quote:

Originally posted by maladat:
Let me preface this by saying I know just enough to be dangerous, but am far from an expert on tube amps.

I think the bias wire is there to provide a medium voltage source that can be rectified and used to DC bias parts of the circuit, e.g., a lot of people report DC biasing the heaters by a few tens of DC volts to reduce AC noise.

Generally speaking, you can just cap unused transformer taps. A lot of purpose-built guitar amp power transformers come with multiple sets of input taps for different line voltages (almost always for 120V and 240V, but some repros have extra, slightly underwound input taps to emulate the lower wall power voltages of the old days) and multiple sets of output taps.

The heaters are normally 6.6V in parallel or 13.2V in series. As far as I know, the only benefit of wiring them in series is less current draw on your transformer (which can sometimes be an issue, some tubes draw a lot of heater current).

You'll get less AC hum in your circuit if you run your heater supply as +/- AC, whether that's +6.6/-6.6 for series heaters or +3.3/-3.3 for parallel heaters. You can add a pseudo-center-tap to ground to do this if your transformer doesn't have a center tap for the heater windings.

quote:

Originally posted by maladat:
There apparently are also some power transformers with bias taps that you apply a voltage to in order to regulate the transformer's output. I don't know anything about them, though.

quote:

Originally posted by maladat:
If you know what tubes were in the original Wurlitzer, it wouldn't give you the max, but it shouldn't be hard to ballpark the original heater current draw.

quote:

Originally posted by maladat:
Hmm. I'm not sure how I came up with 6.6V/13.2V. You're right that heaters are nominally 6.3V/12.6V.

12AX7s are 12V/150mA or 6V/300mA.

12FQ8s are 12V/150mA only.

If there were at least 4 12AX7 tubes with the heaters running off 6V in the Wurlitzer, you should be OK running 2xEL34 heaters off the 6V supply that fed 2x7868 and 4x12AX7.

Then in your new circuit, you can run however many 12AX7s on the 12V supply as there were 12FQ8s in the Wurlitzer.

Another option is that some high-fidelity tube audio amps use regulated DC for heaters in an attempt to reduce induced noise. You could add a separate regulated DC supply running off the wall power for the heaters (or some of the heaters).

Another another option is that you could add a pseudo-center-tap to the 12V supply and run one EL34 off one leg and the other EL34 off the other, if there's enough current capacity on the 12V supply.