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In Reply to: RE: Help on biasing / adjusting - 829B amp posted by Mark Williams on May 11, 2008 at 05:16:55
I think I can help you out, as I have built some push-pull monoblocks running these guys (two per channel, not one ;)), but be sure to follow these steps in order.
1. Determine the value of the resistor from cathode to ground on the 829B tubes. This resistor goes from pin 4 on each septar socket to ground (it looks like it might be the white sand cast resistor).
2. The schematic shows two adjustment pots, one that looks like a balance pot, and the other adjusts the negative voltage. You want to figure out which one is the balance pot, set it to center, and forget that it exists.
3. Secure the leads of your multimeter to each side of the cathode resistor on one channel. Turn the amplifier on. You will want to set the total current for each tube to around 95ma. For a 1 ohm cathode resistor, this would mean that you would want to see .095VDC on your multimeter, for a 10 ohm cathode resistor you would see .950VDC, etc.
4. Repeat for the other channel.
Hopefully this is clear enough. Be careful when you're working under the amplifier, as that 430v will have some bite! If all of this seems ultra comfortable to you, you might return to that balance pot when the bias is set and adjust it a little if there's a significant difference between plate voltages, although I really would just leave it centered. Also, 95ma per tube may be a little low, you can move it within +/- 10% to see if you develop a preference for a specific operating point.
-Paul
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