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

Wed Oct 12 08:53:09 EDT 2022

Jim, Thanks for the detailed explanation about 'cathode poisoning'.  It
makes perfect sense with what I know.  That explains why the early tech
refs before 1960 suggest very short excursions to stand by.

It's interesting about house ac voltages.  I've lived in 5 different areas
in California in the past 45 years and the house power has been quite
stable at a voltage between 119 vac and 127 vac, usually different at each
home, but very stable at that voltage at that home.  Where I'm at now, it
started out at 119 for about 5 years, then a large power transformer blew
and  had to be replaced, and then it was 127 for a short time, but was
changed to 125 (after many complaints).  It's been 125 for about 5 years
now.  What is nice is that the voltage (whatever it is) has not fluctuated
much at all in a 24 hour period, every place I lived.

Now to the 6AK6 audio output tube issue.  It is real, but not as bad as one
might think.  Since it's running at 16 MA (close to it's rated current of
17.5 MA), a slight increase in voltage will increase it's current flow.
So,  in stand by, the voltage increases about 8.2 % and the current will do
about the same, or 17.8 MA.  This is not a significant amount above it's
recommended value, so is not a problem.  This is confirmed by the tech
manuals written after 1960 by not placing a limit on the amount of time in
Stand By.  The 1961 Army tech ref does not mention using the Stand By
position.  Both the 1970 and 1985 Navy tech refs state:

2. 3. 2. 9 Operator's Turn-Off Procedure.
1. When the receiver is not to be used but is to be maintained in a state
of readiness, turn the FUNCTION switch to STANDBY.

I believe that this totally vindicates the R-390A from not using 'Stand By'.

Regards, Larry

On Sun, Oct 9, 2022 at 12:47 PM Jim Whartenby via R-390 <
r-390 at mailman.qth.net> wrote:

> 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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