[Amps] 3CX1500A7 VS 3CPX1500A7

[Tom W8JI]

> >From what I understand the 3CPX1500 5.5v filament is a 
> >PULSE SPEC.  It is
> the same filament as in an 8877.  The guy emailing REID, 
> as that one too...
>
> It is ok with a directly heated cathode to turn down the 
> filament until a
> noticed drop in emission, then turn it up a click...

You have to be very careful there.

It has to be at the very highest PEAK current at the lowest 
anode voltage. If you set for a power drop at LESS than the 
absolute peak cathode current for any band at the lowest 
anode voltage it will be wrong. The peak cathode current 
varies from band to band in an linear, so it isn't so easy 
to waste your time trying gain nothing in life without 
hurting IMD in amateur applications.

Of course those giving the generally bad advice to reduce 
filaments below minimum ratings in amateur service often 
give incomplete answers since they don't really understand 
the full problem.

A broadcast station is a single-band single-anode-voltage 
single-output-power single-mode situation and easy to do 
right. Reducing voltage even, since the tube is run at a 
fraction of ratings and not cycled off and on, improves life 
in the 24 hour operation. In amateur service you would have 
to measure the highest peak current condition to make an 
adjustment that almost certainly will do nothing to improve 
tube life.

> Not with an indirect full voltage

True. Run them AT the rating range. Nothing less. Just like 
Hams should do for all tubes to be safe.

73 Tom