6BE6 mixer circuitry puzzle - UK Vintage Radio Repair and Restoration Discussion Forum

G4BZI Roger · 12th May 2022, 3:54 pm

I'm restoring an M&G 3 ssb transceiver from the 1960s but am puzzled about the receive 1st mixer circuitry.The mixer uses a 6BE6 which mixes the VFO output with the amplified incoming signal to give the IF (8625 kHz) which then passes through a filter.
The VFO runs at 8625 minus signal on 160 and 80m and signal minus 8625 on 20m and is injected to G1. The incoming signal is injected to G3 and this grid has negative cutoff bias applied during transmit. The anode load is 220k.
The puzzle is the screen (G2/G4) - this is fed with HT via a 220k resistor which is decoupled to chassis. The resistor/decoupler junction is connected to the screen via a parallel tuned circuit which seems to be resonant on 4.45MHz if I believe my GDO.
Can anyone explain the purpose of this tuned circuit?
TIA
Roger
G4BZI

Cruisin Marine · 12th May 2022, 4:49 pm

Being a parallel tuned circuit, I am guessing it is rejecting half the IF frequency, 8625 divided by 2= 4312.5, why? I cannot say.

12th May 2022, 3:54 pm	#1
G4BZI Roger Pentode Join Date: Nov 2005 Location: Crewe, Cheshire, UK. Posts: 138	6BE6 mixer circuitry puzzle I'm restoring an M&G 3 ssb transceiver from the 1960s but am puzzled about the receive 1st mixer circuitry.The mixer uses a 6BE6 which mixes the VFO output with the amplified incoming signal to give the IF (8625 kHz) which then passes through a filter. The VFO runs at 8625 minus signal on 160 and 80m and signal minus 8625 on 20m and is injected to G1. The incoming signal is injected to G3 and this grid has negative cutoff bias applied during transmit. The anode load is 220k. The puzzle is the screen (G2/G4) - this is fed with HT via a 220k resistor which is decoupled to chassis. The resistor/decoupler junction is connected to the screen via a parallel tuned circuit which seems to be resonant on 4.45MHz if I believe my GDO. Can anyone explain the purpose of this tuned circuit? TIA Roger G4BZI

12th May 2022, 4:49 pm	#2
Cruisin Marine Heptode Join Date: Jul 2018 Location: Worthing, West Sussex, UK. Posts: 992	Re: 6BE6 mixer circuitry puzzle Being a parallel tuned circuit, I am guessing it is rejecting half the IF frequency, 8625 divided by 2= 4312.5, why? I cannot say.