How does the B+ go high?

[nickb]

Thanks to all for your time.

Enzo, I said a shorted turn(s) on the primary would cause a higher secondary. However, that doesn't go along with same VA on the heater winding. Hence the post.

The 513 vdc with one 22u/450v cap and no further load, and 750 AC unloaded, go together, that was my point: that the high voltage is there with out any weirdness in the circuit.

When normal, the power supply string and the power tube idle load did not sag the power supply more than 15v, if that.

I have a paper schematic but no way to upload it, but the power supply is bone simple: two s.s. diodes to 22u/450v cap, then through a 510 ohm/2w R, to a 22u/450vcap, which feeds to the top of the SE transformer and to the preamp RC's. It was 400V prior to the event.

Thanks for your thoughts.

Dan

You didn't mention the all important 470 ohm resistor that keeps burning up. What is it connected to?

No scanner? Digital camera? Smart phone? Camera on computer?

There are lots of online schematic capture tools where you could draw it in half an hour and then export e.g. https://www.digikey.co.uk/schemeit/project/

[dcoyle]

"You didn't mention the all important 470 ohm resistor that keeps burning up. What is it connected to?"

The 470 ohm resistor I mentioned is the 510 in the above quote. Look at a fender 5c1 champ.

[eschertron]

a 2W/500R resistor is designed to fail when passing more than about 60-ish ma. If the OT is after the 2W resistor, then at that point there is no load at all on the PSU, allowing it to raise to what you see. Swapping a 5W resistor in will allow 100ma at design max, long enough to take some voltage readings assuming all OK with the tube.

If we assume 10% regulation for the PT, then the 500v (513vdc) at the diodes will drop to around 450v, and a 500R resistor will drop the remaining 50 volts with 100ma current. Not that I'd expect 100ma, closer to 40ma at idle, even with SE class A at 100% dissipation. That would allow for the 2W resistor, in a correctly-working amp (no failures).

My guess is a problem 6V6 tube. Did you power up with tubes in or out?

edit: oh, snap. my hypothesis was mentioned in post #2

[Tony Bones]

While your at it, how come the shorted copper strap soldered around the cores doesn't suffer the same fate?

To add to Enzo's reply, the external strap doesn't see (thus doesn't affect) the flux that stays in the core itself, but it does see the flux that "gets away." For that flux, it is exactly a shorted turn. A current is generated in it that cancels the escaping flux. You can think of it as loading down the escaping flux in the same way that a shorted turn inside the transformer would load down the primary current.

[g1]

If we assume 10% regulation for the PT, then the 500v (513vdc) at the diodes will drop to around 450v, and a 500R resistor will drop the remaining 50 volts with 100ma current. Not that I'd expect 100ma, closer to 40ma at idle, even with SE class A at 100% dissipation. That would allow for the 2W resistor, in a correctly-working amp (no failures).

My guess is a problem 6V6 tube. Did you power up with tubes in or out?

edit: oh, snap. my hypothesis was mentioned in post #2

Well it needed to be reiterated anyway.

The 513 vdc with one 22u/450v cap and no further load, and 750 AC unloaded, go together, that was my point: that the high voltage is there with out any weirdness in the circuit.

When normal, the power supply string and the power tube idle load did not sag the power supply more than 15v, if that.

It seems to me the high voltage is there without any circuit at all (aside from the diodes and cap).
You are comparing to a time when all you disconnected was the power tube. With the resistor removed, you have everything disconnected, and the voltage will rise much more.

[nickb]

"You didn't mention the all important 470 ohm resistor that keeps burning up. What is it connected to?"

The 470 ohm resistor I mentioned is the 510 in the above quote. Look at a fender 5c1 champ.

I'm almost speechless...

OK so the problem is after the 470/510 resistor. The 6V6 has been mentioned several times. It could be that, it could be it's cathode bypass cap or...other things.

There is almost certainly nothing wrong the PT so pull the 6V6 and retest for that resistor overheating. If OK check the DC resistance from pin 8 of the 6V6 socket to ground. Take a look to see what the cathode resistor value is and that is what you should measure. If wrong remove the bypass cap and retest. If it's now right fit a new cap. If the DC resistance was OK then try a known good 6V6. Report back with test results and we'll go from there.

[tubeswell]

If the screen supply resistor is burning up, could be a shorted screen in the 6v6, or a shorted smoothing cap (screen supply node filter cap) - assuming the ‘470R’ is the screen supply dropping resistor. Without a schematic, we might as well be whispering in the dark.