High Voltage Power Supply for Linear HF 813

Parts List
Cx 4700 Uf @ 350 volt Electrolytic R1,R2 25 Ohm 10 watt
D1-D4 5000 PIV @ 1 amp Rx 25K @ 5 Watt
F1,F2 15 Amp Slow Blow T1 2600 VAC @ 1 amp
RL1 120 VAC Delay on Operate T2 12 VAC @ 10 amps

lot of the parts used in this project came with the transmitter. As a result I had to get a bit creative to get what I wanted. In doing a bit of research, I found out that several people have run 813 tubes with 3Kv sucessfully with out damage. Based on what I had, I decided to change the power supply filter from a choke input filter to a capacitor input filter. I found a great deal on some 4700 Uf @ 350 V capacitors. So I bought 12 of them, and put them in series (Cx above). This gives me an equivalent capacitor of 390 Uf @ 4200 volts. Yes, I know this is overkill, but hey, they were cheap, and I had the room! This brought me to my next problem. When I wired in all of the caps, and fired it up, I had 3200 volts. I felt that was too much for the 813's so I found a low voltage transformer to put in series with the primary of the main transformer as a "bucking" transformer (T2). This then dropped my output voltage to the 3000 volts I wanted. Once again, this is not the best way, but I was able to find a transformer cheap.

Usually High voltage supplies have a "step start" circuit. This one is no different. The purpose of the step start circuit is to apply power at a reduced voltage so as to limit the initial inrush current. After a period of time, the step start circuit energizes, and applies full input voltage. I felt this was very necessary since my filter capacitor is larger than typical. I was able to find a nice programmable "delay on operate" relay (RL1) at the local surplus store for a good price along with the current limiting resistors (R1 and R2).

WARNING
The voltages inside this project CAN KILL. If you have any doubts about your abilities please seek assistance from a capable amateur radio operator or electronics technician.

Linear HF Tube 813

WARNING
The voltages inside this project CAN KILL. If you have any doubts about your abilities please seek assistance from a capable amateur radio operator or electronics technician




Parts List
C1 150 pF variable 0.125 spacing L2 See next page for details
C2 1200 pF 3 section AM receiver variable RFC 1 (2) 3/4" dia. 3" long 24 AWG (see text)
C3 900 pF 2 section variable RFC 2 2.5 mH
L1 3" Dia. 13 turns - see text RL-3 DPDT relay 10A contacts

L1 is actually two coils in series. The first coil is 3" diameter coil with 0.25" spacing. with taps for the following bands:

80 Meters 13 Turns
40 Meters 7 Turns
30 Meters 4 Turns

The second coil is 4 turns of 1/4" copper tubing 2" diameter with 0.5" spacing. with taps for the following bands:
20 Meters 3 Turns
17-15 Meters 2 Turns
10 Meters 1 Turns
RFC 1 is actually made up of two seperate chokes. Each one is close-space wound on a 3/4" diameter ceramic rod 3 1/2 inches long using 24 AWG enamel wire. The two chokes are mounted at right angles to each other, and have a .01 uF @ 5 KV bypass capacitor to ground between the two chokes. This is an attempt to eliminate any self-resonance near the operating frequencies of the 813 amplifier.


RFC 2 across the output is a mandatory safety item. Should the .001uF DC blocking capacitor ever break down, you will have the high voltage supply connected to the antenna system. This choke will shunt the DC to ground and open the fuses in the primary side of the plate transformers. Under no circumstances is this choke to be left out. Use 28 AWG enameled wire. Wind 150 turns on to a 2" ferrite rod. If you have to, you can wind the wire in 2-3 layers.

The T-R switching cicruit is composed of two 12 VDC relays. I chose to use two relays, RL-3 and RL-4, becuase I was concerned about coupling between the input and output of the amplifier. RL-3 is a DPDT relay that is energized during transmit. The second set of contacts are shown on the Filament Supply schematic. This relay is located in the input section of the amplifier. RL-4 also a DPDT relay and is located in the RF deck of the amplifier near RFC 2.