Bias voltage too low...?

[bobshbob1]

So I've been working on re-building a Peavey Butcher, keeping the transformers and chassis and re-doing all the guts on turret boards/strips, basically as a fun/inexpensive project since I don't really use the amp anymore. It's very close to being a JCM800 2203 clone except for a circuit to light the power and standby LEDs, so I'm using a similar layout to the JCM800 kit I assembled a while back.

The issue I'm running into is with the bias voltage; according to the schematic I should have -56V coming from the power supply board, just before the two 220k bias resistors (R30 and R31 on schem), and -55V just after, where they connect to the power tube grid resistors. I'm getting -54V coming off the power supply board, so close enough there, but -43V after the bias resistors. This kind of has me stumped, and I've tried investigating it myself to no avail. I've powered the amp on fully and it sounds fine at low volume but has a lot more buzz than it used to, which I'm thinking may be related to this loss of bias voltage.

My first thought was leaky coupling caps coming from the PI; I lifted the ends where they connect to the bias resistors (where the voltage drop is happening) and no change, still appear to be losing about 10 volts across those resistors. Lifted the bias winding's center tap (DC supply for LEDs and elevating heaters) and no change. BTW I do have +27VDC after the diode on the center tap, where the schematic shows +26V. Tried swapping in new bias resistors, even tried lifting the leg of each bias resistor where it connects to the power tube grids, and still no change, getting -43V on the ends which are just hanging in the air.

I feel like I'm missing something here...I've double-checked all my wiring and it looks good. It just seems like the bias voltage is finding some path to ground or elsewhere in the signal chain that I'm not seeing. I'm also not sure why voltage would be dropped across a resistor that only has one end connected to a circuit, but I don't really know enough to say if that's to be expected or not.

Anyway, I'd like to get this sorted without pulling it apart and starting from scratch (...again ). Any advice or ideas on where to check next would be much appreciated, thanks!

P.S. Supporting documents:
peavey-butcher.pdf - Schematic
JCM800 Kit Layout - Layout I used, at least for the main board

I can post gut shots if you think they'd be helpful, alas I don't own a nice digital camera so they'd only be iPhone-quality.

[JoeK]

"Tried swapping in new bias resistors, even tried lifting the leg of each bias resistor where it connects to the power tube grids, and still no change, getting -43V on the ends which are just hanging in the air."

That shouldn't be possible. If you have no load on the other end of a resistor the voltage should be the same on both sides of the resistor. Are you measuring at the actual resistor leg on both sides of the resistors?

[caverat]

What kind of meter is being used to measure the voltage? These are 220K resistors, and if a simple VOM is used the load effect of the meter will pull the voltage down. Bias circuits should be measured using a high impedance DVM which won't load it down.

[bobshbob1]

That's the voltage I'm measuring from ground to the actual resistor leg. It didn't seem possible to me either, but I took those measurements with a pretty cheap DVM. There's a nicer one at my band's practice space that I can use. I'm starting to suspect the meter because last night I measured voltage directly across the resistor (not to ground) and got 0 volts where I had expected 10 or so...will check with a better meter and report back.

[bobshbob1]

Well, yes it was the meter reading too low; different meter reads -52 V on pin 5 of all power tubes. Good to know in the future.

Also got the buzzing issue straightened out. The way things are laid out in this amp, the OT primaries come out fairly close to the first preamp tube. Originally there was a separate board over this tube, I assume acting as a shield. I just bolted a vertical metal plate (aka folded piece of beer can) between the tube socket and primary wires and, ahhh...silence.

[The Dude]

I just bolted a vertical metal plate (aka folded piece of beer can) between the tube socket and primary wires and, ahhh...silence.

Plus you get recycling points!