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Re: [Amps] tetrode push-pull amp schematics

To: <>, "'amps'" <>
Subject: Re: [Amps] tetrode push-pull amp schematics
From: "Gary Schafer" <>
Date: Thu, 18 Jun 2015 17:13:39 -0500
List-post: <">>
Push pull and class B operation are two different things. 
You can not run a lower idle current just because the tubes are in push
pull. You will get the same distortion products as if they were operated
single ended in an RF amplifier.

A push pull amplifier can be run in any class.
A single ended RF amplifier can be run in any class with equal results to a
push pull amplifier as far as distortion products go.

Transistors and tubes are also different regarding operation modes.

There are a couple of reasons that transistors are usually used in push
pull. Those same reasons do not generally apply to tubes.

Operation of tetrodes in push pull does nothing to reduce distortion
products in an RF amplifier (other than 2nd harmonics). 
In an audio amplifier it is a different story as the harmonics are in the
base band that you are interested in. in an Rf amplifier the harmonics are
way out of band.

Driving power does not differ with push pull or single ended circuitry. Gain
is no different between push pull and single ended for the same number of

For your efforts you get a more complicated circuit with push pull tetrodes.
That's about it. But like I said before if you want to do it for the sake of
doing it then go for it!

Gary  K4FMX

> > 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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