Problem with 35 : Red plating

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daj12a

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Hi All

A friend has a Mark V 35 which failed on him after a couple of hours on in rehearsal. Since then, I've ran it on the bench, and after checking the amp for any damage and testing all the tubes and re-seating, it now seems to be working OK. However, I have seen some red plating on power tube v7 while in 25W mode only. This occurs irrespective of the actual tube in the socket. This is also the mode which failed him in rehearsal. It is worst (surprise, surprise) when in no-speaker mode with all controls dimed.

My questions are:

1) I know there is not a huge amount to go on, but can anyone suggest a fault with either the tubes or the amp which could cause red-plating on just that tube in that mode?

2) Are there any techs out there who could comment on the -17.5 V on the Control Grid (pin 2) on this tube and its partner- does it sound about right? This goes out to about -25V in 35W mode.

It may be that moving the pre-amp tubes around has made an effect, as today (after all the moving tubes around), the problem has lessened somewhat.

Thanks in advance
Dave J
 

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