tubes age 218
Subject: Re: [R-390] Heater Dropping Resistor R536
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.
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Date: Wed, 29 Apr 2009 16:35:17 -0400
Subject: [R-390] Limiter hum
I've had that problem too. The fault doesn't show up on a tube tester!
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Date: Wed, 29 Apr 2009 13:47:32 -0700 (PDT)
Subject: [R-390] ?A? Tube Filament Voltage
There has been considerable discussion on the list regarding tube shield
efficacy and filament voltage. There has been mention of running the tubes
filament voltages at 95% of the nominal rating of 6.3 volts. Thus 6.3 times .95
equals 5.995 volts. On all six of my R390A power transformers the filament
voltage is stated as 6.1 volts not the 6.3 that at least I had assumed. This
contradicts the Y3K manual chapter 5 values which are given as 6.3 volts. I am
TDY for several months so I haven't been able to make any measurements.
Now comes the question: Is this the actual voltage or a typo? If 6.1 volts is the
actual value then I guess that the Collins engineers realized that there would
be heat issues affecting tube life and used this value to help prolong tube life..
Could some one enlighten the list on the true value?
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Date: Wed, 29 Apr 2009 16:55:28 -0400
Subject: Re: [R-390] Limiter hum
Here's a thought that came to mind. Even if H-K leakage were excellently low, I
could see that some sort of imbalance in the heater wire geometry could
capacitively couple an AC component onto the cathode.
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