Jim Isbell wrote:
On pages 202-205 of the 1956 "Radio Amateurs Handbook" (ARRL) there is
an amplifier described called "Parallel Tetrodes in a High Power
Amplifier" How do I, from that description and schematics etc.,
determine in what class the amplifier is operating? And what is a
National MB-40-L? I assume its no longer available but also assume that
it can be reproduced. This looks like just what I need for a linear to
go with my Viking Ranger to add some boost to the signal when needed.
AND still be contemporary period.
The class of operation is determined by the operating conditions
(element voltages and drive). A pair of 4-250As, grid driven, with 3KV
on the plate would require about -180v bias, 500v on the screen, and
5.2w of drive to produce about 1600w output in class-C. Plate current
about 790 ma and efficiency of 77%. Of course, this is for CW only. If
you want to build a big plate modulator, you could operate them at AM
phone ratings of 3KV/450 ma, 400v screen, biased at -310v with 6.4 watts
drive giving you 1020w carrier output (illegal today).
For linear operation, you could run them grid-driven in in class AB1, at
4KV with -100v on the grid, and 510 on the screen. ZSAC would be about
80 ma., and you could get about 900w PEP.
An MB40L is a small 'multiband tank' used for a grid tank in this
amplifier. It consists of two ganged parallel-tuned circuits, one
covering 3.5-7.3 MHz and one covering 14-29.7 MHz. You just turn one
knob and the entire (1956) spectrum is taken care of (the two circuits
are staggered so that you don't tune to a frequency and harmonic at the
same spot)! The Q varies widely, but for a the grid tank of a class-C
amplifier this doesn't matter much. You can find these on eBay from
time to time.
I don't have a 1956 handbook, but suggest you think, er, more than twice
before building something like this!
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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