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Re: [Amps] tetrode push-pull amp schematics

Subject: Re: [Amps] tetrode push-pull amp schematics
From: "Roger (K8RI)" <>
Date: Sat, 20 Jun 2015 18:55:36 -0400
List-post: <">>
Isn't class A idle current max (tube dissipation) with max signal giving minimum dissipation? (The tube is never driven "into" cut off) Class B is at cutoff while class C is deep into cutoff?.


Roger (K8RI)

On 6/18/2015 7:13 AM, KA4INM wrote:

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.

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