[HBR] Triode mixers

[Ron Barlow]

Hi Ian
 IIRC, I cured a mixer oscillation problem, by adding a 10 pf cap (with very short leads) from plate to chassis ground. The tuning slug, of the 455 khz IF transformer, had more than enough range, to compensate for the added "C", in my case, but your situation may be different.
 If your IF xfmr has a ceramic or mica cap, across the primary of the IF xfmr, simply relocate the cap to the plate connection, on the tube socket, to chassis ground, again using short leads. The important thing is to provide a very low inductance RF path to ground, from the plate. There should be no change in the performance, if the cold end of the IF xfmr primary winding is well bypassed to chassis gnd.
 I believe that I found this idea in Hints and Kinks, but this happened some 43+ years ago, so I dunno for certain!
 Also, add a 33-47 ohm resistor in series with the control grid lead, of the mixer, if one isn't already present, to inhibit vhf parasitics.
                              GL & 73 de Ron    n4gjv 


________________________________
From: Ian Wilson <ianmwilson73 at gmail.com>
To: HBR Receiver List <hbr at mailman.qth.net> 
Sent: Monday, December 17, 2012 11:26 AM
Subject: [HBR] Triode mixers

Working from the back to the front of a dual-conversion RX loosely modelled
on
the 75S-3C, I have reached the first mixer. This is the triode part of a
6EA8 in
the Collins RX. The xtal osc output is injected into the cathode.

I am attempting to use half a 6DJ8 for this, but it isn't working well. I
suspect
that I need to have a cathode circuit that is low impedance except at the
injection frequency. Otherwise this looks rather like an oscillator. The
Collins
circuit essentially takes the xtal osc output from a 100 ohm load in the
plate
of the pentode half of a 6EA8. Across this 100 ohm load is the (untuned)
primary of a transformer, the (tuned) secondary of which provides the
cathode
injection for the mixer. I have no information about the transformer;
however,
the AC injection voltage is shown as 1v on the schematic.

Would welcome general info on cathode-driver triode mixers and/or pointers
to schematics on BAMA of receivers that use this, hopefully without
components
that are difficult to reverse engineer.

73, ian K3IMW