[Amps] TL-922 and overdriving

[Tom Cathey]

Oliver,

You may know the following info already, but just in case...  just setting
the drive power at an arbitrary level is not a good idea for several
reasons...

Lets say the final tubes are getting soft, the power supply sags or you have
a poor match to the antenna. In these cases there is a good chance you could
see premature flat-topping even at 80 watts of drive in extreme cases. But
inversely, lets say you have everything up to snuff - then perhaps even 125W
is a robust level - hard to say.

What I'm getting at is that you need to have a scope sampling RF at all
times looking at your transmitted waveform. A small wire stub inside a 50
ohm connector is all you need for a probe. Be especially focused on the
audio peaks for smooth roundness. The best test for ssb is to put an audio
two-tone into the mic input and adjust the amplifier for maximum output
through loading and tuning adjustments while finding the proper drive level
BEFORE the peaks start to saturate or flat top.
Some suggest to load the amplifier a little heavier after peak power, ie,
decrease/unmesh the loading capacitor value slightly for better linearity.

If you look closely at a simple 1 khz sine wave  audio waveform that goes
directly into a scope, you will develop a good eye for what a clean sine
wave looks like - and be able to see small signs of distortion later when
you look at an RF waveform modulated by a two-tone  sinewave.  Practice this
to train your eye. Notice the fast and straight cross over of a sinewave at
the center line and that the peaks are neither sharp nor flat, but somewhat
rounded.

If you do not have a two tone generator, hook the amplifier up to a dummy
load and play a clear radio with an announcer's voice through the microphone
and look closely at the peaks. You will quickly see how much drive is too
much when the peaks go no higher with increased exciter drive. It is a
juggling act between plate tuning, loading and drive to get the cleanest
looking waveform with highest power peaks on the scope...  It's a visual
thing since the trained eye can see distortion quite well.

Hope this helps, OM.

73,
Tom, K1JJ


> Hello,
> I recently changed my TRX for a TS-950SDX which has 150 W power output.
I'd
> like to know if it is dangerous to drive a TL-922 with 150 W, instead of
the
> recommended 100 W ?
> I ask that because I usually run 150 W and when  I turn on the amplifier,
I
> forgot to reduce power  sometimes...
> Maybe the best solution is to set the TRX max power to 100 W ?
> Regards
> Olivier
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