Crate VC3112 red-plating one tube

[gui_tarzan]

This came to me with one tube (V7) blown (cracked glass) and one (V8) that won't heat up in any socket I try it in. I checked all the resistor values in the power amp section and they test ok.

Someone made some modifications (cut some cap wires, removed a couple of resistors, swapped a cap for a resistor) to this at some point, I put it back to stock before I replaced the two power tubes because the person did some real shoddy work. I assume they were trying to make it sound "better" but who knows? The rest of the circuit looks fine so I put in a new pair of EL84s.

I also touched up the solder joints on those and the tube sockets since there was a lot of pressure on them from the tube brackets that hold the EL84s in place. They have a foam strip that sits on the tips of the tubes but they were really pressing hard. I figure that's why the one tube was cracked so I poked holes in the foam so the tips go through just enough to relieve the pressure.

The new V8 lit up bright blue and the amp was making a nasty crunching sound so I shut it off. I swapped one of the existing tubes into that place and used a different new tube from my stock and it powered up ok. But after a few minutes V8 started red-plating so I again shut it off.

I'm a little confused as to why this one tube is red-plating, although I haven't measured voltages yet (I will, probably tomorrow) but the first things I checked were the plate and grid resistors and they were fine. I'm hoping one of them is breaking down after warming up so it won't be a pain in the butt to figure out.

The more I look at it (and after typing this out) it seems that it could easily be a bad solder joint or tube socket. I'll tear the board back out tomorrow but I wanted to get a jump start on the diagnosis with input from you guys.

[martin manning]

Measure the grid voltage on all of the power tubes to see if it is zero. If not, you may have one or more leaky coupling caps.

[gui_tarzan]

I've got 10-11v DC on all pin 2s, 314-317v DC on all pin 9s, 0v AC on pin 9s. The AC reading jumps all over the place on each of the pin 2s.

[martin manning]

I'd be looking for tenish volts on pin 3's (cathodes), zero on pin 2's, and 300 or so on 7's and 9's. All DC.

[gui_tarzan]

9V DC on all cathode pin 3s.

Double checked the grids on pin 2s and still get 10-11V DC on all four.

[xtian]

So that 10v on your grids is causing your bias to be upset, leading to tube meltdown. How could DC get on those grids? Only bad coupling caps (c27, 28), or a broken reference to ground (jumper J02).

[gui_tarzan]

Ordered replacement caps today.

(sigh)

[xtian]

Hey, Jim, don't forget to measure resistance to ground from pins 2. Should be 222K.

[gui_tarzan]

Yep. It is.

[gui_tarzan]

That fixed the problem. It's amazing how much one little part (replaced both of them) affects the whole operation. The amp works great now. Well, as much as it can for what it is.