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stromberg carlson asr 120 Red Plating

1

1971mark

New Member
View attachment 2487136 Hi, I just picked up this tube amp and the 6v6gt socket 1st on left Is red plating whatever tube is there. I can move the red plated tube to another socket and is fine. Ceramic Drop Capacitor .05 - 400 648 I can't identify with all tubes removed I show the second drop cap from the left shorted.

This amp has the following tubes
1@ 6CA4
4@ 7808 OR 6V6gt
2@ 7199

Any help would be appreciated, however I'm real rusty on electronics took electronics at votech in high school in the late 80's. I hope you have patience

Sorry didn't know how to rotate image to right 90 degrees
 

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Tom Bavis said:
The brown ones? A shorted coupling cap will make one tube red-plate. .047 uF would be current replacement - .05 isn't common any more. That circuit board isn't easy to solder to - might be better to clip the old one and solder to the stubs.

I cut the cap out and it tests good not sure where problem is now
Click to expand...
 
Tom I cut the cap in question out, it tested fine on the bench, I tested for continuity with a ground on both stubs and they check fine and both stubs continuity fine back to sockets. It has to be a bad cap right? Could a resistor cause red plating?
 
Solved, put cap back in and problem fixed i guess my readings were right on my meter and cap was somehow grounded but all is good now ty tom for help
 
Caps don't usually dead short they just get slightly leaky at high voltage. That circuit is pretty high impedance so it doesn't take much leakage to make the tube run hot. Usually won't find any problems with a typical ohm meter either. Need a 400 or so volt supply. Or can test in circuit. Any voltage drop across the grid to ground resistor is cap leakage.

Realistically most people just change them as a preventative measure and pay no mind to the state of the old caps.
 
1971mark said:
Solved, put cap back in and problem fixed i guess my readings were right on my meter and cap was somehow grounded but all is good now ty tom for help
Click to expand...
The cap couldn't have been grounded, it would have blown the fuse. You've simply disturbed it temporarily with the heat of soldering. It will fail again. You need to replace all four of those .05uF coupling caps.

I bought one of those amps years ago on eBay before prices became ridiculous. I used the 7408s in a small SE amplifier and scavenged the power transformer to build a 2x EL34 guitar amplifier. Worth every penny! :)
 
I use a lot of boring yellow Illinois MKP series but for that specific physical packaging, possibly a Vishay 716p would fit better. Just make sure the voltage is at least as high as whatever is there now.
 
Thanks I ordered Vishay 716P .047 400v x16 as I have a number of other tube amps to rebuild, and looking thru them I see that .047 and 400v seems kinda common.
 
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