[AMPS] A "no tune" amplifier output section/operation questio n.

[Geurts, Gerard]

Skip,

There is only one way I know to make a no-tune tube amplifier, and that
is by making a so called distributed amplifier. In this configuration a
large number of tubes, normally tetrodes, are connected across two
transmission lines, each having an impedance higher than 50 ohms. The
capacitive loading of the lines with the tube capacitances reduces the
line impedance to 50 Ohms, and also makes for a low pass filter action
that limits the amplifier on the high frequency side. Each tube is
driven by the voltage on the input line and drives the output line. If
you get the phasing right the signals on the output add up to a large
signal in one direction on the output line and kind of cancel each other
in the other direction (if this attempt to describe the distributed amp
doesn't make sense let me know and I will send you a drawing)  Broadband
amplifiers (10 - 220 MHz, KW output) used in EMC testing are made this
way, typically using something like 16 4cx250's. Before you start
thinking about making one consider the drawbacks: circuit complexity and
low efficiency. I don't think you could design and build one of these
amps without some serious measurement equipment. And by that I don't
mean a GDO.

Broadband matching of a high power tube to 50 ohms is not possible in
the same way as can be done with transistors for two reasons: the big
impedance ratio between the tube output impedance and 50 Ohms and the
large output capacitance of tubes compared to high frequency
transistors. You can imagine (or calculate) that 20 pF across 2500 Ohms
is going to have a much bigger effect that 5 pF across 5 Ohms over the
full frequency range of interest (1.8 - 30 MHz). There is a minimum Q
needed to match between say 2 to 2.5 KOhms and 50 Ohms. This works out
around 8, which also means that for an amplifier tuned at say 10 MHz the
3 dB bandwidth will be 1.25 MHz maximum. This might be no tune, but
definitely isn't broad band.

Now that we have ruled out 'no tune' the only option left is auto tune.
If the load presented to a tube is purely resistive the input and output
signals will either by in phase or 180 degrees out of phase, depending
on the actual circuit used. You could measure the phase difference
between plate and the driving point of the tube (grid, cathode) and use
this to tune the amplifier, either under micro processor control or
using servo's. You will need to measure another parameter to control the
'load' setting of the amplifier. Grid current could be an option here.
One problem here: this only works properly if the tube is operated in
class A.

High power amateur radio 'no tune' amplifiers are only no tune for the
operator, after he/she has set everything up. So, if you change band the
amp will automatically tune to the new band you have selected using
values stored in memory. The Alpha uses conventional networks at the
input and the output. The (air variable) capacitors in the output
network are controlled by electrical motors. When you switch to a band
you have never used before it will use a factory default, probably
established with a perfect 50 Ohms load. If your antenna is different
(as it most likely will be) you will need to hand tune the amplifier
before you can use it on that antenna. If you store the settings the
amplifier will automatically go back to them next time you switch to
that frequency/antenna combination. That's the 'no tune' bit, which is
of course very convenient if you are serious about winning the CQWW
contest in the single operator all band category. An intelligent amp
will be able to interpolate between stored settings for a couple of
frequencies in each band, but everything the amplifier does is based on
what you have stored in its memory yourself. 

Gerard
MoAIU / AA3ES / PA3DQW



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