by dcgillespie » Mon Dec 20, 2010 7:28 am
Cedric -- The buzz you're hearing likely has nothing to do with the transformer voltage taps you are using, unless in changing the taps it caused AC power wires to somehow come in close proximity to the input wiring, which I doubt. An AC buzz is usually due to ground loops in the wiring, or heaters that are not properly referenced to ground. I would check these things first, as most folks report that these amps are dead quiet when built as instructed.
As to your Zener diode option, if you get a 20 volt device, it will allow the best of both worlds for your operating conditions. If you install it, you could then restore the AC power to the original AC taps on your transformer, which would then restore the proper 6.3 vac to your heaters. It would also push the available B+ back up to 400 vdc, but with a 20 volt Zener installed, it would drop this back to an operating B+ of around 380 vdc. Accounting for about a 10 volt drop in the OPT primary winding, and about 14 volts across the bias resistor, this means there will be about 356 vdc develop across the tubes. If you then set the bias for 35 ma. per tube cathode current draw, the plate current component of this figure is about 31 ma. With 356 volts effectively dropped across each tube, this equals 11 watts of plate dissipation per tube, which is within safe operating parameters.
Doing this will also restore your cathode voltage readings (pin#3) to more traditional values. Realize that just as the control grid of any tube (pin#2 in this case) always controls the current flow through the tube, so does the screen grid as well (pin#9 in this case) in pentode tubes. Therefore, since these are UL amplifiers, reducing the operating B+ from 400 volts down to 380 volts by the Zener also reduces the screen grid voltage as well. This then reduces current draw, which then allows more normal cathode bias voltages to be developed when achieving the target current draw.
Good luck with your amp!
Dave