Jim, it is not that simple. The there is about 9000vdc and normal helix
current is less than 5mA.
Right now there is a 12.5K ohm glitch resistor to limit any fault current
but that is not enough to fully protect the tube in case of a flashover.
More R could be added but then the voltage drop during normal operation
becomes too much and changes in the helix current screws up the already
marginal
voltage regulation.
Reducing the amount of filter C would reduce the stored energy but at the
expense of higher ripple voltage. I need to spend some time experimenting
with the filter C value to see just how low it can be for acceptable 120 Hz
hum on the carrier.
The thought of less hum level because of linear operation class was new to
me. The filter C design was based on the tube spec showing .08 dB per volt
change in gain. In that scenario 10v of ripple would cause a gain change of
.8 dB. .8 dB is a power difference of 20% which would be very noticeable
if it was present in the form of hum modulation.
The tube runs class A and is fairly linear but not perfect by any means. It
is run at maximum power output, around 300 watts at 10.4 GHz . Running the
tube this way is considered to be in power saturation. That does no damage
to the tube but is not that great for linearity. We mostly use cw or one
of the digital modes so linearity is not an issue.
A suggestion was made to use three phase 400Hz voltage derived from three
audio amplifiers to reduce the needed C and that would certainly work..
9000v at 5mA is only 45 watts Before taking on a major rework like that I
will
do some homework with existing power supply.
I thought this was all figured out. Wrong!
73,
Gerald K5GW
In a message dated 10/5/2013 4:12:17 A.M. Central Daylight Time,
jim.thom@telus.net writes:
Date: Fri, 4 Oct 2013 08:26:13 -0400 (EDT)
From: TexasRF@aol.com
To: ww1c@outlook.com, garyschafer@comcast.net, amps@contesting.com
Subject: Re: [Amps] Plate modulation from power supply ripple?
My current interest is mainly in microwave eme operation. Traveling wave
tubes are used to generate power. The homebrew power supplies have a lot
of
filter C in the helix supply to reduce hum on the transmitted signal.
After
this discussion I am wondering if there may be more C than necessary.
Normally it would not matter but if there is a flashover in the twt the
helix
structure could be damaged. There are trip circuits to shut the power
supply
down but most of the stored energy would be dissipated in the helix.
73,
Gerald K5GW
## How much B+ ?? How much plate current ?? Just put a 50 ohm
glitch R
in series with the B+.... and wire a fast hv fuse just in front of the 50
ohm glitch.
Anything arcs, the glitch limits the fault current to a safer value. The
HV fuse interupts
the fault current..and will easily do that in less than 2 msecs. But
you have to size the
fusing wire correctly. Add the glitch + HV fuse on top of your
existing protection.
Jim VE7RF
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