[R-390] Heater Dropping Resistor R536

[Jon Schlegel]

Dave,

Interesting.  I now see the 470k resistor feeding the cathodes of the 
Limiter tube.  That's pretty daunting.

I've just started to bring this radio up to speed.  I've recapped, 
re-tubed and re-mech-filtered the IF module.  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.

Best regards,
Jon


At 10:19 AM 4/29/2009 -0700, you wrote:
>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.
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