>
>The resistor isn't to ground..... it's in series with the tube(s) that
may draw 1
>amp intermittently ... therefore 1 amp squared through 30 ohms is 30
watts.
>
Correct, the resistor needs to be rated 30W or more to handle normal
operating loads.
>If the tube draws more amps ... a short or a bad load... maybe a two
amp
>spike... that is 120 watts which will hopefully burn out the resistor (
acting
>like a fuse ) before the transformer or diode stack goes up in smoke.
>
Sorry, NOT correct - two times over.
First of all, if a fuse or resistor is rated to carry one amp in normal
operation, it will not protect against a minor overload like two amps.
Only an electronic trip can do that reliably (we don't design amplifiers
on "hopefully").
Second, the glitch resistor is only there to limit really BIG current
surges like a vacuum arc inside the tube, or a physical short-circuit in
the HV supply or the RF deck. Without surge limiting, the current would
spike at 100 amps or more, which would wreck the delicate internal
structure of a tube in a matter of microseconds. See Eimac Bulletin 17
on 'Fault Protection' [1].
The purpose of the glitch resistor is to limit the current to a
non-destructive level, and to buy time while some slower device
disconnects the power. The reason for using a resistor is that it
provides truly instantaneous current limiting. A fuse doesn't do that -
fuses provide no current limiting at all until after they have blown, by
which time the damage has already been done. On the timescales we're
talking about here, fuses fall into the category of "slower devices"
that follow along behind to disconnect power and shut down safely.
The timescales for a safe shutdown are much longer - lots of whole
milliseconds, even - and there are many other devices that can do that
without destroying themselves like a fuse. Minor tube arcs are often
once-only events, so the aim should be to press a Reset button and be
straight back on-air.
As a contester and DXer, I have always hated having to replace fuses. It
always happens at the worst possible time, and the inside of a HV supply
is no place to be fooling around while sleep-deprived and driven by
urgency. A safe and resettable HV shutdown was one of the prime
directives behind designing my Tetrode Boards and Triode Board [2
-shameless plug].
So here's a final test question: Why did Drake and the Radio Handbook
use a 0.5W resistor in the HV+ line instead of something 100 times
bigger?
Answer: Because they screwed up. This is no example to follow!
73 from Ian GM3SEK
[1] <http://www.ifwtech.co.uk/g3sek/misc/bull17.pdf>
[2] <http://www.ifwtech.co.uk/g3sek/boards>
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