[Amps] 4-400As as Subs for 3-500Z

[Pat Barthelow]

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 at hotmail.com
http://www.jamesburgdish.org
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Jamesburg Earth Station  Moon Bounce Team