tubes age 7
problem seen in most supplies; inrush current. Cold filaments exhibit a much
lower resistance than hot ones; this contributes to the turnon "thump" some
amps have. A conventional filament supply; a secondary winding, is at least
limited by the resistance of the winding. A regulated supply (voltage) wants to
see constant voltage accross its output, so some regulated supplies,
especially if stiff (Zout is low), are probably worse than unregulated in the long
run.
Some amps have a "softstart" feature which is a resistance which limits turn on
currents either in primary or secondary for several seconds. Filaments are
usually warm enough after say 10 seconds to take full voltage with no negative
effect. Clearly, overheated bases and pins, inadequate ventilation/cooling, and
parasitics or mistuning don't help anything either. I am sometimes surprised by
even experienced ops trying to max out a PA. The on air difference between say
1200W and 1.5 KW is nothing, Nada, roomheat. Yet backing off just a little
increases life disproportionately. If you are running tetrodes, regulation of
screen current is vital. Great article in recent QEX on same. For RX or small sig
tubes, filament probably not as important, but you still don't want to overvolt.
Check at pins with a meter; you might be surprised. Substitution of solid state
rectifiers in BA will often result in B+ rising more than is good. Hollow state
restifiers have drop. Also, a point often overlooked is that a solid state rectifier
B+ comes up essentially instantly when rig is turned on. Tube rectifiers don't
deliver much B+ until their filaments heat up and guess what, they heat up at
rate similar to all other tubes in rig. I have heard that B+ applied to a tube with
cold filaments can cause damage. Probably a real concern with TX tubes, but
probably doesn't help RX either.
When I turn on a tube radio that hasn't been used in say 4 months, I run it from
a Variac on mains. Start off at maybe 15 volts and go to full 110VAC over maybe
15 seconds. This is nice for filaments and good for PS filters too. If no smoke
after a couple minutes< I shutoff and plug into the main. Easier on everything.
---------------------------------------------------------------------------------------------------------
From: Jim Haynes <haynes@...>
Date: Sat Dec 6, 1997 2:18 pm
Subject: [R-390] Re: tube failure
I don't claim any special expertise on this, but
We know that when the heater power is turned on the heater resistance is
lower than it will be when the heater is hot. So the initial current is high; plus
there's the mechanical strain from temperature cycling the heater and cathode;
plus maybe there's some magnetic effect proportional to heater current. So
frequently turning on and off is bad, turning on with current limiting is good,
leaving turned on all the time is good up to a point.
(14 pages)
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