At 11:10 AM 9/6/97 -0800, Rich wrote:
>
>
>This is just another case of someone propagating an 'old wives tale',
>Harald. Whenever you use two tubes, the output load R is 1/2 as much as
>it is with one tube, which means that you Need 2x as much C on 10m to
>achieve the recommended Q of 10 to 15. The same thing applies if you use
>three tubes.
If you are going to continue holding yourself out as the final arbiter of
the right and wrong of amplifier design, and deride those with significantly
more experience than you as "recognized experts," at least get your facts
right. Your comments are only true if one is using two tubes to generate
twice the output power at a constant plate voltage eg. 77SX vs 77DX).
1) The design of an output network, be it a simple pi network or a pi-L
is based simply on the tube optimum load impedence which, in turn, is
determined by the anode voltage, anode current, and conduction angle
(class of operation) of the tube(s). Thus, for example, the output network
for 2 X 3-500Z would be identical to that of a single 3-1000Z as long as
the two amplifiers are operating at similar output powers.
2) It is more difficult to achieve the required low "C" for an output
network tuning capacitor with two tubes than with a single tube simply
because one must account for the output capacitance of TWO tubes instead
of one. In the case of 2 X 4CX800A the output capacitance is 22 pF
(11 pF x 2) plus circuit strays where the single 4CX1600B is 12 pF.
This capacitance appears in parallel with the plate tuning capacitor
effectively dominating the minimum circuit capacitance and forcing
HIGHER network Q on 10 meters (often well above 20) contributing to
higher circulating currents, and increased losses in the plate tank.
In the case of this set of tubes, the 4CX1600B operates at a higher
anode voltage (3.3 KV vs 2.1 KV) and lower plate current ... thus a
a higher optimum load impedence, a larger plate tuning capacitance,
and potentially a more efficient output network.
In general, the single tube design is simpler and more forgiving (unless one
is fixated on "tubes with handles" and extra-legal power levels).
73,
... Joe Subich, W8IK ex-AD8I
<w8ik@ibm.net>
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