. Normally I would say remove the power tubes, but the B+ on these amps is very high, so removing the phase splitter is a safer bet. Now you will only hear sound from the twin, and it will be easier to tell whether the fizz is still there or not.

[the_product8]

I appreciate the help, fellas!
I'll let you know if I get it fixed and what it was...THANKS!

The ext. amp jack can be used as preamp out or power amp in. If you have an amp with a preamp out (or FX send) you could run it into the Ampeg's ext. amp jack to test the Ampeg's power amp section. If you hear the fizz you will know it is from the Ampeg's power amp section.

To disable the Ampeg's power amp while doing the test you performed with the twin, (preamp test) try removing the phase splitter tube (usually the preamp tube closest to the power tubes).
This way you will only hear sound from the twin. Normally I would say remove the power tubes, but the B+ on these amps is very high, so removing the phase splitter is a safer bet. Now you will only hear sound from the twin, and it will be easier to tell whether the fizz is still there or not.