[AMPS] Tube Life

[Skip S Isaham]

[snip]
I do know that on one 4CX1000A I own, that increasing the heater voltage
by
5% above normal brought it's emission back up to almost spec. It made an
un useable tube quite useable in normal ham service.
Good luck
Larry - W7IUV
[end snip]

Hello again,

In some of the older tube final broadcast transmitters I've serviced, the
typical method of operation is to preheat a new/rebuilt tube for some
hours, Set the heater voltage to the mfgrs value, then operate the tube
for some hours/days. Upon verification of stable operation over a time,
the heater voltage was reduced to the point where power started to drop,
then increased back up just a bit  above the return of normal power
output.
The tube operated over it's normal life at this value which is often
below the normal mfgrs requirement. The figure of 3% reduction comes to
mind for the many 5CX1500a/b 's I've placed in one Sparta 680 through the
years.
(the 5cx1500 can be rebuilt)

As "non rebuild" broadcast tubes reached the end of their normal life, we
would often see the output drop slowly. If we left the tube in operation
and increased the heater voltage to maintain the "normal" power level,
the remaining lifespan would decrease exponentially compared to the
slower drop in power with the heater at the original value.
Often days to weeks were the useful for leaving the heater alone.
A compensated heater/power level would often reduce the tube life to less
than 3 of 4 days, a week max in the mentioned Sparta
Current requirements would also increase as the tube aged.

To sum this up. If your tube has greatly reduced output and is not a
candidate for a rebuild, you can obtain a very short life at a "usable
power output level".  I'm running a "dud" 4CX1000 in my 30S1 in a similar
fashion now.

cheers
skip may  wv6f
nospam4me@juno.com

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