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Old 6th August 2007, 06:01 AM   #1
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Default quick question on fixed bias

hi, first post in a while. I'm working on a couple of monoblocks and recently decided that I'd like to go the fixed-bias route in lieu of cathode biasing, both to get rid of the bypass electrolytics and to get a bit more power. my B+ will probably be around 425V - one 200mA 1000VCT choke-input rectified with a NOS 5U4GB into either an 80 or 100uF motor run cap. I'd rather avoid buying a separate bias transformer for each monoblock (stupid hammond 2xx series doesn't have the windings).

my idea is this: connect a UF4007 to one of the 500V legs to get a half-wave-rectified negative voltage source. where I go from there, though, is fuzzy. should I tag a smoothing capacitor right there, then use a voltage divider to get negative voltage down to something more manageable, like -70VDC? I'd like to leave some room for one pot to control overall voltage, and one pot to balance the tubes. I'm shooting for around 50mA per tube, for around 21W of dissipation. theoretically around 55mA should net me 24 or so watts, which is around 80% of the 6L6GC's max dissipation of 30, so I might try that also.

another question: when figuring out resistance values, it helps to know how much current each 6L6 (Sov 'WXT+, in this case) will draw.. I figured that a 10K pot in between the tubes might work, and maybe a 50K pot to control overall voltage? if current is low enough I'd love to use some ultra compact 21-turn trimpots to do this adjustment. also, how important is capacitance in filtering this bias supply?

thanks for all the help!
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Old 6th August 2007, 08:07 AM   #2
oshifis is offline oshifis  Hungary
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Hi,

I tried both fixed bias and cathode bias in 807 monoblocks with exactly the same B+ voltage and current. My experience was that fixed bias was not stable, sometimes the tubes run overcurrent. I used separate variable resistors for each grid setting. The cathode resistor (560 ohms in my case) blocked with 2 x 10 uF PP capacitor gave a stable solution.
If you still want to try fixed bias, use a separate small transformer. Since there is no significant current drawn, a 470 uF puffer will do.

Laszlo
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Old 6th August 2007, 08:44 AM   #3
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You could try using capacitors to drop the voltage to the bias supply, using full-wave rectification, as Gary Pimm did. This link is to an article where Gary describes the development of his 47 PP. You have to scroll a long way down to see where he introduces capacitive droppers into his power supply.
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Old 6th August 2007, 02:26 PM   #4
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thanks everyone! the capacitor/resistor divider method as shown in the diytube eiclone schematic seems pretty simple, so I'll give it a go.
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