[R-390] Random tube sub of the day: 6AU6 for 6BA6

[Tim Shoppa]

One tube sub I tested: 6AU6 for 6BA6. They are both 7BK base pentodes
but the 6BA6 is remote cutoff (more appropriate for AGC action)
and the 6AU6 is sharp cutoff.

Sticking the 6AU6 into various places in the IF strip resulted in
changes in AGC action and carrier level meter. The changes were
noticable but not horrendously bad. I did not tweak the GAIN ADJ
for each sub. I only subbed one tube at a time.

What was more surprising was action in the PTO and BFO sockets: no
oscillation at all! I had expected the cutoff details to be
unimportant there, but I suppose it's possible that the 6AU6
is biased deep into cutoff where the 6BA6 isn't.

I have to find a tube equivalent of SPICE or something and start
modeling some of the 390A's circuitry. Of course it's probably
more fun to stick tubes into sockets and see what happens! I get
to stare at computer screens all day at work, why should I think
about doing that instead of playing with tubes and solder?

I think it's time to break out the tube handbooks and some graph
paper. As a boss of mine once told me : "If you don't know how to
do it, you don't know how to do it on a computer."

I know how to do graphical analysis of operating point and gain
vs bias for single stages, but have never really tried it for
a huge RF/mixer/mixer/mixer/IF/IF/IF/IF+AGC chain. Is there a tube
receiver with more stages than a 390A?

Tim.