[R-390] R-390A Heater Dropping Resistor

[Shoppa, Tim]

> I'm working on my R-390A IF module and was reminded of the heater
> dropping resistor R 536 feeding V507 (Limiter).  Looks like this
> drops the heater voltage to V507 by about 1.2 volts.  What's the
> purpose for the dropping resistor?

The limiter is an "oddball" stage in the radio because its cathode is operated at a high impedance (390K) and a voltage not as close to ground as the other stages. Contrast with every other stage where the cathode resistor is 1K ohm ballpark or lower. There's a good reason for V507 to be configured this way so that it does operate as a limiter :-).

In this situation a concern is to reduce heater-to-cathode leakage, and operating the heater at a lower voltage helps here.

Despite this amelioration, it is in my experience not rare for V507 to show leakage/hum and require replacement. (Or more realistically just swapping to any other 5814A socket that isn't running the cathode at such a high impedance.)

I'm not saying that V507 is in any way a magic stage requiring some super-hi-spec specimen in its socket. Just that tubes with some cathode-heater leakage will work well in nearly every other socket, but not in V507 :-)

Tim N3QE