[Boatanchors] UNSTABLE 6K8 OSCILLATOR

[Bill and Liz]

I’m playing with a 6K8 mixer working with a 6J5 BFO in a homebrew receiver which works very well in almost every other respect except there is random drift. This is up and down after a good warmup period, not in one direction, and sufficient to change a CW note significantly or change the voice on an SSB signal.  It is not caused by mechanical instability.

The filaments of both tubes (operating in series) are run from 12V DC regulated; the screen of the 6K8 is zener regulated as is the plate of the 6J5.  The BFO changes frequency by as much as 3 hz while the local oscillator changes a good deal  more than that.  Both move up and down randomly. It appears to me that the real culprit is the local oscillator and not the BFO.

I have other receivers employing the 6K8 and I can sit them on a frequency with no random drift like I am experiencing in this receiver.  So I’m wondering if anyone has any ideas on what might be causing this.  I have substituted tubes with known good ones, checked for cold solder joints, loose grounding lugs etc.  I have tried temperature compensating caps, NPO, N*** and P*** with no success.

I’m stumped!!

Bill VE3NH