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In Reply to: RE: adding a bias tap posted by tubeflash on November 25, 2015 at 10:35:35
I would not suggest tapping into a power transformer unless you really know what you are doing.
Follow Ups:
Agreed! Be safe and use the little trafo for the C- supply.
Full wave bridge rectify the rail with 4X SB190-T Schottky diodes. CRC filter. A Nichicon KZ series part in the reservoir position works well.
Eli D.
I recently bought a large quantity of small transformers for exactly this use. Adding a separate bias transformer is the simplest way to deal with the issue.
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Buy Chinese. Bury freedom.
what voltage/amperage transformer did you purchase?
The transformers I bought are 28V @ 175mA. That's more current than is needed for most bias supplies, but these were available in quantity on eBay at a good price when I was looking. They're small enough to fit under most chassis, they're US-made, and the voltage is about right for many tubes if I use a FW bridge. If more voltage is needed, it's simple to use a doubler as Eli notes below.
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Buy Chinese. Bury freedom.
While you don't use an elephant to squash an ant, just about any low voltage winding of modest current capability can be adapted to bias supply service. Only a few mA. are drawn from a C- rail.
Even windings of 6.3 VAC can be employed. Just use a voltage multiplier, like that uploaded, containing enough stages to accomplish the task. Don't forget a RC reservoir section, after the multiplier. That too is shown in the graphic.
Eli D.
tapping off the B+ would be a copy of the design in the
RCA receiving tube handbook page 696
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