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biasing setup on Scott 222D question

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Posted on March 18, 2017 at 18:07:37
gkargreen
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was poking around on this amp, noted that it has a 10 resistor on the PAIR of output tubes, which means that when you are setting bias, its not between tubes but between tube PAIRS. Wondering if I can change that single 10 by putting 2- 20 ohm resistors in parallel, one to each cathode of each tube where I can measure each tubes cathode current and still have the 10 ohms for the bias setup in the manual. Any ideas/suggestions?

 

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RE: biasing setup on Scott 222D question, posted on March 19, 2017 at 11:28:33
Michael Samra
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Randy
Just put a 10 ohm resistor to ground on each cathode.Then,you simply adjust for 0vdc between each pair of tube's cathodes with the DC balance pot.Then you adjust your bias to the current you want.I think the 7189s are 28ma to 30ma depending on your B+.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: biasing setup on Scott 222D question, posted on March 19, 2017 at 13:11:38
Thermionic27609
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You can't really split the 10 Ohm resistor and keep the stock bias adjustment setup due to the way it's wired. I don't really think you gain that much by this modification. Is there a problem with adjusting the output tube balance for minimum hum?

There are also some places that sell EL84 bias probes.

 

RE: biasing setup on Scott 222D question, posted on March 19, 2017 at 15:08:42
gkargreen
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Thanks, Michael & Therm, that makes the most sense. I may give it a try with the published biasing method, but being able to actually measure each cathode current is best!

 

RE: biasing setup on Scott 222D question, posted on March 19, 2017 at 20:04:55
Michael Samra
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Therm
He just needs to do the Eico bias setup as they do on the HF50s and HF60s.
The reason they didn't on this unit was because of coat and convenience but you have a balance and bias pot for each channel,the 10 ohm for each tube makes sense.This is what I do on Scott 222s and it makes it so much easier to balance.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

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