Date: Sat, 2 Jun 2018 14:31:47 -0500
From: "Jim W7RY" <jimw7ry@gmail.com>
To: "Roger Graves" <ve7vv@shaw.ca>, "Manfred Mornhinweg"
<manfred@ludens.cl>
Cc: <amps@contesting.com>
Subject: Re: [Amps] new 2200m/630m amplifier - distorted waveform
output
"the heatsink is cooler running at 200W output than at 125W output"
<This is typical on both VHF and HF amplifiers. Trying to turn down a
commercial VHF or UHF repeater amplifier (or mobile radio pressed into
repeater service) down to keep it cooler doesn't usually work. It just gets
hotter at low power. Karl, AK2O, has had success reducing the DC voltage
from nominal 13.8 volts down to 10 volts. The PA is much more efficient. A
100 watt PA can easily be run at 90 watts without over heating when run at
10 volts.
Same for an HF solid state PA. Much less efficient at lower power that it
was designed for.
Thanks
73
Jim W7RY
## agreed, I get similar results on all 4 of my HF xcvrs. 50% eff
when run at 200 watts out.
But only 35% eff, when run at 100 watts out. PO drops in half, but current
only drops a little bit.
Diss with 100 watts out and 35 % eff = 185 watts.
Diss with 200 watts out and 50% eff = 200 watts.
So it does indeed run a bit.... cooler, when run at half power, but only by a
miniscule amount.
Your Z changed when running lower power, so eff goes to hell..fast. In other
cases, like what W7RY
noted above, diss gets worse.
## The fix of course is to reduce the voltage. Notice on all the old tube
amps that came with a high + low
B+ position, that the higher V position was always 41% higher. Double the
power, the ratio is 2.
Sq root of the ratio 2, is of course 1.414
## If ur supply voltage is say 13.8 vdc, then use 13.8 X .707 = 9.76
vdc Use a similar calc for
any other supply vdc. Eff will improve, BUT the imd wont improve with the
lower voltage
## On another note, Roger said he used a T type LPF. A L-C-L..with the C
being shunt, will work
as a LPF..barely. I have given up on L-C-L LP matching networks. A PI,
with the normal C-L-C..and both
caps being in shunt, works good.
## still on topic, somebody e mailed me months ago about designing a GG
triode amp for use on 475 khz. Everything
in the final design is quadruple what it is on 1900 khz ! Doable on 475
khz..with a lot of effort on my part.
I would not consider a GG triode or any other tube config...on 137 khz.
Jim VE7RF
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