Angel Vilaseca wrote:
> In my amp, the ten 4CX250Bs are wired in pairs, the plates of each
> pair are linked by thick ( 3 mm dia) wire inductors, with inner dia.
> about 10 mm. The inductors have a different number ( 3 to 5 ) of
> windings when going from one end of the amp to the other. Which end
> should be connected to the RF output and which one to the loading
> resistor? Also, do the small inductances mean that the amp was
> intended for above 30 MHz operation?
I think it doesn't matter which end is the output, and which end is the
resistor - 50% of the power goes into each one. The inductances between the
anodes work with the anode capacitance to work like a transmission line
(L-C-L-C-L-C and so on). The value of the inductor is set by the capacitance
and the Zo of the line.
>
> Of course, wasting half of the output power in a resistor is not very
> satisfying. What if I replaced all of the inductors with a wide copper
> strip and used the amp as a 10x parallel 4CX250B, with a classical
> tuned switchable multiband input and output circuits?
>
> Instead of wiring the output (and input) at the original end of the
> row of tube pairs, I could re-wire it halfway between. The pairs ot
> tubes furthest from the output would then be about 20 cm away from
> it. That is 2% of the wavelength on 10 meters. Would this cause
> problems, or would it be beneficial, like making the amp more
> broadband?
This might work, but you have to remember that the 4CX250 has plenty of gain
to 500MHz and beyond so even thick copper straps become significant
inductors in both the anode and grid circuits and there's a high chance of
the whole thing turning into a V/UHF oscillator.
Steve
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