Yes John, and it turns out to be a fairly simple equation if you make a
few assumptions. Around 1958 or so and a student [sophomore], I spent
the summer with a small crew building KPFK on Mt. Wilson in Los
Angeles. The TX was almost totally homebrew [REL serrasoid exciter],
with 4 4-1000A's in the PA [4 parallel class-C 4-1000A's at 90.7 Mcs was
a bit of a challenge [:-) ]. About to return to school, I was reviewing
the physics I'd supposedly learned the previous year and realized I
should be able to calculate the efficiency of the PA from the smattering
of thermodynamics I knew. Tx engineers don't have much to do until
something breaks.
I knew the pressure difference between the air inlet and the exhaust
[quite small, there was a differential manometer], the filaments
dissipated 600 W, nearly all in heat, and I knew the temp difference
between the gozinta and gozouta air so I could calculate the mass of the
air moving per unit time or so I thought. Then, with the PA on and
stable [3.5 A @ ~5.8 KV], I could do it again only this time calculate
the power lost to heat the air. Subtract the 600 W from the filaments,
and I got 69.4%. I assumed the air was heated at constant pressure, my
calculus hadn't progressed far enough to do otherwise.
After I went back to school, they finally measured the efficiency on a
2-stage oil/water cooled dummy load and called me and said they got
71.3%. If it doesn't go up the 3 1/8" hardline, it comes out as heat
... somewhere. There's only two options.
I'm a little surprised at the obsession with fan noise ... I'd like to
hear about how much more DX everyone is working with a KPA1500.
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
On 6/2/2018 1:06 AM, John Perlick wrote:
Bill Schmidt is right--you have to get rid of a bunch of heat--no matter what
device you are using to amplify (tubes, transistors, or magic). The amplifying
device must dissipate considerable heat because we are running Class AB and
because of all electronic devices are naturally lossy.
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