Gary Schafer wrote:
Push pull doesn't really buy you anything other than reduced 2nd harmonics,
which isn't a problem as the tank circuit will take care of that for ham
purposes.
It is sometimes used on higher frequencies if the plate capacitance of the
tube is very high and it is a problem getting the tank circuit Q right. Push
pull allows for a lower capacitance plate tuning capacitor.
But on HF you don't have that problem with that tube.
Push pull tubes will not reduce intermodulation products.
You do not want to run a 4CX1500 in anything but AB1 or class A. normal
operation of that tube in AB1 is close to class A anyway.
All you buy with push pull is more complicated circuitry for no practical
reason.
If you just want to be able to say "I did it" then that's another story.
I disagree, it allows you to sit at a very low idling current with
little or no signal. Every 100 Watt Japanese HAM transceiver that I
have looked at except for the latest units with LDMOS final amplifiers
(the ones with a Class "A" button) use a push pull class "B" stage.
You do not need to change the bias or the amount of high Voltage to suit
different modes of use. (modulation) That includes AM.
You also get the power of two devices.
Someone hear-a-bouts said the devices need to idle at about 30%
current, but the transistors in my FT-990 idle at 100 mA with a peak
current of 10 Amperes.
Second harmonic cancellation gets the transceiver manufacturers a low
pass filter that is much smaller on each band and a fuller utilization
of the output devices power capability.
If you use grid driven tetrodes your gain is so high that grid swamping
is normally or frequently used, even then low driving power is used.
At higher frequencies cross neutralization is easy for extreme
stability. If you don't need as much drive power the intermodulation
products from your exciter is reduced.
The only rub is the balanced output tank circuit.
For your effort you get something, if it doesn't suit you don't do it.
Why anyone is happy with a pair of 3-500Z's operating class "A" plate
current beats me.
--
Ron KA4INM - Youvan's corollary:
Every action results in unwanted side effects.
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