At 12:04 30/10/98 PST, you wrote:
>I think the really ultimate home brew amp would have different PI-tanks
for each
>band arranged around the tube. (If size isn't a BIG matter)
Hello John, Einar, and group,
This method is used in the "Aerocom" 6 channel 1kW SSB/CW HF transmitter.
The exciter drove the 4CX1500B tetrode with 3 watts in a passive grid
circuit with broadband matching network to the grid ... from memory (this
was 15 years ago) it was a simple 9:1 toroid transformer with a 450 ohm
termination/grid damping resistor.
The 6 pii tank circuits were switched by 5kV rated solenoids to the anode
blocking capacitor/anode choke, and regular open frame relays to the
output. In each pii circuit C1 consisted of a 30pfd 5kV variable with "850
series" capacitors to pad for the low frequencies; coil stock of different
diameters/wire sizes for high and low frequencies with taps to select L;
and a bank of "850 series" capacitors selected by links for the correct
value of C2.
The solenoids were remote controlled from the operating position some
distance away by a rotary wafer switch and a multi pair cable ... real
simple and reliable stuff compared to the motor driven vacuum variables and
their control systems in the other multi channel HF transmitters at the
station.
The only drawbacks I can see for this in an amateur situation is the
bandwidth needed to QSY from CW to SSB frequencies in bands like 80/75, 40M
(in the USA), and possibly 20M ... and getting this to work on the high
bands with the stray C and L in the connections to the solenoids.
73 ... Bob, ZL1RS
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