[R-390] 5814A/ 6C4 tube placement

[Tim Shoppa]

"Ed Zeranski" <ezeran at ezeran.cnc.net> wrote:
> Not sure how it relates to R390s but I had several brand new (NIB) 6C4s used
> in osc ckts that had cathode/fil leakage causing noise under strong sig
> conditions in Hammarlund communications receivers. Not mixer but osc
> circuits in my HQ-170AVHF and the 6C4s were from new sticks of tubes.

Was there something odd about the Hammarlund oscillators using a
high cathode-to-filament potential? In simple oscillator implementations
it's just a few volts difference for grid bias BUT sometimes they do
funny tricks like putting cathode followers in there or putting a
very large resistor (same size as the plate resistor) in the cathode
to pick off a signal of opposite phase without using a transformer, and
in those cases you can end up with 70 or 100V real easy (and that
is pushing the limits of the spec.)

Speaking of which the 390A has a remarkable lack of buffer stages
between oscillators and mixers. I have not yet tracked down which
oscillator seems to pull under CW reception strong AGC action.

Tim.