The G2DAF HF amplifier! HV and filament voltage required, just as in a grounded grid amplifier. Screen voltage is derived from the input RF and the grid is at DC ground. Check the circuit description below. The HF station, amplifier rack to the right The QBL 5-3500 tube.
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 |
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 zenerdiode provides the proper bias for the 813 tubes. On receive, R5 does the biasing, and puts the813's into cutoff. This helps keep heat down. D6 - D8 are glitch protection diodes for themeters in case of a negative going High voltage spike. F4 provides further protection in theevent 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 themwired in series so they will run at a slower speed. This is an something I am trying inan attempt to keep the fan noise down. If it doesn't keep the tubes cool enough, I will try something else.