[R-390] R-390A Use of Stand By on Function switch

[Larry H]

I think the 'Stand By' position on the Function switch has gotten a bad
rap.  Yes, there are some issues with using it, but it's not that bad.  The
problems that I know of are: 1. cathode deterioration on those tubes that
are not powered with B+ (IF deck and some of the RF deck (the
oscillators are normally powered)).  Since the BFO and Cal oscillators are
off most of the time (B+ removed from the screens and plates), the cathode
deterioration issue is really not much of a consideration.  2. Higher
current draw on the 2 audio output tubes, 6AK6's.  In order to assess the
impact on these tubes, we need to look at the increased voltage on them,
it's 17 volts at C606A. That is, of course, if your 390A is working
correctly.  The voltage at C606A only goes up 17 V, from 205 V to 222 V.
The voltage on the 2 output tubes is less than that, so the voltage
increase is also less.  But this will still shorten the tube life only very
slightly.  3. The 0A2 150 V regulator is not affected.  4.The capacitors in
the rx are not going to be affected by the small voltage increase in stand
by because it is still way less than the startup voltage of 242 V.

So yes, don't leave it in stand by for more than a few hours, if you can
help it.  The recommended maximum length of time in stand by is 30 minutes,
but I think that is a little conservative.  I feel comfortable with a
couple of hours a day.

Remember, this is all based on the rx having the correct input voltage, 115
vac.  I feed my 390's and some of my other toys from a large bucking
transformer for this reason.

Regards, Larry