[AMPS] RF amplifier test

[John Lyles]

Hi,

I've been kind of busy turning on the 200 KW  peak push-pull tetrode
amplifier at 2.8 MHz. As project leader, there are numerous tasks beside
just pressing the HV on button. Such as writing a detailed procedure with
photos of how to short out the plates before reaching in, and a test
procedure before just turning it on. All part of the beauracracy around
safety documentation working for US Govt projects. Since  a worker almost
got killed with a microwave oven HV supply a few years ago, nothing gets
done without a lot of paper to make sure you understand the hazards.

Now the good news:
The new stage came on with the first application of HV. We were up to
nearly rated peak power, at 4-8 pulses a second. This was last week. The
amp is class A, so we can remove drive and just pulse the plate current and
learn a lot. Then turn RF drive on, and swing the plate. The plate currents
are 35 and 40 Amperes peak, when the pulse is on. We pulse the control grid
bias from -800 (cutoff) to -130 for conduction. Screen voltage for both
tubes independently supplied at about 1.5 KV DC.

We started getting arcing, and it was DC related, no parasitics found on
the scopes or analyzer. But that is hard to find in a pulsed amp. On a
hunch, we guessed that the Thomson tube socket had the spark gaps set too
close, and we were getting screen to ground arcs which were tripping the
plate crowbar. Sure enough, the gaps (built into the sockets that we
bought) arced at 1800 with a hi potter, and we were running around 1500.
The gaps were marked in French that they were set at 2450, but we are at
7000 feet above sea level, and the factory is much lower. Sure enough that
was the problem.

Once this was fixed, the amplifier was turned up, and we found some
interesting things. For instance, the two tetrodes are not matched for DC
or RF gain. But we have enough lattitude to compensate. Setting the plate
current identically, using the screen voltage, results in greatly differing
RF output. So a compromise was found, with one screen at 1300 and 1600 on
#2. With that, the RF was close on each side. The individual tetrode
outputs are not combined into a transformer or load, but are driving
individual Pi networks, which feed individual 500 Ohm dummy loads. The
ability to adjust the Pi capacitors allowed me to tweak the phase to
exactly 180 degrees between tubes, and peak the output (no plate dip in
class A). Also, the RF output could be fine tuned better than just using
screen voltage (coarse way).

Linearity - We did not perform an absolute linearity test,right now just
ramped the input drive and overlaid the input/output curves on a good
scope.

Harmonics - Only second was measured from each output, about -50 dbc. Pi
network doing fine. Other harmonics below -80 dbc.

Stability - With plate current at 40 Amperes pulsed, varied the input
tuning, loading, and the pi networks, about 10 turns on the vacuum caps. No
RF drive present. No breakup, just sits there. No parasitics have been
found with this amplifier so far. We had spent about a week building a
parasitic suppressor device, which we were going to install if the
amplifier were to jump to 40 MHz, where the dominant parasitic resonance
was seen on a  network analyzer. We have not needed this device, and will
put it into the drawer for that rainy day...

The gain of the amplifier is high, power gain is 1176. Sorry, I am on the
road right now, and don't have a calculator with LOG on it. About 85 watts
in, 100 KW out! I need to check the calibration of the input wattmeter, on
the RF Power Labs amplifier. Even if it were off by 2 or 3, it is still a
very high gain class A amplifier.

RF leakage nearly zero, a little Xrays from the center of the plate, and
cooling appears adequate, with 13 GPM of water per tube, and air on the
sockets. The tetrodes required about 3 days of occasionally internal
arcing, as we raised the duty factor of the RF and bias pulser. Finally the
arcs cleared as the outgassing was minimized, and the thing runs at our
full DF and peak now without trips.

Thats all I have to report. Sometime in the future I will have some digital
photos that I can share, but right now, gotta finish testing it and get
online for operations.

John
K5PRO










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