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In Reply to: RE: Bias source from B+ or seperate trans, which is better? posted by Lee of Omaha on August 08, 2014 at 10:21:49
Lee,
What you write seems reasonable to me, but I was specifically told to do fixed bias with a separate PT and a FWB by an audiophile tenured EE Professor, a genius. No EE got through his class until they built a successful sounding tube amp !! This E.E. PhD. was in charge of USA's early NASA project of putting a monkey into space.They were using an EARLY version of Jeremy Epstein's DC 2A3 amp circuit. To calm the mammal, in the capsule, they installed WE 755s, played Bach and Beethoven, using an amplifier that was intially called "Monkey on a Rocket ".
Seriously, there are all kinds of high frequency nasties in the modulating HV secondary, and I could underdstand intuitively - a separate PT for fixed bias would be better. Rather than..."that will do" designing. The EE Professor was specific about this !!
Jeff Medwin
Edits: 08/08/14 08/08/14Follow Ups:
Jeff, why is it that all these people to whom you have stories like this are long dead and buried with absolutely no published documentation for the claims you make.
Do you have some reference as to this professor and the requirement for students to build an audio amp to pass the course? As this is a rather unique credit requirement I'm sure there would be something documented of this practice.
What was the professors name and the school where this was practiced so some of us may research this unique course requirement further?
monkeys in space. that made me laugh.
I *heard* through a brilliant engineer that cathode bias was superior. Said engineer had actually developed apparatus to help pigs fly.
I believe the spikes from rectification not only affects the forward B+ supply but is also reflected back into the power transformer and cross contaminates all th secondaries.
To that end I placed three snubber caps, .1 uF, across the legs of the filament leads and ground, hoping that the effect would have some change in the B+. It certainly did. Everything became significantly warmer and fuller with an increase in midrange dynamics and detail. Cheap and very effective.
I only had 120 VAC rated caps so I couldn't place them on the B+ leads.
Also one of the major issues with bias circuits is to adequately buffer it from the ground. In most cases, the adjustable pot ( thinking Dyna circuits here) simply has a a cap to ground. This allows the bias voltage to swing a bit more than I prefer. I place a simple cap across the resistor to ground with great results, better resolutions and dynamics. The cap ( a small one, 5 UF is OK and electrolytics certainly work very well) buffers the bias circuit extremely well. You can see this in metering out the bias. Turn the knob and the voltage moves significantly slower, thus maintaining better control over the signal, IMHE.
Using this cap actually eliminates the need for a separate full wave bias bridge, although it should work there too.
YMMV and certainly FWIW
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