[R-390] Conversion of R-390 IF Deck

[Jim Whartenby]

I count seven 12AU7 tubes in the R-390/URR: V507, V510, V511, V601, V602, V901 and V902.  Only V511 is in series with two 6.3 volt heaters, all of the others are in series with another 12AU7.  So 5814s could easily substitute for all of the 12AU7s except for perhaps V511.  

I have not tested the voltage seen on V511's heater with a 5814 in the socket so I do not know how much this will upset this heater string.  But adding a 240 ohm, 1/4 watt resistor across the heater of the 6.3 volt tubes would allow a 5814 to sub for V511 there as well.  But this might cause other problems down the line.

A lot is made about cathode poisoning but I think what is really meant is cathode sleeping sickness which is caused by an interface resistance between the cathode sleave and the cathode coating.  That is, as far as I know, only a problem with vacuum tube digital timing circuits when a tube is held in cutoff for long periods of time.  

The crystal calibrator in the R-390 is operated for long periods with no B+ applied.  This should be the same as operating the tubes with a high negative grid bias.  In either case, no plate current is flowing.  I am not aware of any reported problems with the R-390/URR crystal calibrator not working when it is needed.  

If the tubes in the crystal calibrator start conduction in a microsecond or in several milliseconds, I doubt that anyone would notice any difference.  Do the tubes in the calibrator fail more often then other tubes in the R-390/URR?  If not, then cathode poisoning is a non issue.
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Monday, January 27, 2025 at 02:03:35 PM CST, Jacques Fortin <jacques.f at videotron.ca> wrote:   

 Hi Byron,
All depends from which R-390A transformer winding you plan to power the filaments.
The R-390 IF deck (and all the other decks, btw) are powered from the 25.2V winding in the R-390.
But in the R-390A, almost all filaments, except the PTO-BFO sting and the 26Z5 rectifiers, are powered from the 6.3V winding.
Maybe it will be preferable but more job to do,- unless you want the R-390 IF deck being able to go back in a R-390 frame again -, to modify it for the 6.3V.

For sure the 5814A should not be used in a R-390 because of their different filament current requirement (175mA @ 12,6V instead of 150mA for the 12AU7).
But they are the original tube type used in the R-390A.
They will be OK to use only if your R-390 IF deck is converted to 6.3V filaments power as a whole.

On another note, the 12AU7 and 5814A can be replaced by 5963s, which is a "computer" version of the 12AU7 made to better resist cathode poisoning than the standard 12AU7.
The more rare 7316 can be used also.

73, Jacques, VE2JFE in Montreal

-----Message d'origine-----
De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la part de Byron Tatum
Envoyé : 27 janvier 2025 14:08
À : R-390 Mailing List <r-390 at mailman.qth.net>
Objet : [R-390] Conversion of R-390 IF Deck

A quick question regarding the filament string re-wiring in R-390 IF deck to allow use in an R-390A. I am assuming 12AU7A tubes should be used (as marked on IF deck chassis) due to their slightly lower filament current draw. Are 5814A tubes to be avoided in regards to this string re-wiring?Byron W5FH ______________________________________________________________
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