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Discussion Forum for Mesa Boogie Products
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PostPosted: Thu Oct 05, 2017 1:26 pm 
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Mark IV

Joined: Thu Feb 21, 2008 1:51 pm
Posts: 580
My Mk. III (blue stripe) needs a little love.

Ages ago, I had a bias pot installed along with a pair (inner) of SED 6l6GCs and (outer) JJ KT77s. Checked the bias and it's coooooooold. Like, 28ma. By my calculations (using 25-watts to 'balance' the inner/outer tubes' max. dissipation), life would be better if that was in the 36-39ma range. Especially for the 30-watt 6L6GCs.

Voltage on pin 5 is -43.3v and -53.3v on the outer and inner tubes, respectively. Within just a couple of volts of what the schem says. And 28ma is all the hotter I'm able to get the bias (pot cranked fully CCW).

I'm thinking that the bias pot value may be wrong. It's 10k and I think I've heard that 50k is what guys are using.

What do you think? Am I on the right track?


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PostPosted: Mon Oct 09, 2017 9:03 pm 
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Single Recto

Joined: Sun Feb 19, 2006 11:43 am
Posts: 1459
Location: NYC
Gotta look at what was done when they did the mod.Some techs put a resistor in series with the bias pot.When I do a bias adj mod,I dont use a resistor in series with the pot.As a rule of thumb,I use a pot that is 2x's the value of the stock resistor that is there.i.e if theres a 50k stock resistor,I use a 100k pot.Makes the stock bias voltage more or less in the middle of the pots range.


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