5150 vs. 380V

[fentemple]

Hi, during a gig a generator broke down and gave me nice 380volts, should have been 230V (dutch i am). Main fuse was blown, replaced it, no sound. The poweramp is ok, using the effect return, the preamp is silent. Opened her up, no visual damage, filament and HV are ok, relais switch ok, jfet's were blown, replaced them, still nothing. Hints and tips are welcome!
thanks john

[Enzo]

Apply a signal to the input and follow it through the stages, see where it goes away.

All those 12AX7s - if you have heaters running, and there is B+ on all the pins 1 and 6. And the cathodes on pins 3 and 8 are not at zero, then it ought to make noise.

I suspect your JFET circuits are not yet repaired. You may have replaced them, but what about the control signals at their gates? For that matter how about the power supply for them and their circuits, is it still present and more or less what it should be? DO the relays work, they use the same 22v rail as the JFETs.

DO you have a 5150? Or is it a 5150-2? The 5150 only has one JFET I believe, while there are two in the 5150-2.

In any case, in addition to the 22v supply, JFET Q7 is a clamp on the signal line right after the post controls. There are a pair of diodes CR21,22 that prtect it. If one of them shorts, DC will be on the signal path there.

If triac CR23 shorts, the thing will stay muted.

It resistor R84 1meg opens, you will stay muted.

[fentemple]

Enzo, thank you for your reply! I should have said that its a combo, sorry. Replaced both jfets, there's also one in the combo for the reverb. Will check the diodes and the triac. The relais are switching fine. Thanks!
john

[fentemple]

Update. Had my readings wrong, B+ was to low, 80V, on the preamptubes. The 5w powerresister was more than 10meg, replaced, preamp is fine! Switching is working. Now poweramp volume to low, B+ is ok, cathode resister 200r so it's fine aswell, tried different tubes, tried the send/return cable test, what next to do?
thanks john

[Enzo]

It is good that you found all those things, but none of them negate the possibility of those other small parts being bad too. The triac, those diodes...

[fentemple]

Checked the diodes and the triac, they're fine. Connected the preamp into a poweramp of a different amp, works perfect. Tried the poweramp of the peavey with a different preamp, low output. The precense and resonance are not working, so there's something wrong with the feedbackloop aswell.

[Enzo]

OK, then stick a nice strong steady signal onto the FX return and follow it. Use a scope, or if no scope, set your voltmeter to AC volts and use that. Is it unreduced at pin2 grid of V3A? Stronger at pin1 plate of same? And at that level at pin 2 grid of V4 PI tube? How about at the plates of V4? And the same at the grids of power tubes pins 5? And plates pins 3?

Your mains fault would have dumped an extra 300v or more on top of the existing B+, so your OT could have arced and shorted a turn or two. Disconnect the OT and clip in some sort of substitute. Most anything will work, it doesn't have to be a close match, all we want is a go/no-go test. I use an old Fender Bassman OT for this on any amp.

Over at RG Keen's Geofex site is a transformer tester you can make easily.

[Steve Conner]

If the OT is shorted, that would explain why the presence and resonance don't work, as the OT is of course part of the feedback loop.

For a quick and dirty test, you could just crank the power amp with a signal generator. If the OT is shorted, the power tubes will start to redplate and generally show signs of distress. The OT may smell and smoke too. (Don't do this for too long!)

Did you check your screen resistors? If they had burnt out, that would cause similar symptoms, but the power tubes would stay cold in the above test.

[fentemple]

Thanks for all the help! Replaced the ot, same result. There don't seem to be any screenresistors. I attached the schem, could it be those diodes at the plates? I cannot find those diodes, could they be at the underside of the powertube pcb? There's no redplating of the tubes and the ot doesn't get hot. thanks john

[Enzo]

Those diodes are flyback protection for the transformer. You can lift them to see if they are involves, but when they get leaky, they generally blow fuses by shorting your B+ to ground. They are on the power tube board.

Your link isn't working for me.

OK, this amp has no screen resistors, but what voltage is on the screens? A damaged R86 400ohm 10w could reduce the screens too far.

And then I default back to tracing a signal through the loop return buffer and hte PI stage.