DSL100HR Red Plating!

Cadorman

Cadorman

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Just to set the stage this head is 4 months old. One day last week I was playing as usual and hadn't changed anything since April. All of a sudden I was getting drops in volume and the loud popping scratchy sounds like dropping a needle on an album. I have the channel volumes maxed and master on 2.5ish. So yelling loud, but nothing is really being pushed. I look behind the head (I keep the back panel off) and the left (from the back) two tubes are red plating! The right pair look completely normal. I shut the amp down and check all my speaker connections (I'm using 1960a/b full stack) and everything is fine. I turn it back on and everything is fine. I think this was last Thursday. I have played the amp every day since then for a total of about 5 hours and had no issues. Just now it did it again. I grabbed my voltage meter and check the bias. Right side 75ish mv pretty much where I set it 3 moths ago. Left side 73ish mv, okay wait. It starts climbing and goes up to .7 volts! I put it back in standby and the red plating stops. This amp is completely stock other than the tubes. What do you think I'm looking at here? I really don't want to ship it back to Zzounds.
 
Cadorman said:
Just to set the stage this head is 4 months old. One day last week I was playing as usual and hadn't changed anything since April. All of a sudden I was getting drops in volume and the loud popping scratchy sounds like dropping a needle on an album. I have the channel volumes maxed and master on 2.5ish. So yelling loud, but nothing is really being pushed. I look behind the head (I keep the back panel off) and the left (from the back) two tubes are red plating! The right pair look completely normal. I shut the amp down and check all my speaker connections (I'm using 1960a/b full stack) and everything is fine. I turn it back on and everything is fine. I think this was last Thursday. I have played the amp every day since then for a total of about 5 hours and had no issues. Just now it did it again. I grabbed my voltage meter and check the bias. Right side 75ish mv pretty much where I set it 3 moths ago. Left side 73ish mv, okay wait. It starts climbing and goes up to .7 volts! I put it back in standby and the red plating stops. This amp is completely stock other than the tubes. What do you think I'm looking at here? I really don't want to ship it back to Zzounds.
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And when you swap (left right pairs) the tubes around, does the problem stay on the same side, or does it move with the pair of left right tubes?
See if the problem follows the tubes.
Or if the problem stays at the same sockets no matter which tubes.
Then report the result please?

But I sort of suspect a bad tube...therefore the problem will follow the tubes when swapped around left right.
 
Cadorman said:
Thanks for the recommendation, but I quit playing over there once I found this place.
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Well, over at the Marshall forum you have a bunch of people who think all problems are caused by capacitors....so they race to replace capacitors without knowing what's really wrong.
But that sells a lot of capacitors. :pound-hand:

Heat the amp up really good, and let it stay on for a while.
It usually won't happen unless the amp is heated up.
Play it, stop, and let it idle for a while. Then measure it...

It could be a tube or it could be a bad circuit board.
 
I swapped the tubes around and played for a while. Even took the volume up above 3 which starts to pound the concrete pretty good. I had reset the bias after the swap right at 80 mv. Now the left side is sitting pretty solid between 79.9 and 80.1. The right side now acts like it's had too much caffeine. I've seen 77.0 up to 86.1 at this point, wait make that 88.2 but hasn't red plated yet. I'm thinking you are right about a bad tube. It's as if one of the tubes resistance is drifting.
 
Cadorman said:
No smell, nothing burning. They were E-H and whichever side I put the bad pair in the bias wandered all over the place quite rapidly +/- 20 mv. After switching the bias is holding rock solid on both sides barely fluctuating .1 mv.
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O.K. all dicking about asside I do hope you got it sorted out -- and all is well -- sux when one of our babies is ill and not working properly !!
 
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