red hot el84s DC3

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greenmeanh1

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Hey guys, New here with a question.
I have a DC3 amp that is giving me some problems. Two of the power tubes (EL84s) are glowing red hot and humming after amp has been on for about 5-10 minutes. I was told that more than likely it was a tube problem so i changed out the two tubes with new ones but it didn't help.
So im thinking the issue must be internal to the amp like a bad cap or blown resistor. I also don't have a schematic so,, i got nothing to go on.
Hoping for some advice on where to look/test inside the amp. Thanks
 
You probably lost a grid resistor. Get thee to a repair guy.....

BTW - these amps benefit from the mod that lowers the bias of the power tubes. My tech did it and the tubes now run much cooler.

Al
 
So is that a similar mod to what the earlier black face fenders had with a 10k resistor and a pot??
I need a source for a schematic. I have only been able to find owners manuals.
 
greenmeanh1 said:
So is that a similar mod to what the earlier black face fenders had with a 10k resistor and a pot??
I need a source for a schematic. I have only been able to find owners manuals.

Try this; http://www.thegearpage.net/board/index.php?threads/changing-negative-grid-voltage-mesa-dc-3-fixed-bias-el84-amp.822840/

I do not have the schematic...

Al
 
So it appears i have fixed it. All the voltages were just below spec on the 400 300 and 200 v busses but prior to testing i was looking over the circuit and found a loose chunk of solder that had found a lodging place in the circuit. I removed it and it seemed to solve the red hot tube issue. Amp is on the bench with standby on for over two hours and things seem stable. I sprayed out some of the pots that were crackling. The EQ sliders were pretty bad.
Two notes of interest. The pin 2 spec of 11v was low. I was getting 9v.
Power transformer is quite hot,, not sure if that is normal. I assume so.
I will keep you posted on how it cranks after the bench test.
 
amrose57 said:
You probably lost a grid resistor. Get thee to a repair guy.....

BTW - these amps benefit from the mod that lowers the bias of the power tubes. My tech did it and the tubes now run much cooler.

Al

Bias mod works fine - I took my dual cal from -10.8 to -14, doesn't eat valves now and sounds just as good as before. It's a 5 minute job to solder a piggy back resistor. No need for a pot.
 

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