>>
>>Using a spreadsheet I got from G3SEK. I am using the following values,
>3000V
>>on Anode, max of 1A DC anode current and planning on getting 1500W
>output,
>>using a K factor of 1.5. So the implied efficiency is 50% and a plate
load
>>of 2000 Ohms, all well and good but 50% seems low and I would think I
>would
>>be closer to 60%.
>
To Dave:
The spreadsheet does NOT predict the efficiency the tube. In that part
of the spreadsheet, efficiency is an INPUT value that you provide, and
its accuracy is only as good as your personal guess.
The "K factor" is widely touted as a method of estimating RL, the
optimum load impedance that the Pi- or Pi-L network must offer to the
tube. But the whole method is very dubious because it claims to be
generic... as if all tubes were the same, which they very obviously
aren't!
That is precisely why the spreadsheet recommends that RL is estimated by
drawing load lines on the characteristic curves for the specific tube.
The "K factor" method is a very inferior alternative.
To Bill:
>
>For what it's worth: I have built two amps using the GS35b and had the
>same
>low efficiency you indicate above. Several European hams have reported
>the
>same. It appears that 50% is all the tube is capable of. If you can
tolerate
>that, otherwise it works well and is very inexpensive. Going to a much
more
>expensive tube gains you only about 10%.
>
>As to what causes the low efficiency, I can only guess. I suspect the
grid
>structure does not have complete control over the electron flow and
allows
>some current to "leak" past it. That's just a guess, I am willing to
stand
>corrected if someone has a better answer.
>
>73, Bill W6WRT
There is a major difference between the Western high-mu triodes and the
Russian family of low-mu triodes. High-mu triodes like the 8877 are
designed to deliver good gain and efficiency at relatively low anode
voltages, but the GS35b requires much higher voltages. Performance at
anything below 3.0kV is sure to be disappointing, and the best overall
performance is generally obtained at about 3.5kV on load, rising closer
to 4.0kV off-load. Some specimens will tolerate higher voltages but with
a rapidly increasing risk of arcing.
73 from Ian GM3SEK
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