tubes age 217
Subject: [R-390] Heater Dropping Resistor R536
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. For the
moment assuming that the heater's resistance doesn't change over temp, this
means that V507's heater draw is about 250 mA @ 5.3 VAC instead of the
nominal 300 mA @ 6.3 VAC. Heater power is about 70% of nominal. Anybody
know what's going on here? Isolation from the rest of the heater string? Does
the Limiter depend on soft emission? Don't think we're at a frequency where
cathode back heating is an issue <G>. Also R536 looks to be about a 5 watt
WW and it's dissipating a cool 1/4 watt. Guess they wanted the stability of a
WW.
----------------------------------------------------------------------------------------------------
Date: Wed, 29 Apr 2009 10:19:46 -0700
Subject: Re: [R-390] Heater Dropping Resistor R536
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.
--------------------------------------------------------------------------------------------------------
Date: Wed, 29 Apr 2009 13:51:56 -0400
Subject: Re: [R-390] Heater Dropping Resistor R536
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.
---------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------------------
Date: Wed, 29 Apr 2009 16:14:33 -0400
(14 pages)
(10 pages)
Manymanuals.com
Manymanuals.de
Manymanuals.fr
Manymanuals.it
Manymanuals.pl
Manymanuals.cz
Manymanuals.es
Manymanuals-pt.com
Commentaires sur ces manuels