[BC] The last of the tube 1000 watt transmitters

[RichardBJohnson at comcast.net]

In that time-frame Eimac just introduced the 4-400. They
had been making 4-250, 4-125 and 4-65 bottles since WWII,
but they decided that the 4-250 base could take a larger
envelope and internal structure. Some hams like Bill Orr
(wrote the care and feeding of power-grid tubes) made
rigs with the new bottle, but no broadcast transmitter
manufacturer would commit to trying them even though Eimac
published both a push-pull and a parallel tube schematic
that seemed to look impressive.

A bit of background… Up until the rules for Type Acceptance
changed, every broadcast equipment manufacturer's complete
schematic, parts list, and test results were available at
the FCC's public reference room. This meant that if a
manufacturer made something that was better or different,
EVERYBODY would be showing a similar design during the next
NAB Convention.

I decided to make a 1kW AM broadcast transmitter to replace
the aging Western Electric transmitter at WDEW. I didn't know
that the rules had been changed so that composite transmitters
could no longer be used in broadcast service. I used the Eimac
design with the help of Bill Orr for the final amplifier, plus
a pair of 4-400s for modulators. He recommended 4-250s, but I
wanted beefy tubes there. I also used a brand new kind of crystal
newly available from Northern Engineering. This was an AT-cut
crystal mounted in a vacuum-tube envelope. These had never been
used before in broadcast service so I got mine free. I also
used a 6146 as a driver for the finals, and 12BY7 tubes for the
crystal oscillator, a buffer, and two Class-A drivers for the
Class AB modulators. The result was an excellent transmitter.
I tried to get it approved for radio station use and the FCC
said NO! I needed to have it Type Accepted and for that,
I needed to be a company!

So, I made a company called Johnson Associates. I named the
transmitter the RBJ/1-C. Professor Don Howe from Worcester
Polytechnic Institute performed the measurements for Type
Acceptance that required equipment and skills that I didn't
possess. The result was a great performing transmitter using
the "latest-and-greatest" vacuum tubes, Type Accepted by the
FCC. Don Howe helped get me into college at a later time.

Strangely, the next year's NAB showed the Collins 20V-3.
They had previously produced a 500 watt transmitter that used
the 4-250 tubes so it wasn't much of a gamble after I had
showed that it could be done. In the same time-frame, RCA
came out with their version. They made the mistake of mounting
the 6146 horizontally and from a PC board no less!

Many years later, while talking to the Collins Chief Engineer
at a NAB Convention, I mentioned that their 20V-3 was practically
a copy of an 18-year-old high-school dropout's design but they
should have gone all the way and gotten rid of the octal
receiving tubes, the obsolete 807, and the crystal oven.
He was not impressed --probably why I never worked for Collins!

The hardest part of the design was the modulation transformer.
It was built by Campbell Electric

 http://www.electrotherapymuseum.com/Campbell.htm

>From my drawings --probably the first and last audio transformer
they ever produced.

I built another RBJ/1-C, serial number 002. It went to WFGM
in Fitchburg, MA. The station was sold before I got paid so my
short-lived company went out of business because I couldn't
afford a lawyer. Their purchase price was $4,000 and my
parts cost was about $2,000. They had made a $500 down-payment
which paid for the magnetics.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: dynotherm at earthlink.net

The Collins 20V3 was basically the 20V2 in a cheaper cabinet. It
used the same 4-400's as the 20V2 etc. Pretty box but IMHO never
lived up to its name tag. I suspect it forced RCA to bring out 
the BTA-1R's when they had a very good BTA-1M triode (833) design.