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

[Jon Schlegel]

Thanks Tim,

Because of the high value cathode resistor, copious electron emission 
wouldn't be necessary and could tolerate the lower cathode temperature.

Thanks again,
Jon

At 08:59 PM 5/14/2011 -0400, Shoppa, Tim wrote:
> > 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