I will check the resting plate current. My bias circuit has been
modified to cut off the tubes in key up. There is a zener, and there are
some other components that might be changing value, so that's a possibility.
I don't think it's a component in the tank, because a) there are no
padders in the circuit on 20m, and b) the tank is tuned for maximum
output, so a change in value would be likely to reduce the power rather
than increase it.
73,
Victor, 4X6GP
Rehovot, Israel
CWops #5
Formerly K2VCO
https://www.qsl.net/k2vco/
On 04/06/2022 15:03, 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.
73,
Jim W8ZR
Sent from my iPad
On Jun 4, 2022, at 3:35 AM, 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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