[R-390] Heater Dropping Resistor R536

[Tim Shoppa]

Jon writes:
> I powered the unit last 
> night and with no IF input signal (coaxes disconnected), I was 
> hearing hum controllable with the Local Gain control. 
>  I'm now 
> building an extender cable so I can work on the module easily.  You 
> can bet that I'll head for the limiter as a possible source of the 
> hum.  Thanks for the info, Dave.  Never expected this to fall out of 
> a casual question about the heater resistor.

When I've had hum from the IF module, it in fact was caused by
heater-to-cathode leakage in the V507 limiter tube.
This happened to me I was using an Eastern European 12AU7 there.

I blamed the tube, but the limiter design is certainly unusual compared
to most tube usages, because the cathode is at such a high
impedance (470K) to the limiter level control, and it may be more
susceptible to heater-to-cathode leakage as a result.

Maybe there are places where hi-spec 5814A's are desirable and
leakage in lesser grade tubes are more noticeable.

Tim N3QE