Tom says;
>By far the biggest stress in amateur service is the
> constant thermal cycling of the filament
That's where the rheostat comes in handy - assuming proper proportions, it
limits surge current.
Of course, so does a suitably designed surge limiting filament transformer, but
that can be a bit more expensive.
>provisions would have to be made for monitoring voltage
>accurately
I assumed external metering, but point taken, especially on cost.
Of course, there's the BBC(?) experience with the 4CX250B, showing that some
filament voltage reduction can help life. GW4FRX has chapter and verse on that,
and, I seem to remember, has published the info here on occasion. Whether, in
amateur service, you'd notice the difference is another matter......
The real cost of a component in a piece of gear can be quite high. I even
convinced purchasing on one occasion that a 22p cermet trim pot was cheaper than
a 3p carbon one, because 15% of the carbon ones broke or otherwise failed in the
test department - and it cost just 8 times as much as the component cost of the
cermet to replace the failed part. Even adjusting a preset in the factory costs
a lot of money.
73
Peter G3RZP
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