Bias drifting LOW!

[TooManyProjects]

Hey everyone! I have an amp that I built a about a year ago. It's something like a deluxe reverb with a lead channel. I haven't had any issues with it lately, but the bias is suddenly way off. Normally, I keep it set to around 20mA. Yesterday, I checked the bias because it had been a while, and it measured at 16mA. I was using a cheap meter because my good one was elsewhere, so I figured the meter just wasn't giving me a good reading. I made sure nothing was red plating and it still sounded good so I left it alone. Today I turned it on and I noticed that every note was crackling as it died out. I checked the bias again and measured only 3mA! Just to be sure I didn't have a supply issue, I double checked the HT, which was around 425V (probably a little low because of the bias) and the bias supply was somewhere around 38V. If I had a leaky coupling capacitor off of the phase inverter, it would always cause the bias to be HIGH, right? What could be causing this?

[xtian]

I would suspect the power tubes first. But first, pull them out, and measure your plate and bias voltages with no power tube to make sure they're in order.

[TooManyProjects]

Okay, xtian. I just pulled the power tubes and measured the plate voltage @ 430V and the bias supply voltage at -49.2V. The bias voltage at the grid load resistors was -39.9V, if that matters. I have some new tubes on order.

[xtian]

Can you clear up your reading at TP22? Put the power tubes back in and measure. That's where you should see ~0.040v DC, indicating that the bias current thru the power tubes is set correctly.

[TooManyProjects]

What's TP22? Do you mean the power tube grid?

[xtian]

It's on the schematic--it's the cathode of the power tubes. Basically, you're measuring DC voltage drop across that 1-ohm resistor.

[TooManyProjects]

Ah. I should clarify that this is NOT a Deluxe reverb. It is a scratch build that uses a DR transformer set. In this case, there is a separate 1 ohm resistor for each cathode. So, yes. Normally I would be seeing around 20 mV drop per tube (40 if the PT cathodes used a shared resistor). As I mentioned in the original post, I measured 16 mA of cathode current the first day, which was a bit low but I assumed I was getting a bad reading because it was a cheap meter. The next day it was all the way down at 3mA. This reminds me: I'm not sure if I measured both tubes or just one. I think they both measured low but I'll have to double check.

[xtian]

Strike one! Ok, snuck that one past me. No second chance.

[TooManyProjects]

Here's the schematic, in case you're interested. I forgot to draw the FS jack and the filament fuse. There is also currently a switch to defeat the negative feedback. Not sure if I'm going to keep that feature. Other than that, I think the schematic is accurate.

CC2H_Schematic.jpg

[TooManyProjects]

Okay. Forget what I said. Turns out there were several errors. Here's an updated one.

CC2H_SchematicV2.jpg

[TooManyProjects]

Just installed the new tubes and they biased just fine. Everything is good now. Thanks!