KT77s in a Marshall 9100

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KT77s in a Marshall 9100

Postby sitka_spruce on Sat Jun 07, 2014 2:26 am

I realise I was a touch naive thinking they'd just go into the amp and work as intended, espec. biased quite hot. Had one develope a slight case of red plating, although lowering the bias pretty much took care of that. But it made me realise it doesn't run KT77s well as is and I believe part of that is the screen-grid resistor of a mere 470 ohms, which most octal tetrodes prefer but not those designed to sub the EL34.

Reading the specs sheet for the KT77s it states the screen voltage must not exceede 300V, and this being a Marshall my estimate would be that even w. a 1k resistor the voltage would be in the neigbourhood of 350V, so it has to come down a bit further than that. Does anyone know what would be ideal in this situation? I know some late '80s Marshalls had 2.2k resistors here, even its predecessor the 9005. Though this may make the amp sound compressed it leaves me little choice.

http://www.drtube.com/library/schematic ... emas#M9000

Any input welcome - cheers!
sitka_spruce
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