Filament Voltage Supply

Parts List

C1 - C3   .01 Uf @ 1000 V	M3          0-15 VAC Meter
D5            5 V 10 W Zener Diode	RH1      120 VAC @ 6 amps Rheostat
D6 - D8   1N5400 3A Diode	R3,R4   25 Ohm @ 10 Watts
F3            3 Amp 120 VAC Slow Blow	R5         50K @ 10 Watts
F4            1 Amp Fast Blow	RL2       120 VAC Delay on operate
M1            0-1 DC Amp Meter	RL3       12 VDC DPDT relay
M2            0-50 ma DC Amp Meter	T3          10 VAC @ 12 amps

---

I was very fortunate when it came to this part of the project in that the filament transformer was still working. 813 tubes require 10 volts on the filaments, and it is very hard to find a transformer that will supply the ten amps that both tubes will draw. In doing some research, the experts thought it best to put a "step-start" circuit on the filament supply as well, so that is what RL2 and the associated resistors are for. RH1 is used to adjust the filament voltage to the tubes, which is read by M3.

In the lower right part of the schematic, is the metering circuit. M1 is used to measure the plate current of the 813 tubes, and M2 measures the grid current. The bias circuit for the tubes are the Zener diode, (D5) and R5. The K3 relay is part of the transmit - Receive relay. In the transmit mode, the zener diode provides the proper bias for the 813 tubes. On receive, R5 does the biasing, and puts the 813's into cutoff. This helps keep heat down. D6 - D8 are glitch protection diodes for the meters in case of a negative going High voltage spike. F4 provides further protection in the event of a catastrophic failure.

Also notice on the schematic that I have two fans wired in series. These are typical 4" muffin fans which are mounted about 6" away from the 813 tubes. I have them wired in series so they will run at a slower speed. This is an something I am trying in an attempt to keep the fan noise down. If it doesn't keep the tubes cool enough, I will try something else.