Hi All,
I'm working on a Sunair GSL-1900A amplifier - 4CX1500B, passive
grid AB1, 1.6 to 30mHz auto-tune. Anode 2500v at 0.650 amps,
requires 40 watts drive for 1kW out.
Getting close to a kW into a dummy load (a Heathkit 'Cantenna', so
the tests are brief) on 160 and 80. About 800w on 40, dropping to
600w on 20, 500w on 15 and almost nothing on 10 metres.
In the Pi-L network, C1 and C2 (vacuum variables) are coupled by a
toothed belt - motor driven by a servo amp. The manual describes
C1 and C2 maintaining a capacitance ratio of 2.5:1, L1 has taps
selected by the bandswitch while L2 is 17uH variable to some
minimum inductance - the continuously variable part of L2 is a
'pancake' inductor, with minimal lead length. L2 is motor driven by
another servo amp.
>From Bill Orr's handbook (23rd Edition), the Pi-L table C1 and C2
values show a ratio of between 4.5 and 5:1 for an anode impedance
of 2000 ohms - close enough for 2500v @ 0.65A.
L1 and part of L2 have factory-fixed taps - only the 'pancake section
of L2 is continuously variable. C1 and C2 are variable but their
capacitance ratio is fixed.
Test conditions :-
Amplifier running - no HV. 1900 ohm resistor from anode to ground.
MFJ analyser connected to output side of L2.
C1/C2 and L2 adjusted manually band by band for minimum SWR
reading - all bands less than 1.2:1 except for 10 metres. The highest
frequency that will yield less than 1.5:1 is 25.5 mHz.
Question - Did anyone on the the list ever use the GSL-1900A amp on
10 metres?
Also, I may have missed something along the way, but why do the
amp manufacturers use 2.5:1 ratio when Bill Orr's (Eimac's) tables
show an average of 4.75:1?
Question - Observing the serious decrease in power output with
increase in frequency, should I be looking at the C1/C2 ratio as a
likely problem?
Disregarding the difference in recommended ratios, it's possible that
some previous owner has "played around" with the toothed belt.
The manual calls for a Vector Impedance Meter to set the capacitors -
I don't have access to one. Maybe there's a work-around?
Your advice is welcomed.
73, Ken ZL1AIH
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