[Milsurplus] Powering a GP (long)

[Jack Antonio]

Last night, Dennis W7QHO and myself
had a GP-7 to GP-7 QSO, from Reno
to Glendale, about 50 watts AM on 75 meters.
We had good conditions for about a half
hour, then the band shifted, and I talked to
Al W6GER in Oregon for a bit. (This was
on the West Coast Military Collectors net,
3983KC on Saturdays at about 8:30).

The biggest problem with the GP series is
that it runs on 120V 800 cycle power. What follows
is how I powered mine.

Robert Downs mentioned on the list a few months
ago that he had some PE-206 inverters, 28VDC in
at about 50 amps, with an output of 80V at 800
cycles. I obtained one with the idea of coming
up with some form of transformer step up system
to get 120V.

After a couple weeks of failure experimentating with transformers
and 60 cycle variacs, I found that using the 380 to 1000
cycle variac listed on Fair Radios's web site, worked best.

This variac is listed as a 120V unit, but is actually
a 145 volt unit, with a center tap. I fed 80V to the
center tap, and took the output off the wiper. I adjusted
the output for 120V at a 300 watt load, the voltage then
varied from 123 at no load, to 116 at a 550 watt load, falling
off rapidly above that. The inverter itself is rated at 500 watts.
Testing was done with light bulbs.

If you are tempted to repeat my experiments,
please do it safely. During testing, there
are plenty of opportunities for fireworks,
think about what you are doing, keep the
workbench uncluttered. I have no idea of
what an 800 cycle shock feels like, but doubt
it is a pleasant experience.

Acoustically, this inverter is loud. But it really doesn't
scream. There is a high pitch whine to it, but overall it
is about as loud as my yard leaf blower. I am going to have
to build a sound deadening box for the inverter.

After all the above, getting the GP itself running was a bit anti-climactic.
Just the normal cleaning, resistance checks, replace one bad tube and it 
was good to go.

So far I have discovered two problems with this setup. One
is that the GP draws around 700 watts in high power CW, and this
does lug the inverter down. At the half power position, the transmitter
still puts out 80 watts and the load on the inverter is reduced.
On voice, there is no problem at the full power position,
as carrier output is around 50 watts.

The second problem concerns the compensating capacitance switches
on the transmitter. They are to be adjusted for no change in filament
voltage when keying the transmitter, the best I can get is a half volt
drop. I can attribute some of this to the variac voltage dropping, but
not all. I don't know if this is due to the inline variac or not, but
have no way to test this until I get a "real" 120V/800 cycle source.

Just for the record, I have no financial relationships with either
Robert Downs or Fair Radio, except as a satisfied customer.

73

Jack Antonio WA7DIA
scr287 at sbcglobal.net