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

[Jim Whartenby]

Larry, I agree with you, it is a nonissue.
The issue of "cathode poisonings" or "sleeping sickness" affected early tube based computers.  Operating a tube at cutoff for extended periods of time, like in a flip-flop, resulted in reduced plate current when normal conduction is first resumed.  This problem was addressed by the late 1950's by requiring that the cathode nickel sleeve be made of high purity nickel.  It was the trace elements found in the nickel that contributed to the interface resistance issues between the cathode oxide and the nickel sleeve.

All tubes manufactured after 1960 or so, depending on the manufacturer, have incorporated this high purity nickel cathode sleeve material so if there is a problem, perhaps changing the tube to one with a manufacturers date after 1963 or so will eliminate the problem.  If the tube is not replaced, normal plate current should be restored anyway after a few minutes of operation at normal plate currents.  This sleeping sickness is not a permanent condition and does not cause tube failure.

The BFO, 1st Crystal Oscillator and the 100 kc Calibrator do not operate continuously in the R-390 or R-390A.  In fact, it is possible that any one of these three stages may not be used for hundreds of hours at a time but are still available at the flick of a switch.  If Sleeping Sickness was a real problem, then many more complaints would be posted about these stages failing more often due to tube sleeping sickness, don't you think?

On the continuing line voltage discussion:
Preliminary Manual, 1953, for the R-390 states on page 13. paragraph 10 that the PP-621 power supply will operate on either 115 or 230 vac +/- 10%, 48 to 62 cycles.  
TO 32R1-3URR-412 calls out a bit tighter voltage for testing of 115 vac +/- 5% 60 cycles on page 130.  
NAVSHIPS 0967-063-2010, 1970, lists 115 or 230 vac +/- 10%, 48 to 62 cycles on Page 1-4.
All of the other R-390 and R-390A manuals that I looked at go no further then to call out 115vac or 230vac with no AC voltage or line frequency tolerance provided, or at least I didn't easily find it.  <grin>

I suspect that those who use a bucking transformer to lower the nominal line voltage will have tubes and components that may last a bit longer over the operating life of the receiver but I don't know how noticeable this will be.  The reduced voltage will still vary over the present +/- 5% line voltage tolerance unless the VARIAC is automated.  Heat being the enemy of reliable electronics, I would think that better air circulation to remove heat would accomplish the same thing if not more.  This is especially true in the R-390 with it's regulated B+ power supply.

Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy

-----Original Message-----
From: Larry H <larry41gm at gmail.com>
To: R-390 Forum <r-390 at mailman.qth.net>
Sent: Sun, Oct 9, 2022 9:52 am
Subject: [R-390] R-390A Use of Stand By on Function switch

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
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