An Improved Three Tube Superhet

During the 2001 AWA OT CW Contest Niel, W0VLZ, used a homebrew three tube superhet (plus external power supply and AF amp/speaker) with a 6L6 transmitter running approximately 7 watts. New York, Indianna, Illinois, Louisanna, Oklahoma, Georgia and Texas were all worked on 80 meters from southeast Minnesota using just a 100' end fed wire antenna.

The receiver is an improved version of the 1941 Beginner's Receiver. A major improvement was the addition of a crystal filter. It gives this simple receiver single signal selectivity. At 6dB down, the bandwidth is only 250Hz. Inserting the crystal into the middle of the regenerative IF stage allowed regeneration to make up for crystal insertion loss without adding an extra tube. Also modified from the original was changing the regenerative detector from 1/2 of a 6C8 to a 6J7, adding better bandspread, capacitively coupled headphone output and a front panel BFO adjustment. The attenuator on the antenna input was added following the contest after experiencing problems from strong signals overloading the receiver.

About the single crystal filter. It is in the signal path and the regen feedback path. If used as a typical regen it peaks the signal on the same frequency as the regen beat. The BFO is required in order to get an audible CW tone.

To set the BFO (once and forget):

  1. Turn off the BFO
  2. Adjust the regen control for CW reception
  3. Tune a CW signal to zero beat
  4. Turn on the BFO and back off on regeneration until the the set stops howling
  5. Adjust the BFO control for a comfortable note

Adjust the regen control for desired level of selectivity (but not enough that the set goes into regeneration)

Left to right, top: Input tuning, Bandspread, Bandset

Left to Right, bottom: Ant. Attenuator, Regeneration, BFO Tuning, BFO On/Off

Left to right, Back: 6C8G, AF Choke, 6J7

Left to right, Middle: Osc Coil, 6K8, Input Coil

Left to Right, Front: Bandset Cap, Bandspread Cap, Crystal, Input Cap

Bottom View

C1, C3 100 uufd. variable (Hammarlund SM-100)
C2 .1 ufd.
C4 15 uufd. variable (Hammarlund SM-15)
C5 220 uufd. silvered mica + 3-30 uufd trimmer
C6, C9 0.01 ufd.
C7 0.005 ufd. mica
C8, C10 100 uufd. mica
C11 200 pf. (approximately) to set center BFO frequency
C12 10-90pf. (approximately) trimmer/variable
C13 10pf. (approximately) to set BFO range
C14 1.0 ufd
C15, C16 100 uufd. mica
R1 47K ohms, 1/2 watt
R2 47K, 1/2 watt
R3 1M, 1/2 watt
R4 10K potentiometer
R5 10K , 1 watt
R6 470K 1/2 watt
R7 33K 1/2 watt
R8 10K potentiometer
RFC 2.5 mh. r.f. choke
L1 Audio transformer, interstage type, 3:1 ratio, with windings in series
L1-L4 See 1941 Beginner's Receiver coil description/table
L5 55 turns No. 30 d.s.c., close-wound on 3/4 inch diameter form (National PRF-2); inductance 40 microhenrys
L6 18 turns No. 30 d.s.c., close-wound on ground end of L5 form
X1 Crystal at IF frequency
S S.p.s.t. toggle switch
R3 47K ohms, 1/2 watt
L7 55 turns No. 30 d.s.c., close-wound on 3/4 inch diameter form (National PRF-2)
L8 18 turns No. 30 d.s.c., close-wound on ground end of L7 form

Not shown is my external Power Supply/Audio Amp/Speaker. See the 1941 ARRL Handbook, page 117, for a description of the AF amplifier. I did simplify it a little by replacing R6 and C11 with an interstage AF transformer.

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