[Amps] General question on filament life

[Tom W8JI]

> ...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