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813 Amplifier Schematic Diagram


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.

Schematic for 813 amplifier

Note: All unmarked capacitors are .01 uFd @ 1KV.
All unmarked resistors are 4.7 ohm @ 2 watt

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

As you can see, this is a typical 813 amplifier schematic with the exception of the input circuit. This is known as a "bifilar tuned input". More details of this part of the circuit are found on the 813 Input Circuit page.

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 Meters13 Turns
40 Meters7 Turns
30 Meters4 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 Meters3 Turns
17-15 Meters2 Turns
10 Meters1 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.