I'll re-post here the notes for DC heaters on Parabees:
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DC filament power for ParaBee amplifiers
Parts for a stereo pair:
8 1N5820 Schottky diodes, 20v 3A. Don't substitute!
2 6mH 2A chokes, Hammond 155B, available from Antique Electronics
2 10,000uF 6v capacitors, I used FC types from DigiKey
4 standoffs, 1" long internally threaded for #8 screws. I used
aluminum, but brass or nylon should work too
4 #6 screws
4 #6 lockwashers
2 5-lug terminal strips, from Radio Shack(?)
First, find the 300B filament power wiring (2S, 2F, 4S, 4F) and disconnect the wires from the power transformer.
Now locate the 6v heater winding, terminals 3S and 3F. Those are the power transformer terminals that feed the driver filament. One end goes to the juncture of a 240k and a 24k resistor which sets a 40v bias on the 6v heater winding - remove this connection, but leave the two resistors connected.
Now you are going to mount a choke on the standoffs, using the power transformer bolts at this side of the transformer. Mount the terminal strip to the choke foot nearest the 6v terminals. (There is an unsolved problem here, I had to use #6 screws because the terminal strip hole was too small for #8. It seemed to work, but obviously it's not the right way.)
This next part is mechanically tricky, because the diodes are bulky and have stiff, fat leads. You want to make a fullwave bridge from the transformer terminals to the two closest lugs of the terminal strip, right above them. Mark one of the lugs + and the other -. I'll attach a circuit diagram.
The end of the diode with the stripe is the cathode. You want to have two diodes from each power transformer terminal, one with cathode to "+", the other with cathode to the transformer and the other end to "-". Solder at the transformer ends, retaining the driver power connection. Yes, I know, there's not enough room to do this properly on the transformer connections.
Now you can mark the two free lugs on the other end of the terminal strip ++ and --. Connect - to -- with a bit of wire, then connect the choke leads from + to ++. Solder at the diode terminals. Now install the cap with the negative lead to -- (of course!) and the positive to ++.
Ideally you would then solder the 300B filament leads to the ++ and -- where the capacitor is attached. I don't know if they will be long enough, so you may have to extend them or replace them.
There should be about 4.8vDC on the 300B filaments; all the other voltages and resistances should not change.
You can remove the Bench hum cancellation circuit if you want, or just leave it in and turn it down all the way, which might be easier. You'll need to retain the large hum pot in any case. To remove the Bench cancellation circuit, remove the 2uF cal, the 10k trimmer, and both 1N914 diodes.
Be sure to adjust the large hum cancellation pot for minimum hum. This power supply has about 30dB less hum than AC filament power, but it still has a little, which can be trimmed to a minimum with the pot. The hum will be at twice the previous frequency, so the 120Hz component can be trimmed out leaving a slight 240Hz component.
