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Re: [Amps] Why does power output increase?

To: Amps reflector <>
Subject: Re: [Amps] Why does power output increase?
From: Victor Rosenthal 4X6GP <>
Date: Sat, 4 Jun 2022 16:51:26 +0300
List-post: <>
1. I have often wondered whether the filament voltage should be measured under full power or in standby. I operate CW only, so even during a transmission, 50% of the time the voltage is at the key up value. And probably more than 90% of the time the amp is on, I am not transmitting. That means that the filament voltage would be high most of the time.

I guess the question is whether the high filament voltage condition during standby would reduce tube life -- or if slightly low voltage in key-down would be worse. Another interesting question!

2. See my previous message.

Victor, 4X6GP
Rehovot, Israel
CWops #5
Formerly K2VCO
On 04/06/2022 15:23, Shane Youhouse wrote:

1.  You want 5.0 volts under full bore TX, not at receive.

2.  I'd suspect the filament primary resistor.  If it's heating up and dropping resistance, more current will be allowed through.  This will cause the voltage on the filaments to rise su sequently increasing emissions from the tubes.

Is the resistor in the airflow to keep it cool?

--WP2ASS / ex KD6VXI

On Saturday, June 4, 2022, 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.
-- 73,
    Victor, 4X6GP
    Rehovot, Israel
    CWops #5
    Formerly K2VCO <>
    Amps mailing list <>

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