[R-390] Test: 6DK6 sub for 6DC6

[Tim Shoppa]

I got a bunch of 6DK6's for my Tek scopes, and thought as long as
I was bringing them up to speed that I'd drop one in to the 6DC6
socket of my 390A and see what happens.

Note: this is a quick 5-minute test. No other part subs, not
even zeroing the S-Meter or setting IF/RF gain balance.

Two obvious things: S-Meter reading approximately doubled on fairly
strong BC and SW stations. e.g. 60 DB readings before the sub
resulted in pegging the meter after the sub, 20 DB readings before
became 40 DB readings after, etc. AND the zero-point setting of
the meter went way off to the left.

Without seeing the actual tube curves, I'm guessing this means
that for the same "strong signal" AGC voltage, the 6DK6 has a lot more gain.
And that maybe at "weak signal" AGC voltage, the 6DK6 has less gain?
(Not so sure about the second conclusion.) This seems inconsistent
with what a sharper-cutoff pentode should do but I never claimed to
have a deep understanding of AGC system balance and design! In any
event there was no rebalancing of gain so making heads or tails of
S-meter readings is probably impossible.

Two more obvious things: Intermod of the local blowtorch AM station
in my backyard was now EVERYWHERE, when it never occured with the
6DC6. And AGC action seemed to mostly
work, at least on AM modulation. On CW the AGC action seemed oddly
out of kilter, like it wasn't kicking in fast enough even on
the SLOW setting.

I did not have enough time with the tube in to evaluate anything
resembling weak-signal performance. With that intermod
it would've been hard anyway.

OK, that's enough with messing with Art's tube choices for the day :-).

Tim.