[R-390] R-390/URR heater circuits.

[Renee K6FSB]

The series resistor is in the 12au7 being used as a diode. It improves 
the diode action by lowering the temperature (emission) of the cathode 
making it more sensitive. method was/is used on Regen and TRF radios . 
similar is in the 390A for the same reason, IIRC 3.9 ohm on the 
appropriate fil portion.
Renée

On 8/10/26 3:16 PM, Jim Whartenby via R-390 wrote:
> The tube heater supply is 25 volts and change in the original R-390.  So there are several series heater strings that add up to 25 volts or so.  Interesting that the two 12AU7s, V507 and V510, have a 120 ohm resistor connected from pin 9 to pin 4 while the other two 12AU7s, V601 and V602, do not have this resistor but they do have a series 27 ohm resistor to drop the heater voltage a tad.
>
> I have checked the tube manuals and did not see any reason given for this 120 ohm resistor.  Does anyone know why Collins added this resistor?  One triode then has about 150 mils flowing through the heater and 100 mils through the other.  Why?
>
> On the same heater string topic, I notice that some heaters in a string are capacitive bypassed to ground while other strings are not.  I have the feeling that the physical size of the capacitor dictated this decision.  I would think that a cap across each heater would be most effective in removing line noise since it would offer the lowest possible impedance to anything much greater then 60 cycles.
>
> Regards,
> Jim
> Logic is a systematic method of coming to the wrong conclusion with confidence.  Murphy
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