> From: Peter Chadwick <Peter.Chadwick@gpsemi.com>
> To: amps <amps@contesting.com>
> Subject: [AMPS] splatter etc
To: <amps@contesting.com>
> Date: Thu, 12 Jun 97 08:51:00 +0000
> Rich asks why Eimac, Svetlana etc don't suggest regulated supplies.
>
> My guess, from my professional background, (I'm a radio engineer, who
> has moved around through transmitter and receiver design at Marconi,
> Decca, Racal, etc to being a systems and standards (and some
> applications) engineer for an integrated circuit manufacturer) is that
> as regulated supplies cost money, you make sure that the data sheets
> offer the CHEAPEST possible application - thus no regulation!
Questions like that often just turn out to be "busy work" Peter. When
we have no idea what was in the mind of the people putting the
sheets together, we can only guess.
Another guess might be Eimac and others may not specify a regulated
supply simply because they assume design engineers use the sheets,
and those people generally know what they are doing.
Other examples? They don't TELL you to use filtered dc supplies. They
don't say you should not immerse the operating tube in cat whiz
(phew). They don't say not to return the B- to one side of the AC
operated filament of a directly heated tube. The data
sheets also don't say we should NOT use 100 k ohms in series
with the anodes.
They only tell you some of the things you should NOT do that could
damage the tube or make the tube unreliable, like having MORE than a
certain grid return resistance.
> So do any of you have ideas for a session at Dayton 98 on Amplifiers? If
> some of you can volunteer to do say 20 to 30 minute sessions, we can try
> to put together a forum of say 2 hours or so. I don't mind putting the
> thing together and proposing it to the Hamvention organisers, but it
> needs input from you guys PLUS a commitment to turn up. How about it
> guys?
I'd be happy to bring test equipment for you poor long distance
travelers, if you want to measure IMD. Gosh, when we are done
with that we could even measure feedthrough power and show how to
neutralize a grounded grid PA!
Someone asked about why tetrodes aren't used. Rich gave a cost
example. Let me give a technical reason.
Two things are of end importance in the tube. One is the Rp (it is
the NON-dissipative plate resistance, or impedance if you have
problems with the concept of a non-dissipative resistance), the
other is the actual anode to cathode resistance (this is a
traditional dissipative resistance, but the heat is all generated in
the anode by electrons hitting the anode).
Just like in the VR tube question, the slope of the anode resistance
change with anode voltage change determines Rp. Rp is measured as
as the ratio of plate current change to anode voltage as the
anode voltage is varied with constant grid to cathode
voltages. Rp relates to Mu, higher mu tubes have lower Rp. (That's
why a high MU tube is more stable in a regulator, and why we want
VR tubes with low ""Rp""). Like Ian said, Rp= delta Ep/ delta Ip but
in the case a a tube it is with constant grid voltages.
Triodes have higher Rp.
That means the time varying (as we drive the grid) anode resistance
is more stable with a pentode. While IMD can be improved, tuning
becomes more critical with a pentode. It is also more difficult to
obtain high efficiency with a pentode because any slight de-tuning
of the tank hurts efficiency more than it would with a lower Rp tube.
Even if cost isn't a factor, most engineers would pick the
highest Rp tube available (and also the lowest Mu) for an amateur
radio PA that has to be re-tuned frequently, or is likely to be
mis-adjusted. While the pentode might have better IMD performance
when tuned exactly correct, tuning is much more
critical....efficiency falls off quickly, and the tube's currents go
out of whack faster. That's OK in a single frequency BC station since
it sits on one frequency, but not in a band hopping Ham operator's
PA.
So you see even with cost issues aside, there was a very good
TECHNICAL reason why triodes were almost exclusively used in SSB
amateur linears, and lower Rp tetrodes and pentodes were avoided.
73, Tom W8JI
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