I wonder how many BC stations up to 5KW or so still use tubes. My 4-1000A
sources around here dried up at least 10 years ago.
Carl
KM1H
----- Original Message -----
From: "Pat Barthelow" <aa6eg@hotmail.com>
To: <amps@contesting.com>
Sent: Wednesday, September 05, 2007 1:06 PM
Subject: [Amps] 4-400As as Subs for 3-500Z
>
> 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
>
>
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