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 Post subject: red hot el84s DC3
PostPosted: Sat Nov 21, 2015 5:20 am 
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Bottle Rocket

Joined: Sat Nov 21, 2015 5:03 am
Posts: 3
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


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 Post subject: Re: red hot el84s DC3
PostPosted: Sat Nov 21, 2015 11:57 am 
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Mark II

Joined: Sat Feb 09, 2013 5:30 pm
Posts: 141
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


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 Post subject: Re: red hot el84s DC3
PostPosted: Sun Nov 22, 2015 3:09 am 
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Bottle Rocket

Joined: Sat Nov 21, 2015 5:03 am
Posts: 3
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.


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 Post subject: Re: red hot el84s DC3
PostPosted: Mon Nov 23, 2015 11:14 pm 
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Mark II

Joined: Sat Feb 09, 2013 5:30 pm
Posts: 141
greenmeanh1 wrote:
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. ... mp.822840/

I do not have the schematic...

Al


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 Post subject: Re: red hot el84s DC3
PostPosted: Tue Nov 24, 2015 8:38 pm 
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Bottle Rocket

Joined: Sat Nov 21, 2015 5:03 am
Posts: 3
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.


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 Post subject: Re: red hot el84s DC3
PostPosted: Wed Nov 25, 2015 8:12 am 
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Single Recto
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Joined: Sat Jan 12, 2008 12:32 pm
Posts: 1271
Here's the schematic for future reference-

http://www.classictubeamps.com/schematics/Mesa/Mesa%20Boogie%20Dual%20caliber%20dc-3%20schematic.pdf

I have a schematic for the DC-3B if you need it.


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 Post subject: Re: red hot el84s DC3
PostPosted: Sun Dec 06, 2015 6:13 pm 
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Bottle Rocket

Joined: Sat Jul 05, 2014 2:04 pm
Posts: 19
amrose57 wrote:
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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