Ampeg v4 vt22 bias problem

[Marc]

Please help! this amp has me stumped.

this is like the Sunday afternoon puzzlers Charles from swamp amps used to post.....the only problem is I don't know the answer, yet.

Here's the back story.
I am able to get it to bias up (-60V) and it behaves fine with a single note signal. As soon as I strum the bias drops out and goes positive until the power tubes redplate.

The B+ is about 540V. The filter caps are original. It is an early V4 with the single 100mfd cap in the bias circuit.
I have replaced the bias feed cap (ac divider), diode and filter cap. The resistors tested fine.
I replaced the diode on the standby switch.

I have replaced the PI 12AU7. I have replaced the coupling caps (tropical fish at .47mfd) with .33mfd nos mylars.

The power tubes are 2 weeks old. sovtek coin base 6l6WGC's. they test OK on my Hickok.

I tried deriving the bias supply from the AC windings rather than a capacitance divider and the bias simply collapsed to positive a little faster.

I'm open to any and all suggestions....
Thanks,

Marc

[Enzo]

Isolate the problem.

First, try different power tubes. I don't care what the tube tester says, chances are it is not putting full B+, full load and a strong signal through them. Remember, when they are in the amp, they don't have the problem until you strum, right? No strum on that tube tester. They don't have to be new, ANY working power tubes.

Other tests:
Monitor that bias voltage. It seems to be going away at the tubes - at ALL of them? So go back to the start of the bias supply. is it collapsing there too?

Pull the power tubes. Now fire up the amp without them and strum the guitar. Monitor the bias voltage at the grids - stick your probe down the empty pin 5 of a socket. Does it still go away, or stay?

FIlter caps are original? There are guys who want to tell you just to automatically replace all the filters, but I am not one of them. If the B+ is holding up and not real hummy, then leave the main filters for later. Make the amp work before upgrading everything. But I would replace that bias filter cap, THAT circuit is having problems, unlike - I assume - your B+.

I just think when an amp has a problem, it is best to sort out that problem before swapping out a lot of parts. That way the potential for creating new problems is separate from the repair process.

The PI tube couldn't cause this, but the coupling caps from it could. You replaced the old caps with other old caps - try some new caps. I mean caps made within the last couple years, not NOS. You can remove them later, but why assume old parts are always good? I would normally suggest just lifting those caps, but your symptom seems to require the signal coming through.

[Marc]

yep, I bypassed the first rule....

Thanks Enzo,

I swear, I know better than to trust my tube tester.
Besides that's a violation of R.G's rule#1-suspect the tubes first!!

I tried a pair of bogen 2027's in it tonight and the bias stayed steady and the amp sounded fine.

I just couldn't fathom that a quad of new power tubes would go bad at the same time, so I kept trying a pair of the old tubes. Duh.

The bias diode was open and one of the screen resistors read 10K instead of 1K, but I'd assume that happened after/during the failure.

The tropical fish 0.47 coupling caps were not leaky and have been reinstalled.

The main filter caps seem to be healthy-the amp is very quiet. The bias filter cap which I did replace measured 50% out of tolerance (154mfd instead of 100mfd) just in case it was 'weak'.

So, would the tubes failing open up the bias diode or would it be more likely the other way around?

The owner said he mis-matched the impedance on the cab he was playing through about the time it started acting up. He set it on 2 ohms for an 8 ohm cab. Would that have stressed the tubes enough?

I can't in good conscience tell Mojo that the tubes all failed on their own without a reasonable explanation of why.

Enzo, an you give me a possible explanation from this detail?

Thanks,
Marc

[Marc]

Tube reccommendations?

What power tubes are people using in these amps?
It ate the sovtek 5881 / 6l6wgc's.....

any other current tubes like the 540V?

Marc

[52 Bill]

Marc:
Glad to hear that you got it up and running again.

I would tend to believe that the loss of bias voltage took out the tubes, and not the tubes taking out the bias supply. But who knows, anything is possible.

I've had good luck with either 6550's and new 7027A's (Sovtek or JJ), in these amps. Bass players seem to prefer the 6550's.

[Bassman3]

Biasing

Iv heard you can use 6L6s in an old V-4, but those 7027As are the "goods" on this thing. I own 3 V-4s, I wouldn't put 6L6s in it for nothing.

[J Martin]

I only use the JJ tubes in the V4 family amps. After many retubings and repairs, they are the most consistant in performance. Modern 7027 tbes are actually relabled 6L6 tubes. Angela instruments still has some originals in stock if you can afford them. When a V4 comes in for a refurb, I always rebuild the bias circuit. If its going out on tour, I'll do the filter caps too. For some reason people seem to run these amps into the ground. They still sound amazing after a rebuild.