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.