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Sunn A212 Bias question

3/19/2004 10:14 PM
BWilliamson	Sunn A212 Bias question Got stuck working on one of these for a buddy. Needed some 6550's, had a pair of pulls from another amp said he could have. Drop them in: Plates: 550v Screens 540v Bias V: -58 MA reading: 86ma Quite high and the plates are starting to cherry, and there's no pot to adjust the bias. Start tracing out the Bias supply on the PCB and it's not really my thing as I stick to very normal stuff when doing my own thing. Anyway, looks like: Diode-->50uf to grd--1K--50uf to ground-->pretty standard till there and then it splits at a junction of three resistors: 2--100K resistors that goto the power tubes 1--1.8K that connects somehow into the 1st channel preamp cathodes for the 12ax7. Get lost trying to trace the route on the PCB. Anyway, thinking just ditching the current bias supply and building a new one and leaving what is already there intact for that preamp thingie. Just cutting the wires after the 100K's and connecting that to a new adjustable bias supply. any you real techs got an opinion on this? thanks bwilliamson

3/19/2004 10:14 PM

BWilliamson

Sunn A212 Bias question

Got stuck working on one of these for a buddy. Needed some 6550's, had a pair of pulls from another amp said he could have. Drop them in:

Plates: 550v
Screens 540v
Bias V: -58
MA reading: 86ma

Quite high and the plates are starting to cherry, and there's no pot to adjust the bias. Start tracing out the Bias supply on the PCB and it's not really my thing as I stick to very normal stuff when doing my own thing.

Anyway, looks like:

Diode-->50uf to grd--1K--50uf to ground-->pretty standard till there and then it splits at a junction of three resistors:

2--100K resistors that goto the power tubes
1--1.8K that connects somehow into the 1st channel preamp cathodes for the 12ax7. Get lost trying to trace the route on the PCB.

Anyway, thinking just ditching the current bias supply and building a new one and leaving what is already there intact for that preamp thingie. Just cutting the wires after the 100K's and connecting that to a new adjustable bias supply.

any you real techs got an opinion on this?

thanks
bwilliamson