Bill,
Has anyone see or used negative feedback to better obtain lower IMD
in a passive (resistor terminated) input tetrode amplifier?
Any thoughts or references or experience?
I can offer only some thoughts. I have no experience with such negative
feedback in tubes, nor references.
My thoughts are that direct resistive plate-grid feedback should work,
but would incur in high loss. The idea is this: In a normal tetrode
grid-drive scheme, you would resistively load the grid to ground, and
directly apply drive power, maybe through a broadband transformer or an
L or Pi network, if the tube requires more drive voltage than the
exciter provides into 50 ohm. When adding negative feedback, part of the
resistive loading to ground is replaced by "resistive loading to plate".
This provides the negative feedback, but the resistor value must be much
higher than the value of the resistor to ground it replaces. The factor
by which it must be higher is
(grid RF voltage + plate RF voltage) / grid RF voltage
And the power dissipation in that resistor is higher by the same factor.
A typical tetrode might run with a plate RF voltage 20 times as high as
the grid RF voltage. If we would simply replace a 50 ohm grid load
resistor by a 1050 ohm feedback resistor, the amplifier would work, the
input SWR would remain fine, the IMD should improve, but if the grid
load resistor previously dissipated 30W, the feedback resistor will now
dissipate 630W! That's not acceptable.
So this simplistic scheme isn't workable. What we can do, of course, is
raise the value of the feedback resistor, use a series resistor between
the exciter and grid, and add the proper loading resistor at the input,
to provide 50 ohm. This might produce a workable amplifier, but the
higher the resistance values are, the more likely become HF and VHF
self-oscillations due to additional phase shifts between those
resistances and the tube's and stray capacitances.
Many solid state amps solve this problem by using a feedback
transformer. This keeps the feedback and gate circuitry at low
impedance, and also keeps the dissipation in thefeedback resistors low
enough. But the drain of a typical solid state amp carries only 9 to 35V
RMS voltage. Even a drain-to-drain primary winding in a 50V,
kilowatt-class amplifier only sees 70V RMS. It's reasonably easy to make
a clean-performing broadband transformer for that voltage. But tubes
work with roughly 2000V RMS on the plates! I don't see how to make a
broadband transformer for that voltage, that behaves cleanly (negligible
phase shift) over the entire spectrum at which the tubes have gain.
Winding enough turns on a ferrite core to handle 2000V RMS would incur
in too much interturn capacitance and too much wire length. So I don't
think that such transformer-based feedback can be used with tubes.
Maybe one should look into using a capacitive voltage divider for
negative feedback in a tube amp. Basically the tuning cap of the output
Pi would be tapped close to ground. For example by placing a capacitor
in series on the ground side, that has 10 times the capacitance required
in the tuning cap on each band. The grid would be connected to that
point by a low value resistor. Power loss would be modest. But it would
require more complex bandswitching.
In any case, only a relatively small improvement in IMD should be
expected. At most 10dB or so.
There are better options: The gain of a tetrode is easily controllable
via the screen grid. And the IMD produced by tube amps comes mainly from
amplitude distortion, not from phase distortion. So it is possible to
apply linearization by means of two detectors (connected to drive input
and antenna output), an error amplifier, and a high voltage amplifier
that modulates the screen voltage. Such a scheme should be able to
produce a dramatic improvement in IMD, like 40dB or more. It would be
more complex, but would have very little power loss.
Also predistortion can be applied to earlier stages. SDRs like the
Hermes, ANAN, Red Pitaya, with PowerSDR software, can do that easily.
A catch is that external distortion reduction, be it via screen grid
modulation or via predistortion, is effective only over the bandwidth of
the distortion correction system. A screen modulation system can
probably be made to cover several hundred kHz, while PowerSDR at this
time has only about 40kHz correction bandwidth. Outside that bandwidth
the IMD becomes HIGHER than it is without such correction! So, to use
such distortion correction systems, one must start with an amp that is
plenty clean enough by itself at frequencies well away from the
operating frequency. The correction systems only work for the IMD that's
relatively close-in. Plate-to-grid feedback instead works at all
frequencies, but is far less effective.
Enough thoughts for now...
Manfred
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