tubes age 39
Found on the Nostalgia Air Technical Discussion Forum: The Stancor P-8130
is rated 12.6 volts @ 2 amps. You can clean up some oxide coated cathodes
by increasing filament voltage. In receiving tubes, by making grid #1 slightly
positive and forcing extra plate current, emission is often increased. I found this
almost always works with some older tubes like 26. Once the tube is forced to
draw excessive current then voltages are brought back to normal, emission
may be normal. The method you mention, only increasing filament voltage
without other elements connected, works well on Thoriated-tungsten filaments.
Norm
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Date: Wed, 16 Feb 2000 12:54:33 -0400
Subject: [R-390] Tube Testing
Just a note from a recent experience. I have a simple Heath TC1 tube tester
that I had been noticing as being too harsh on tubes declaring many as fair or
bad. I had a gut feeling that something was wrong here and a quick check up
confirmed my fears. All the resistors in the TC1 had changed value by a lot!!.
Upon replacement with exact original values the response of the tester
changed dramatically giving a lot of tubes a passing grade. Luckily I never
disposed of any of the previously tested tubes. I give you this info because we
sometimes tend to be more careful about the 15% change in value of a resistor
in the RF deck of a 390 than a 100% change of value in a resistor located in
one of our testing intruments. In fact many of us tend to think that since the
testers are not used so often, everything must be alright. Nothing could be
further from the truth as my experience shows. Check your tubetesters
resistors regardless of manufacturer!!
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Date: Wed, 16 Feb 2000 16:59:02 -0600
Subject: Re: Re (n+1): [R-390] 6DC6 TUBE SUBS + semantic soliloquy
Alignment is needed to compensate for the varying capacitances of tubes as
you change them. Good design would maximize the tuned circuit capacitance
to minimize the change in tuning when tubes are changed. The rule of thumb
(Radiotron Designer's Handbook, section 26.7) suggests 100 pf will minimize
the need for alignment and for the change in alignment with changing AVC
voltage. But in wide range RF tuned circuits sometimes that much C won't
allow tuning to 30 MHz.
I don't think there's any way around realigning a stage when the tube is
changed. Means you may not have to realign the whole radio, but that stage
needs to be peaked, often to learn the effect of the tube change. A tube may
seem to be lower in gain and also quieter when it had a larger change on the
stage alignment which killed gain more than the tube's inherent gain.
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