Hi guys,
I think Rich is rather extending my comments on grid current. I do not
say that more grid current is better, merely that, IN A SUITABLY
DESIGNED SYSTEM, grid current to certain amounts is not necessarily
disastrous, and it can improve distortion at the maximum output by
cancelling distortion introduced by the plate characteristic.
As far as triodes in AB2 linear grounded cathode, the CAT12 in the
Marconi SWB8 is an example - albeit from the depths of history.
For measurement purposes, ITU rec. SM328 says that you measure IMD
relative to one tone of a two tone signal. For straight voice, as
opposed to anything using privacy or scramblers, they recommend 3rd
order IMD to be -25dB: for ISB, MCVFT, or scrambled speech, -35dB.
Although Tom's method is valuable for evaluating the linear on its own,
there is something to be said for evaluating the whole system by using
the exciter with two audio tones. Those of us too mean to buy new rigs
(i.e. those with old tube PA's and thus clean exciters!) don't get as
much of the problem of the signal to garbage ratio getting worse as the
level is run down - the FT102 is very good in this respect - can get a
good look at the whole system performance. Alternatively, you can do the
test with white noise - very interesting. I've also seen tests called
for with a tone spacing of 25Hz - that one really looks at the
performance of the power supplies. Where a tuned choke is used in the
power supply, the use of a tone spacing equal to the reonant frequency
of the choke can be interesting.
The comments about 'never' are apt - there are a few things you will
never get - like power gain out of a tube with no filament, bias screen
or HV supplies, but otherwise, remember the Shakesperian quotation
'There are more things in heaven and earth, Horatio, than are dreamt of
in your philosophy'.
141 spectrum analysers are around. The price over here is dropping to
around $750. You can't get the extension cables for love or money, and
the ones for splitting the RF and IF units are even rarer. I believe
that the particular variety of plug isn't made any more - I bough two
scrap units at Dayton to raid for the plugs and sockets. I've also got
to make the extender cards for the units.
73
G3RZP
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