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Some background info - compliled from manufacturers' design guidelines

Posted by mb-de (A) on January 11, 2002 at 12:18:25

In Reply to: Re: Tube lifetime posted by dave slagle on January 10, 2002 at 09:09:14:

Hello Dave,

the issue of over- or underheating indirectly heated small signal and
power valves was discussed ad infinitum in the design guidelines of
the manufacturers.

Some points:

(1) Philips (and other European) SQ valves were warranted to reach
their specified lifetime of > = 10000 hours only if the heater voltage
was kept within +/- 5 % of the nominal value, or - in serial heater
circuits - within +/- 2 % (!) of rated nominal current - for the
latter the inrush current had to be limited, too.

(2) The manuals usually stated that for small signal or small power
valves no inrush current limiter was required, if the voltage at
turn-on did not exceed the nominal value by more than 5 %. For
higher values, inrush current limitation to two times Inom was
mandated.

(3) For many industrial and military SQ valves a warranted acceptance
criteria was defined for the number of sudden power-ups with 20 %
Overvoltage at the heater (7.5V for 6.3V valves) and no current
limiter - one minute on, one minute off. Usually the valve was to
survive several thousand of these cycles...

(4) Telefunken published a study in 1964 in which they had done
long term survival tests of some small signal and power valves for
MIL and for consumer applications - not SQ models.

For these, usually a heater voltage variance of +/-10 % was permitted
in consumer gear - to reach the 1000 h rated lifetime.

Most of the small signal valves benefitted from 5 % under-heating.
The average life was highest with this value. More than 15 % under-
or any overheating killed the valves off quickly.

Small power valves would hold out longest at nominal heater voltage,
or in the bracket +5/-2%. Any further underheating killed the valves
quickly, whereas up to 10 % overheating would just reduce the lifetime
by about 20..50%.

None of the tested valves survived more than 10000 hours at rated
power and environment conditions, even with optimised heater voltage.
Regular power cycles (1 h on/1 h off) had no statistically significant
influence.

(5) Valvo recommended the shutdown of small signal heaters for more
than 20 minutes of idle operation of equipment - to preserve life,
and strongly recommended against standby circuits leaving the catode
idle at full or reduced heater voltage, by switching off B+. Catode
poisoning was cited as risk.

However, the same sources recommend for thungsten or thoriated
thungsten catodes in directly heated transmitting valves a alingment
of the heater voltage to the minimum value necessary to reach the
necessary emmission and gm - to preserve heater life. This was not
true for oxide coated thungsten filaments in directly heated catodes
- for these, nominal +/- 5/10% heater voltage was recommended. All
DH catodes benefit from soft start, according to these documents.

Svetlana make similar statements in their technical notes on power
valves (DH) for transmitters.

The text is a bit long, but I hope it gives some interesting info
in the background...

Regards,

---mb---
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