[R-390] Re: 6C4? 6C4W? 6C4WA?

[Tim Shoppa]

Barry Hauser <barry at hausernet.com> wrote:
> I seem to remember a thread some years ago about the 6C4  and it being best 
> if the tube is not too "hot" -- as if a mediocre-reading (middling, not 
> weak) tube was the better choice -- or one that had been worn in elsewhere. 
> I don't recall if it got into s/n ratio or whatever.
>
> Anybody else remember that one?

I believe that argument is most often invoked for the 6BA6W/5749 in the PTO.

There the argument is that constant emission/mu there is most relevant
for long term stability, with some hints that "hot" high emission/high
gain tubes may actually be more likely to have their gain depend on
other voltages/circuit elements/have their gain change more rapidly
over time as the tube breaks in.

Contrast this with classical oscillator theory where high gain is
used as a Q-multiplier. That works well for Meecham bridges but for
better or worse the 390A oscillators are not so idealized!

Me, I use 12BA6's that probably saw many years of use in AA5 sets :-).

One of my summertime projects may be measurements of PTO and
the three crystal oscillators for frequency changes as a function of
AC line voltage/AGC voltage/PTO filament currents etc. After a
burst of Cosmos PTO activities a few months ago I am a little burnt
out with frequency measurements at the moment, though :-).

Tim.