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PostPosted: Sat Nov 15, 2014 10:05 am 
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Bottle Rocket

Joined: Mon Jun 27, 2011 2:49 am
Posts: 10
Location: The Netherlands
Hi,

The outer right power tube of my Mark III starts red plating as soon as I take the amp off standby. It also affects the tube next to it, which will also start red plating a bit.
Thought it was a faulty power tube. Ended up replacing both the power tubes and preamp tubes (they were due to replacing anyway). I switch it on, wait a couple of minutes, take it off standby, and boom... the same tube starts red plating again.

El34s are Mesa TS red. 6L6s are Mesa TS grey. Conclusion is that it's probably not the tube's fault. Could it be the caps leaking voltage or something like that? Would replacing the caps fix this?
I'm happy to hear if someone experienced this, and/or if you know a solution.

Thanks a lot and sorry for my English :-)

Philippe

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Mark III red stripe DRG medium head
DIY 1x12 cabinet with vintage EVM12L


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PostPosted: Sun Nov 16, 2014 4:08 pm 
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Mark II

Joined: Tue Jan 07, 2014 10:54 pm
Posts: 70
a bad socket or a broken bias voltage path...

The audio coupling capacitor between the phase inverter output and the power tube grids is shorted.
I'm pretty sure.
The original capacitor is rated 400 volt, replace it with 600 volt.

These are the 2 coupling capacitors between pins 1 and 6 of the phase inverter and pin(s) 5 of the power tubes.
http://tubefreak.com/mk3-2.gif
These capacitors should be changed to: .1 uF 600V

If you are not qualified to work on PC boards, be sure to hire a professional.
The circuit board is easily damaged by inexperienced soldering.


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PostPosted: Mon Nov 17, 2014 9:46 am 
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Bottle Rocket

Joined: Mon Jun 27, 2011 2:49 am
Posts: 10
Location: The Netherlands
Thanks man..

My soldering is not very good, so I will leave it to a professional.

_________________
Mark III red stripe DRG medium head
DIY 1x12 cabinet with vintage EVM12L


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PostPosted: Tue Oct 24, 2017 12:41 pm 
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Bottle Rocket

Joined: Tue Oct 24, 2017 11:11 am
Posts: 1
Hopefully nobody minds me bringing this post back to life....

I am having the exact same problem where the two far right power tubes (6L6's) on my Mark III (Purple stripe) head (not Simul-Class) are red plating. I have tested with new tubes, and still overheating. I have it opened up, and don't see any signs of burning/browning/cold solder joints on resistors/caps/diodes. Next step, multi-meter.

Any tips would be greatly appreciated as I'm hoping to fix this myself. Anybody experience this? Thanks.


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