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Old 3rd August 2008, 06:05 PM   #1
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Location: Indiana
Default Bias winding to power sand

I have literally hundreds of small signal BJTs on hand and am considering using them for the meter driver in my 6EM7 project. The PT for the project has a 45VAC bias winding that I am not using for its intended purpose. I am wondering if it would be reasonable to use it to provide power for about 1/2 dozen of these little guys.

The schematic of the PT makes it appear that the "winding" is just a tap off of the HT winding which also has 275V and 325V taps. If so then I suspect the current of a few mA should not bother the winding at all. Would you agree? I am somewhat concerned however that there might be some interaction from the rectification and filtering that would be reflected back into the B+. Is this a legitimate concern? If so what precautions would be indicated?

Over all would I be better off to just take a voltage divider of the B+ for the tubes to derive the PS for the transistors?

BTW does anyone have a nice BJT circuit for a VU meter driver? I am thinking of adapting the Elliot Sound Op Amp circuit for BJT.

The meter would be switched between microphone input and output power monitoring.
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Old 3rd August 2008, 08:06 PM   #2
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Location: Monroe Township, NJ
Quote:
I am wondering if it would be reasonable to use it to provide power for about 1/2 dozen of these little guys.
Sorry Mike, I think you are asking for trouble. Bias taps, as opposed to true bias windings, can't be bridge rectified. Other than the tiny draw that occurs when the grid potential approaches 0 V., the tap is intended to provide no appreciable current.

It's not like separate, low VA, trafos will break the bank. AnTek 10 VA toroids cost all of $10 each. $6.29 buys "Rat Shack" catalog # 273-1366, which you could Greinacher voltage double to power "gobs" of BJTs.
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Old 3rd August 2008, 09:29 PM   #3
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OK. I thought there might be trouble in paradise. I actually probably have an appropriate tranny lying around as I have salvaged a lot of solid state gear. I just didn't want to waste anything if it wasn't needed.
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