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Re: [Amps] Why does power output increase?

Subject: Re: [Amps] Why does power output increase?
From: Jim W7RY <>
Date: Sat, 4 Jun 2022 22:23:09 -0500
List-post: <>
No fixed loading caps in a TL-922 OUTPUT on 20 meters... The tuned input is another matter though..

73, Jim W7RY

On 6/4/2022 7:03 AM, MU 4CX250B wrote:
Victor, do you notice any change in the resting plate current? I don’t
recall the circuitry of the TL922, but it’s possible the operating
bias point has changed if there is a zener diode in the cathode bias
circuit. Another possibility is heating of a fixed capacitor in the L2
part of the tank circuit, causing the loading to change. I’ve seen
that happen on the lower frequency bands in one of my homebrew
amplifiers which uses fixed caps in parallel with the variable load
cap. THese are switched into the circuit on 80 and 160m. I understand
that it’s not a problem, but it’s definitely unusual (and therefore
interesting) behavior.
Jim W8ZR

Sent from my iPad

On Jun 4, 2022, at 3:35 AM, Victor Rosenthal 4X6GP <> wrote:

This isn't a problem, just a phenomenon that I would like to understand.
I have a TL922. Mods include directly grounded grids. I also measured the 
filament voltage with my Fluke true RMS meter and put a small resistor in the 
primary of the filament transformer to provide exactly 5.0v in standby.

It has been running with the same genuine Eimac 3-500Zs in it for about 6 years.

So here is what happens: after a longish CW transmission, the power output 
INCREASES. On 20m, for example, it may go from 1200 to 1400 watts. When it 
cools, the power drops back.

I've noticed this with several wattmeters. I have observed no change in SWR 
when this happens (it's 1:1 thanks to a Johnson KW Matchbox).

It happens when the amp is driven with an Elecraft K3 and also a TS890s.

I'm interested in hearing theories about why this might occur.
Victor, 4X6GP
Rehovot, Israel
CWops #5
Formerly K2VCO
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