Tom,
I think that the confusion on 'maximum plate current at zero grid volts'
occurs because the statement should say ' maximum plate current in the
RF cycle flows at at zero grid volts.' The RCA Tube Manual TT5 (1962)
uses the term as Rich put it, and that's what I read it as, but looking
at it again, I can see that it easy to cause confusion. In AB1, where
there isn't any grid current drawn, obviously the grid doesn't go
positive, so the maximum voltage on it is zero. Minimum voltage (which
is actually the maximum negative voltage excursion) is thus twice the
bias.
Rich asked where the curves of IMD against grid volts were in The Care
and Feeding of Power Grid Tubes. My copy is the third printing,, and
it's on page 72. Actually, it graphs the calculated IMD products for the
ratio of RF grid volts to bias volts over a wide range. It assumes a
perfect tube, (which it says) and NO limiting at the output (which it
doesn't say). If the amplifier limits, presumably at its output, the 3rd
order IMD products will be 13dB down on either tone when it is hard
limiting. (See S. F. George and J. W. Wood, Ideal Limiting, Part 1,
Washington D.C.: U. S. Naval Research Laboratory, AD266069, October 2nd,
1961)
The guy who asked about two tone testing didn't get a reply.
Some naive questions:
1. I heard somewhere ( and I can't remember where) that running power
tube heaters (4CX250's etc) on DC leads to failure because of an
electrolytic type mechanism. This sounds unlikely to me: anyone got any
information on this?
2. Has anyone heard of any successful use of Doherty or Terman-Doherty
linear amplifiers on HF SSB?
3. Has anyone used (or heard of the use outside the laboratory) of Polar
Loop techniques for low IMD high efficiency linear amplifiers at HF?
4. Has anyone used the Cartesian feedback technique for an HF linear?
The Cartesian or Polar loop techniques suggest possibilities for high
efficiency solid state linears, at lower cost. It's interesting that you
can get 10KW solid state UHF TV transmitters now at prices which are
claimed to be competitive with klystrons.
For K6GT.
Tom, try10 off 150 watt light bulbs in a series - parallel
arrangement.i.e 5 lots of two bulbs in series all parallelled. That will
look like about 35 ohms, so a simple L network tuner will have a low
enough Q to not be too critical to match up to 50 ohms. The impedance
will vary with power, so be carefull with close spacings of tones for an
IMD test. I don't pretend that it's an ideal approach, but it's better
than nothing.
Alternatively, you could use 14313 - there's so much garbage there no
one would notice the difference!!
73
Peter G3RZP
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