[Collins] Collins High Power

[Dr. Gerald N. Johnson, electrical engineer]

The 250 KW AM started in about 1962. I started at Collins July 1, 1963
and it was already being designed and I immediately started working on
it. JFK died late in 1963. Radio Australia received the second model,
the 821A-2. VOA received the initial design the 821A-1. I built parts of
them and I shipped them after I put together the installation kits of
all the racks, the modules, and loose parts (like transformers up to 5
tons each), and the necessary wire. VOA stored them so long there was a
concern about aluminum electrolytic self destruction. When I departed
Collins in August 1966, the 821A-2 had a PDP-11 for the computer. But it
was a year from the first assembly.

The RF driver for the 821A-1 was derived in parallel with and by the
same group as the 208U3 linear using many parts in common from cabinet
to servos and power supplies.

The 821A-2 was begun in Dallas in 1965. Fabrication may have been
contracted to Cedar Rapids, certainly I had to buy my panel silk
screening from Cedar Rapids because the Dallas division model shop
couldn't make readable silk screen text from the same artwork that CR
made look like offset press printing.

I've been around 205J parts, we used its power supply to run the first
RF tests of the 821A-1 in Cedar Rapids.

Collins' standard for linears required full output with 3 volts of drive
to a 50 ohm load. The 608 and 606 would do that.

I wasn't aware of a Collins 250 KW linear. The tube line up of the
821A-1 and 821A-2 would have made a good 200 KW linear, but it was
marginal for screen dissipation when trying for 250 KW carrier in class
C and I think wouldn't have had the output for a linear. I started the
design of a single tube 100 KW linear with the 4CV100,000 C building a
mock up of a plate tank and worked at getting it to tune all the way up
to 30 MHz and to still have enough coil OUTSIDE the boiler to be
variable. The capacitance of boiler and tube made that very difficult. I
knew they built a complete prototype but at the time of my last contact
several years later, none had been sold.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.