[R-390] 26Z5W Tubes - Other Devices?

[Jim Whartenby]

Jordan

Thanks for the quick reply.

I checked page 5-50 of the Y2K manual and found no mention of adding voltage dropping resistors when replacing the 26Z5 rectifiers with solid state rectifiers.  I then checked "NAVSHIPS-0967-063-2110-FC6-1966 R-390A Internal Heat Reduction" and it too has no mention of adding voltage dropping resistors.  AFAIK, there is no military sanctioned modification that adds these resistors to any military receiver.

If the purpose of FC6 is to reduce the heat in the R-390/URR by eliminating the heat generated by the 26Z5 rectifiers, then just over 10 watts of heater power is saved by eliminating the tube rectifiers.  Not mentioned is the worst case heat caused by the 26Z5 "internal resistance" of 2.2 watts per tube plate when the B+ current is 100 mA per rectifier plate.  So for the R-390, as a whole, it is just about 19 watts of heat that is eliminated by using solid state rectifiers.

Eliminating the voltage regulator would save three times the above in waste heat.  In fact, the voltage regulator and the rectifiers account for some 40% of the power used by the R-390/URR, not including the oscillator heaters.  Since the B+ transformer winding specification for both the R-390/URR and the R-390A/URR are the same and both receivers have the same B+ current requirements, then eliminating the voltage regulator and using the power supply designed for the R-390A/URR will greatly reduce the operating temperature of the R-390/URR.

Regards,
Jim

fLogic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Friday, April 17, 2026 at 12:34:20 PM CDT, Jordan Arndt <outposter30 at shaw.ca> wrote:   

 Jim and the group...you can find the info in the Power Supply diagram on 
page 5-50 of the Y2K manual....

I also added ceramic caps shunting the 1N5408 diodes to kill any rectifier 
"hash".  don't recall offhand the value, but they're either .1 or .01uF 1kV 
ceramics.

73...Jordan VE6ZT


----- Original Message ----- 
From: "Jim Whartenby via R-390" <r-390 at mailman.qth.net>
To: "R-390 Mailing List" <r-390 at mailman.qth.net>
Sent: Friday, April 17, 2026 10:57 AM
Subject: Re: [R-390] 26Z5W Tubes - Other Devices?


> JordanWhich manual are you referring to?JimLogic: Method used to arrive at 
> the wrong conclusion, with confidence. Murphy
>
>    On Friday, April 17, 2026 at 11:00:00 AM CDT, Jordan Arndt via R-390 
> <r-390 at mailman.qth.net> wrote:
>
> While I do have a couple of spares, I found that a 1N5408 will simply plug
> in to the tube socket very very tightly, so I added the dropping resistor 
> in
> the Audio deck as per the manual...I have one 390A with the diodes and 2
> with the required tubes...
>
> 73...Jordan VE6ZT
>
> ----- Original Message ----- 
> From: "Alex Hopper via R-390" <r-390 at mailman.qth.net>
> To: "R-390 Mailing List" <r-390 at mailman.qth.net>
> Sent: Friday, April 17, 2026 9:16 AM
> Subject: [R-390] 26Z5W Tubes - Other Devices?
>
>
>> Hi All,
>>
>> I am going through an R-390/URR at the moment, and recently experienced a
>> shorted (filament to cathode) 26Z5W rectifier tube in the power supply.
>> Thankfully I had a spare on hand, and was able to find a few more online
>> to
>> keep in my 'stock'.
>>
>> But this led me to wonder - were there any other devices that used the
>> 26Z5W? I know they are a fairly rare/uncommon tube these days (hence the
>> outrageous prices for them), but I have never seen another piece of
>> equipment use the 26Z5W.
>>
>> Just curious.
>>
>> -Alex
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