Mark III power tube red plating

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pvandendungen
Bottle Rocket
Posts: 10
Joined: Mon Jun 27, 2011 2:49 am
Location: The Netherlands

Mark III power tube red plating

Post by pvandendungen » Sat Nov 15, 2014 10:05 am

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

soundguruman
Mark II
Posts: 70
Joined: Tue Jan 07, 2014 10:54 pm

Re: Mark III power tube red plating

Post by soundguruman » Sun Nov 16, 2014 4:08 pm

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.

pvandendungen
Bottle Rocket
Posts: 10
Joined: Mon Jun 27, 2011 2:49 am
Location: The Netherlands

Re: Mark III power tube red plating

Post by pvandendungen » Mon Nov 17, 2014 9:46 am

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

cheapsk8
Bottle Rocket
Posts: 1
Joined: Tue Oct 24, 2017 11:11 am

Re: Mark III power tube red plating

Post by cheapsk8 » Tue Oct 24, 2017 12:41 pm

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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