Ampeg VT-22 slight mishap

[tubby]

A friend of mine ordered some NOS 7027As on the net and put them in his VT-22. Next thing he knew there's smoke coming from the chassis.

Turns out on one of the NOS tubes the big plastic socket pin in the middle of the base was broken off. So I guess he put the tube in the wrong way (because of the missing alignment notch).

Ouch! Big short...

I opened up the amp, but no burnt resistors, no popped fuses, no visible signs of burns or melting or cracking.
I put in some new 7027As. Checked the OT, seems fine. Checked the voltages, seem fine. No sound on the speakers, though. Only some crackling when I used the probes to check the voltages.
No signal on the pre-amp out. I used the power-amp in and there's the signal on the speakers, but very, very low volume and it's starting to distort when I crank the signal. I haven't replaced the preamp tubes and the PI yet, cause they're somewhat more exotic (I haven't got a spare 6AN8A)...

Any ideas where to look before I start checking every single component in there?

Thanks,
marc

[Don Moose]

Since the output is weak using the power amp input, I'd say your problem(s) are in and around the power amp. That the replacement 7027s aren't redplating suggests that the bias network and PI coupling caps are intact.

Have you used R.G.Keen's Tube Amp Debug Pages before? Fantastic resource.

Yank the power tubes and measure the bias voltage at the 7027 grid pins - does it seem extra low (super-cold bias)? If it's cathode-biased, verify that the cathode caps aren't cooked (unhook Rk and look for very high DC resistance across the caps)

Check the screen supplies/resistors.

Remeasure the OT primary DC resistance to the primary center tap - it should not be more than a few tens of ohms (Enzo, please correct me?)

Verify the resistance from OT primary to all secondary windings and to the core is extremely high (effectively zero continuity).

Hope this helps!

[Enzo]

Transformer primaries measure all over the map, I don't know this one. Marshall amps tend to measure about 15 ohms, but a PV classic 30 measures more like 100.

If you smelled smoke, a resistor went up, so measure all screen resistors. And as to the problem, check the voltages. are ALL the screns up to B+? Are the preamp stages served by B+ now? COUld be a dropping resistor in the B+ string burnt.

So I'd agree, yank the tubes and see what voltage lives in their sockets.

[tubby]

Enzo & Don thanks for your help!

Finally found the prob: a barely visible connection on a 10W B+ chain resistor was loose (probably due to the big current). And that was all...