[Amps] How do I determine Class

[Vic Rosenthal]

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