Joe Subich, W4TV wrote:
Hold it down to 600 watts RF output and you can run it all day long.
600W output is close to the safe CW output! Eimac's data sheet gives
600W CW or PEP as the "useful power output" for a single 3-500Z with
2500V on the plate.
Considering that it is the continuous current in the plate transformer
that causes the overheating and both power/heat are current squared
effects, reducing the current by 30% would reduce the heat by 50%.
A 30% reduction in current (to ~280 mA) would be roughly 1/3 reduction
in power out (if the plate tank can be adjusted to maintain efficiency)
or around 400 watts.
Even at 400 W, I would look for ways to add auxiliary cooling.
The SB-1000 has the same problem as the Alpha cooling layout: the
transformer is upstream of the fan, sitting in a smooth (laminar) flow
of air which doesn't provide particularly good cooling. A stagnant layer
of hot air still clings to the surface and acts as a blanket against
removing the heat.
The tubes are much better cooled because they are immediately downstream
of the fan, where the turbulence scrubs away that layer of hot, stagnant
air.
The main purpose of the auxiliary inlet fan in the Alphas is not to
increase the total air flow through the cabinet, but simply to blast
turbulent air directly at the transformer. This improves the cooling of
the transformer dramatically.
If modifying an amplifier in this way, make very sure that the auxiliary
fan doesn't restrict the total flow rate through the cabinet, because
that is still needed to cool the tube. (The total air flow rate can be
measured from the time required to inflate a lightweight plastic bag
clamped over the air outlet. Just make sure it's the same or better than
before, but no worse.)
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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