"Long life" 144 MHz, 750Watt PA
MADE BY KOTEL
With GU78B tube....
As our club is very active in VHF/UHF contests there has always been
a need of a reliable high-power PA for 2m band. Although the current
power limit in OK is just 750 W, we decided to construct a brand new PA
with a big margin of power which would last "forever" and can be
possibly used for EME as well.
Since 2 pieces of GU78B were lying for longer time on the shelf in my ham
shack
I chose this tube. It's a tetrode designed for linear PAs with sufficient
power dissipation.
Besides I was able to get a socket for it on flea market at reasonable
price.
Also driving such a tube to just a little part of its potential power
ensures a clean signal
in comparison to smaller tubes driven to their maximum.
This is very important in big VHF contests with high number of stations
in a small area (a very frequent case in OK).
For fundamental construction problems at high power level (especially the
anode isolation
capacity) I droped the usual lambda/4 design and developed a construction
with the
lambda/2 resonator. Of course there's a big minus in its dimensions,
not only due to
total cavity length of almost 1m, but also due to the high voltage (about
105kV) at the end
of the resonator. However, there are some important advantages in this
construction. No
anode isolation construction capacitor, small tuning and coupling
capacities and the last but not least
the long and massive copper pipe improves the temperature stability of the
tube. The higher
intended power the bigger significance of these advantages. From certain
power level the
classic lamda/4 design is not feasible.
PA is not for sale !!!
Technical parameters:
Ua - 3100v - no load.
Ua - 2500v - at full load.
Ug2 - 315 V
Ia - 1.5 A
Ia PEP - 4A
I fil - 3.5 - 4A
U fil - 27V
U fil - EME - 28V
P out - 3000W linear (intended)
P out MAX - 4000W peak maximum to limitation
Voltage regulator for G1-- CZ -- EN
Voltage regulator for G2-- CZ -- EN
http://www5a.biglobe.ne.jp/~jh2clv/ - GU84-6m
Poslední update
:12.4.04 12.50