Crate GT-50 Issue

[David Raye]

Hi everyone, I've been a member for awhile but this is my first post in years so I'll try to be brief. I have a 1992 Crate Stealth GT-50 head I purchased about ten yrs ago of ebay. The only issue I have ever had with it is the effects loop sticking so I use a cord to loop them with no problems (The sound came back) and I replaced a signal cap a few yrs back that had broken from it's solder during a transport. The other day I turned it on and it crackled a little when warming up and then I watched as the two inner tubes (6V6 JJ's - I keep the other two outside ones pulled to cut the volume for the home) began to glow orange brighter and brighter until I feared they would actually blow up. I immediately switched it off and tried the other two tubes I had in storage on the inside and then outside tube sockets at different times with the same result. They actually got so bright that I turned it off every time before something blew up. There was no sound during this but the amp really never had a chance to warm up. I'm at a loss about this and I only paid a couple hundred bucks for the head so if I could get some troubleshooting advice and try to repair it myself I'd rather do that because my funds are limited and I really want to keep this old amp alive and kicking. I've attached a schematic for anyone interested. Thanks

[Jon Snell]

Firstly, check you have -45v at the bias point, midpoint between R75 and R76.

[Jon Snell]

C35 and C36 are the next culprits that would cause incorrect -ve bias.

[David Raye]

I appreciate your response. I’m getting -30v at those resistors. Any advice from there would be appreciated.

[Helmholtz]

I’m getting -30v at those resistors.

Ok, that's not enough and explains the redplating. You might try to adjust AP1 to get more negative volts, but most probably either C47 or C46 is leaky and loading down the bias supply.
Also check R93/94/95.

[David Raye]

I noticed on the schematic that it suggests a bias of -32v to -37v for 6V6’s which I have in mine and -45v to -50v for el34’s which are in the gt-100. I also had purchased a video from Lee Jackson on servicing the gt-50 and noticed he biased it around -25v

[Helmholtz]

I noticed on the schematic that it suggests a bias of -32v to -37v for 6V6’s which I have in mine

Correct, but you only have -30V. I recommend to install 1R cathode resistors (or use a "bias probe") and measure idle current. You might be able to adjust AP1 for suitable bias.

[David Raye]

Ok. Thanks a lot.

[Helmholtz]

...and don't forget to test C35/36 for leakage as suggested by Jon Snell. To test measure DCV across each R75 and R76, should read zero on both.

What's your meter's input resistance in DCV mode?

[David Raye]

Well, I was able to get it to -34v using the ap1 and no more red plating. It sounds pretty good now but a little weaker than usual. I think I really need to replace the effects loop jacks. When I wiggle the patch cord, the sound goes in and out. I tried cleaning it but one might be stuck. I probably need to recap it also cause it’s got some age and wear on it. I was afraid to touch it for months. I’ve never seen tubes redplate before.

You guys are the best!!!

[David Raye]

I’m gonna test those caps also since you suggested it. It was biased at -35v by a shop a few yrs back and evidently has drifted a lot

[g1]

Yes, be sure to also check those bias filter caps, C46 & C47. You can check for AC volts at C47, should be in the millivolts AC. (no signal applied).
It's not likely that it just started to redplate due to bias drift. There is probably a fault somewhere.
C35 and C36 can be tested by lifting one end of each. If bias voltage changes, cap is leaky.

[David Raye]

I'm not sure if the pics uploaded correctly, but thought you might be interested in seeing the board of a Lee Jackson-designed 1992 Crate Stealth Gt-50. Lots of small commercial components and opamps, diodes, etc. in the gain channel but does have a tube tone stack and phase inverter. I haven't checked the caps yet because they are mounted flush on the board. Probably will have to take the board out and just replace all of them in the power amp section. The amp isn't red-plating now but is very weak volume-wise and not the same at all. Thanks again for the advice. I feel better knowing it can be fixed and isn't a loss.