Tom W8JI wrote:
<snip>
> Filament or emission life is virtually never an issue for
> amateur service. All the worry comes from commercial
> applications, where the tube runs 24/ 7/ 52 weeks a year.
>
> In amateur service the primary problems are turning the tube
> filament off and on over and over again, extended periods of
> non use allowing gassing, and just plain bad tubes. In
> amateur service we are lucky to see 500-1000 hours a year.
> It'll be long gone for some other reason before the emission
> rolls out.
>
> In commercial service the tube sits and runs never being
> cycled, usually running at a small fraction of ratings. A
> tube often sees over 8000 hours a year and they might try to
> milk it out for a few years, so emission or filament life is
> a concern.
>
> 73 Tom
I agree with everything you've said here Tom but that doesn't dissuade
me from using the switcher on my indirectly heated filament on the
Russian GS-35B triodes. As you stated before, price can be a factor,
but more important to me was maintaining a stable filament voltage on
that indirectly heated cathode with line voltage variations of plus or
minus ten percent. Add to that the soft start feature of the switcher,
the adjustable voltage and the weight to get technical justification.
But, there is also that "I want to do this" factor. One of the joys of
this great hobby is that we can do things to our liking and so long as
it doesn't cause major problems for anyone else it can give us a lot of
satisfaction along the way. I know you have that satisfying feeling with
many of your special projects. I haven't found anything to show this
approach is technically incorrect.
If we wanted to get entirely technical, then we would all run regulated
supplies on indirectly heated cathodes but few of us do. As you said,
the ultimate life expectancy of the tube is unlikely to be determined by
that factor unless the soft start improves the life significantly. With
as little filament heat as comes from these Russian triodes, there's no
reason not to leave it on all the time, or at least all day while in the
shack. It consumes less power than most reading lamps.
One last comment in response to yours about the switcher sustaining the
voltage from a tube arc. This switcher has a small high frequency
transformer isolating the line from the low voltage operating side of
the circuit. My guess (yes, I said guess since I have not tested the
breakdown of that transformer) is that the B- rail clamping diodes will
keep things to a tolerable level when the big bang comes visiting. Good
clamping across the bias circuit and in parallel with the filament
supply could help save the bacon there also.
73, Tony W4ZT
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