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Re: [TCML] Taylor TUBES
Ed Phillips wrote:
Comments at end.
jiml
The LANL report "High Power Microwave Tube Transmitters" (or
similar..) from William North (available in various places on the web)
covers all this stuff in detail. (FWIW, North just recently died,
presumably not from HV, although I didn't ask)
Purpose of the note was to say let the buyer beware! The guy who
wrote that particular note is in the business of "audio equipment" and
tubes therefor [in other words, high power receiving tubes] which most
definitely are not expected to arc over or short under any circumstances
although equipment using them is usually fused in the input line to
protect against catastrophic failures.
Context is everything, I suppose.
The same can be said of
relatively low power transmitting tubes of the 833 class [< 5 kV plate
voltage, < 1 kW plate dissipation]. Such failures would not be expected
and equipment designed using them wouldn't have any stored energy
constraints. I'm not aware of any higher power tube transmitters which
have power supplies deliberately designed to protect against arcing but
there may well be some.
Funny that should come up. Sometimes, the protection came as a
byproduct of standard designs in days gone by. Consider something like
a linear amp for hams. The power supply likely used a thermionic
rectifier and used a choke/cap type filter and had terrible regulation
(by modern standards) because of the series resistance, but that same
terrible regulation has the side effect of limiting the fault current in
a flashover (either internal or external: the spider crawling in).
Today, though, with semiconductor HV rectifiers, and cheap capacitors,
someone might "improve" the design to stiffen up the regulation and
reduce some of the losses in that choke, etc. Maybe even use a FET
series pass regulator that's floated (after all the regulator only has
to deal with a few tens of volts of ripple/sag). Now when the flashover
or fault occurs, you've got a lot more stored energy, a lower impedance
path to the energy source, and maybe a series regulator that can't take
the full supply voltage across the pass device so it fails shorted.
The guy back in the 60s probably never really thought about all the
inherent protection in the circuit... just designing with rules of thumb
and following what folks had done before (the same way we design today,
really), and doing some typical analyses to get it right.
Several members of the TCA group from which this note came are
ex-Eimac and they have often described the great lengths to which Eimac
went to pre-condition high voltage tubes to prevent sparking and arcs at
voltages well above ratings. Shorts are never expected.
Not in the tube, but in the circuit around it, I think they're something
that, except in a sort of experimental or cost sensitive (e.g. hacking)
environment, one might casually expect. Perhaps not to the point of
formally addressing it in a design, but just "not do something stupid".
Even the ARRL handbook (which is sort of the epitome of cost sensitive
hacking design: the unspoken assumption is that if you built it, you can
probably fix it) talks about the value of a resistor in series with the
B+ supply in case of shorts downstream.
I am very
familiar with the design of high power MICROWAVE transmitters for radar
purposes. Cathode or anode pulsed transmitters are inherently protected
against arcing. However, with the advent of high power gridded TWT and
Klystron transmitters which may operate continuously with applied HV
matters become entirely different and such protection is mandatory. A
rule of thumb we used at Hughes [in connection with X-Band radar
transmitters] was the "aluminum foil test". The supplies were designed
to dump [with crobar switch] HV fast enough in the event of a short that
if the output was shorted to a sheet of kitchen foil the size of the
resulting hole was very tiny.
Indeed. And now, with switching supplies, you can get HV with small
stored energy, and it's almost self protecting.The primary side can shut
down instantly, and there's just not much stored energy... if the PWM
rate is 50kHz, you only need enough to supply the output power for
50-100 microseconds or so. Even at a kilowatt, that's 0.1 Joule. Hardly
even exciting if you short it.
With VTTC's, particularly those built by enthusiasts without a lot of
tube experience, failures due to circuit design can be expected and the
consequences are merely monetary if they do get poor tubes. The general
reputation of cheapie Chinese transmitting tubes is very poor with
respect to gassing and low emission and I was trying to suggest that
guys look for something better if they can find it.
I agree.. and because of the generally terrible and uncertain quality of
tubes these days, more sophisticated designs (or at least ones that
tolerate failures without catastrophe) are a good idea.
It's the "pole pig to diode string to 10 uF, 10kV filter capacitor"
sorts of the designs that are doomed to excitement.
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