> ...and the 3 minute time delay on the amp comes in here as
> well - begging
> for the "leave it on" scenario - I have lost opportunities
> due to this wait
> time more often than one would think!
>
> the more general result of this conversation - if there is
> an objective
> result - is to aid others in knowing the answer as well...
> instead of being
> told "don't cycle your amp too much"!
I would leave the amp on.
No matter how you start the filament any thermal cycle
results in filament stress. The stress might change if you
bang a 15 amp filament with 200 amps for a dozen power line
cycles, but make no mistake about it heating red hot and
cooling to room temperature causes things to physically move
no matter how slowly it is done.
This is the BIG difference between tubes used commercially
that run day after day at the same temperature and amateur
tubes that constantly heat and cool. It's why we see all
sorts of failures that totally dominate rather than the
emission just giving out, and this is why it is borderline
nut-case to get all worried about being on the high end of
rated voltage. In amateur service you will almost NEVER find
a tube that has lost emission.
You'll find lots of elements that have shifted in position,
welds that crack or break, gas, melted grids or anodes from
mistuning or excessive long duty drive, and faulty seals but
very few filaments that fail from excessive voltage or
cathodes that run dry from too much temperature.
As a matter of fact looking back at the failures I've seen,
ALL of the indirectly heated tube emission field failures
I've seen came from running low filament voltage. It was
always interesting to listen to the reaction when I called
someone and said "your 8877 is emission-dead and the
filament voltage is just over 4 volts because of the
resistor you added".
One guy I know went through several 3CX1200D7 tubes in his
Henry amp and the only thing unusual was he turned the
filaments off and on several times a day. He would get about
six months out of a tube when the normal 3CX1200D7 would
last 15 years.
Other than that one fellow I never knew anyone who turned an
amp off and on, but there was nothing wrong with the amp
other than open filaments every several months.
Thermal cycling is not good even if it takes 5 minutes to
bring the thing up, although 50 times the rated current for
several power line cycles is probably worse.
If you ask every Ham who has worn a tube out from boiling
off too many electrons to raise their hand, you wouldn't
count many hands in the whole country. If you asked how many
have had elements move out of position or a dozen other
things related to thermal cycling or heat problems unrelated
to constant temperature, almost every person with a failure
would raise their hand. This is why commercial stations
running tubes at steady temperatures have to be very
concerned with filament voltage and reduced emission life
and we do not.
We need to worry about the other ways we hammer the tubes.
73 Tom
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