AMPEG VT-22 phase inverter imbalance

[Wes]

I've seen a number of threads concerning this issue with the floating paraphrase inverter that Ampeg used on amps from that time period. The amp in question isn't red plating; the idle is very low, and most of the pertinent circuit components have been replaced. It behaves as I would expect with one side of the power amp working much harder than the other. The owner of this amp is driving the dog #$%& out of it and I'm concerned about the amp being able to handle this abuse.

The flybacks have been blown once, I've replaced them, and this time he's brought the amp to me with the internal 10A fuse blown. The power supply was rebuilt some time ago as well as the bias supply.

The only thing I noticed after replacing the fuse is that one of his power tubes seems to be much weaker than the others. Of course this tube was on that side....

What can be done to equalize this phase inverter? The half that is driving hardest is the half that receives signal directly from the DW7. I have reduced the plate resistor there down to a 33K and it didn't seem to make much change. Would changing the plate resistor of the DW7- R24- 220K help with this?

Also- on this schematic you can see where someone hand drew some sort of circuit between the plates of the PI. What would that be about?

[Dave H]

Don't change the value of the plate resistors. It's R33 which sets the balance of the PI. If the output from C11 is greater than the output from C13 reduce the value of R33. If C13 output is greater than C11 output increase the value of R33.

The hand drawn circuit looks like it could be a cross-line master volume.

EDIT: I've just simulated it in LTSpice and best balance was with R33 set to 850k so 820k isn't far out. I'd try a new PI tube and check the resistor values (R32, R33)

[octal]

The drawn in circuit looks like an attempt to add a master volume. This is more or less a floating paraphase inverter circuit. Adjusting the ratio of R32 and R33 will trim the AC balance. If you decrease the value of R33 or increase the value of R32, you should get more signal out of the side not driven by the 12DW7. First, I would check that the 12AU7 is well matched and all resistors are in spec before tweaking the circuit.

[octal]

Well, it looks like Dave H beat me to it. :-)

[J M Fahey]

That cross plate Master volume won't work well here, because the lower triode is working closed loop with huge NFB and unity gain, also having very low output impedance.

In principle it will try to keep its gain even if heavily loaded partially defeating the MV concept.

Best will be to use the 2 pot MV, where each section is a true volume control for the corresponding power tubes half.

[Wes]

All the resistors are in spec. I've tried different AU7s with the same result. I lowered R33 to 620k and that helped alot.

Thanks for your help, guys!

[g1]

I think C12 being in tight spec would make a difference also.

[Wes]

So things were looking ok with this amp. I fire it up to play and it has developed this sizzling sound over the top of the signal.
At low settings it does not do the sizzles, but if you crank it up- it's nasty.

It seems in the past that whenever I've experienced this, I have singled out the OT as being the culprit. Taking resistance
measurements on the leads usually never revealed anything, but when I would put a small signal into the secondary and measure
the primary- it showed as bad there.

I have tested this transformer that way and it shows to be ok and doesn't measure any opens or shorts. The output doesn't
sound weak- it's healthy but it's got static sizzle over it.

What else could cause this sound? The amp is quiet at idle and good sounding at low volume.

[Wes]

So the transformer was not bad per my testing. Just in case I missed anything there I decided to hook up another OT. Same results.

I noticed one thing- Whenever I plugged in my inline ammeter test socket to V8 the problem was not present.

I figure the capacitance of the lead running from the socket to the ammeter was somehow affecting this situation. I moved the grid stoppers (R37, 38, 43, and 44) to the pins of the sockets and that eliminated the problem.

So this was some sort of parasitic oscillation? Prior to the issue arising- I am not aware of any leads being moved around the area of the power tubes. I don't know how this came about, but this seems to be the case.

Does this make sense?