Dual Bias For JTM

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Re: Dual Bias For JTM

Postby High IR » Tue Aug 24, 2010 1:02 am

Hi everyone. New to the forum.
I'm about to start my second amp, JTM 45 with RS this time. Finished a Metro 50W last year. I'm looking to have one of the bias circuits above, not sure which, but would also like to add a Lar/Mar master volume. Can anyone comment on how either the circuit described by Alex or SDM above would need to be changed to accommodate the master volume? Thanks in advance for any advice. IR
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Re: Dual Bias For JTM

Postby Brian Wallace » Sat Aug 28, 2010 5:53 pm

The circuit would remain the same except you would run the bias feed from each bias circuit to each pot.
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Re: Dual Bias For JTM

Postby Steve_Hogan » Thu Apr 18, 2013 8:39 pm

chubs wrote:Here is a Steve (SDM) version that is 100% independant... Works like a charm in my JTM45/KT66

The completely independent Bias circuits in this layout retain all the original circuitry including the half-wave rectifiers. HOWEVER -- because each bias circuit is fed from the opposite phase of the HT transformer, the ripple on each circuit is 180 degrees out of phase with the other circuit.
For this reason, the output tubes will not cancel the remaining ripple appearing on the bias voltage. For this reason, I would suggest feeding both 180K resistors from the same side of the HT winding to solve this problem.

Alternatively the circuitry could be reworked to be a full-wave rectifier instead of 2 half-wave rectifiers. The full-wave rectifier would have 120 Hz ripple instead of 60 Hz ripple. The full-wave ripple would be identical on both tubes, lower in amplitude, and, at least in theory, it should cancel out in the push-pull output circuitry. Other bias circuit values may have to be changed to get all the voltages right with a full-wave approach. The full-wave rectifier will charge up the first 10uF cap to a higher initial voltage because it gets charged up twice as often as the half-wave version. I have a full-wave dual bias circuit figured out that I will be implementing in a JTM-45 build later this month. I will tweak the values and report later.
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