Back in the olden days, as a poor college kid building an amp, I checked in
with the local FM broadcast staitions esp those that ran a KW or two on FM
broadcast. I inquired about possible extra PA tube pulls, that might be
donated to a worthy cause. Often the friendly station engineers DID have
some, and donated same to the Amp Project, gratis. 4-400As were the most
common. The design and circuits for a 4-400A grounded grid amp were very
close to most current and older 3-500Z pair designs. Bill Orr, W6SAI, rest
his soul, had a classic Eimac App note, showing a 4-400A design. Recently,
I did a direct swap of an old pair of 4-400As into a Drake L-4B amp, to just
run a test. Worked fine, but had a little less output than the 3-500Z
originals in the Drake.
I don't know if lower output than the 3-500Z is due to tired used 4-400
that has seen a few thousand hours on FM broadcast, or if there is less
gain, inherent in the design of the 4-400 tetrode, used as a GG triode.
In any event, most of the 4-400A pulls that I had were Eimacs, and for
those seeking reilable tubes for a home brew amp consider your local
friendly Broadcast Engineer, who, likely, is also a ham.
With a pair of the old used, 4-400s that were almost expendeble, I ran a
key down test at high power levels, into a dummy load, curious about visual
ques of CCS service, and beyond. The test got the plates to a brilliant
orange color, and they seemed to stay healthy. One time, they demonstrated
the phenomenon where the tantalum plate became transparent, at some
temperature. You could clearly see the spiral, hot glowing filament,
THROUGH THE PLATE. It was interesting, and a little scary. The tube
handled that overload without apparent problems
There will always be folks looking to build on a budget, so if Eimac 4-400
Pulls still sit on the spares shelves of Broadcast Stations, you might
inquire about them for Ham Radio use.
Perhaps in Modern times, though, Spare 4-400As are no longer trivial
decisions to pull, and collect dust on the spare locker....I have a 70s
vintage Eimac price list showing new Eimac 3-500Z for around $70. Those
were the days...
All the Best, 73,
Pat Barthelow aa6eg@hotmail.com
http://www.jamesburgdish.org
Subscribe: http://bambi.net/jamesburg.html
Jamesburg Earth Station Moon Bounce Team
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|