That's a good take on a likely reason.
A voltmeter across the resistor then monitored, would confirm it,
if VD drops.
vk4tux
On 4/6/22 22:23, Shane Youhouse wrote:
Vic,
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 <k2vco.vic@gmail.com>
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
https://www.qsl.net/k2vco/
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