[R-390] Heater Dropping Resistor R536

[David Wise]

The limiter is running dim to reduce noise and hum.

Those cathodes are at the input of the audio chain,
and they're floating around at high impedance;
the slightest H-K leakage would cause hum.  In some
positions in old tube-type Tektronix oscilloscopes,
a tube might be selected for low leakage.  The other
day I noticed hum in my R-390A when the limiter
was on.  I replaced the limiter tube and the
hum stopped.  The original tube did not register
any leakage on my Hickok 600A.

Tube audio preamps run dim to reduce noise.  If
the cathode emits enough electrons to satisfy
the maximum requirement of the circuit,
then it's bright enough.  The requirement
here is very small.

Dave Wise

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Jon Schlegel
Sent: Wednesday, April 29, 2009 9:55 AM
To: r-390 at mailman.qth.net
Subject: [R-390] Heater Dropping Resistor R536

Hello All,

I'm working on the IF module from my R-390A and just noticed that 
V507 (Limiter, 5814A) has a 4 ohm resistor (R536) in series with the 
heater.