JCM 900 4100 DR DCV at pins 5 high

KVH

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Shouldn't pin 5 have a negative voltage on it? Sounds like a similar problem that I just had with my 4500 JCM900 that I made a post about last week. But mine has EL34 not 5881 tubes. So I'm not familiar with the different wiring diagram. They should be similar though right?

https://www.marshallforum.com/threads/jcm900-4500-el34-red-plating-run-away-bias-current.122500/

I found my problem by checking for voltage at D1
It was a negative voltage on pin 5 I just forgot to put the - in place. I did happen to read your post a few days ago. Glad you got it fixed.
 

KVH

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I put some Mega Boogie 5881-6L6 STR425s in today that was part of a package deal with a pair of Mesa Baron amps I bought. I just picked up an old Heathkit Tube Tester
and used 4 that tested good. I'd rather sacrifice those before using new tubes. Anyway I haven't ran it up long but so far things are good. Does anyone know what the power rating is on these tubes? 30W maybe. Can't find it anywhere.
 

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Pete Farrington

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If they are, 30W will be a design max rating. The designer is expected to apply a suitable de-rating to design max limit, eg to accommodate for variations in mains voltage and PT and OT winding tolerances.
Whereas design centre limits already include such a de-rating for those.
Using the ratios seen in other valve types, 30W design max is equivalent to about 26W design centre.
https://i.ibb.co/SdPwBVr/Design-Centre-Max-Absolute-Systems
 
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66 Kicks

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I put some Mega Boogie 5881-6L6 STR425s in today that was part of a package deal with a pair of Mesa Baron amps I bought. I just picked up an old Heathkit Tube Tester
and used 4 that tested good. I'd rather sacrifice those before using new tubes. Anyway I haven't ran it up long but so far things are good. Does anyone know what the power rating is on these tubes? 30W maybe. Can't find it anywhere.
Those are Russian 5881/6L6 WGC tubes. They were originally designed and built for the Soviet military and, as such, the specifications were classified secrets. They remain classified secrets to this day so that only those with a need to know in the Russian military have access to them. Nonetheless, many people claim that you can treat them as 26W tubes (without saying anything about what rating system they are using).
 

KVH

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Those are Russian 5881/6L6 WGC tubes. They were originally designed and built for the Soviet military and, as such, the specifications were classified secrets. They remain classified secrets to this day so that only those with a need to know in the Russian military have access to them. Nonetheless, many people claim that you can treat them as 26W tubes (without saying anything about what rating system they are using).
Interesting, I've heard the Russian jets only use tubes and not SS. Looking forward to Mesa's response tomorrow.
 

Pete Farrington

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I recall reading that RCA (?) set up a valve manufacturing plant in the Soviet Union, possibly as part of post WWII reconstruction / Marshall plan, which included a 6L6 line.
So that may have been the start of the 6P3S, which then was developed in the 6P3S-E that I suppose these may be, or be based on?
 
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