As a matter of practice, in the interest of protecting a very expensive
tube, in my home brew 2 meter 8877 amp, inrush current limiting is
provided. A correctly designed, not over rated, filament transformer
will do the job nicely. However, if the current rating of the filament
transformer is too high, certainlly inrush current limiting is strongly
advised. (EIMAC makes a point of this.)
Additionally, having the filament float above ground in a cathode driven
amp makes for less filament to cathode signal loss, especially at VHF
and thus required drive is a bit less. In my case, both sides of the
filament are connected to the transformer, isolated with suitable RF
chokes. All of the IR losses must be taken in to account to assure
correct filament voltage at the tube socket.
In my case, there's one switch, ON. All circuits are timed such that
the blower starts first, the air switch closes, the filament primary
voltage is applied and ramped up over 45 secs. (3 x 15 sec steps) after
which HV is applied and it too is stepped up over 1.5 secs. (3 x 0.5 sec
steps) In my experience, manufactures won't go to the expense of this
much circuitry. It's too expensive and the costs must be passed on to
the user.
The result is an amp that is easy to drive, will produce more output
than the legal limit on 2 meters, with tube life in excess of 20,000
hours. That translates into 2.28 years, 24 hours a day, 7 days a
week......key down if I so desire. Can you imagine what tube life in
amateur service could be? Perhaps a human lifetime for one tube.
Thanks to Peter Dahl for transformers that are CCS rated. -:)
73
Bob K4TAX
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