Custom Hiwatt 50 power supply

[Euthymia]

Been a while since I've had a tube amp that was a stumper, but I have this Hiwatt on the bench.

The schematic seems to be closest to this: http://mhuss.com/Hiwatt/Schematics/DR504OL.gif

When it came in it was blowing fuses, so I found the shorted rectifier diode and replaced it (and its partner while I was at it). One of the output tubes was shot, which is no surprise.

This is where things start turning baffling.

With the tubes out, the voltage on pin 3 of the tube sockets sits at about 250V. The schematic wants 500 there.

The PT is putting out a jolly 775VAC.

When the amp is powered up, the DC on the upstream side of the standby switch slowly (about 30 seconds) rises to 500V, then when I flip the standby switch, it sags to about 200V, then creeps back up to 250. Throughout all this, the AC on the secondary of the PT sits at 775.

The above measurements were all taken with all tubes pulled.

I've lifted the connections to the preamp section, I've checked every resistor on the power supply board, I've tested every power supply cap up to its rated 350V on my HV power supply.

What am I neglecting to consider here?

[ken]

You have a leaky power capacitor...

Ken

[Amp Kat]

Or a faulty standby switch

[Euthymia]

I've tested every one of them out-of-circuit to 350V (their rating) with no current leakage measured.

Also, I've subbed new caps in place of the ones downstream of the standby switch and got similar results.

I thought that was a sure test. Is it not?

You have a leaky power capacitor...

Ken

[Euthymia]

Jumpered across the standby switch, no change in behavior.

Or a faulty standby switch

[Ray Ivers]

I'd check for continuity between the PT HV CT and ground (note that this is not the same thing as checking the HT fuse), and also that the replacement diodes - along with any associated wires - are good.

Ray

[Euthymia]

Ray, you get the prize on this one. Checked from the center pin of the HT fuseholder to ground, and it measured about 1.5M!

And BTW, Ken, after I replaced said fuseholder and powered the amp back up, one of the cap cans started ticking. Arcitty arc arc. I guess what wasn't revealed by testing on my HV power supply showed up after the power-up inrush.

Lot of problems for one power supply. Must have taken a big zap.