Ron Hooper wrote:
> I have used 10 ohm 25 watt resistors in some of my amps in the B+
>leads to limit the current. I do not use them anymore after a short
>circuit. It blew the resistor to little pieces made a mess and allowed
>the wire to fall down aginst the chassis, shorting out the supply. I
>just quit using anything at all. However you can buy a fuse that will
>work in that application with the proper ratings.
Unfortunately that still leaves another fuse, namely the grid wires
inside the tube. It's difficult to guarantee that the "real" fuse will
blow before letting through enough energy to damage the tube.
The answer seems to be to persevere with the glitch resistor. I feel
sure that the right way to solve the problem with the resistor is to
make it higher in value and physically longer - not to quit using it.
10 ohms is on the low side. For tubes up to 1500W dissipation, Eimac
recommends limiting the maximum possible current to 40A, which implies
using 50 ohms for 2kV, 75 ohms for 3kV etc... or maybe a little less if
you take some credit for the series resistance of the smoothing caps.
The power dissipation in normal use is quite considerable but of it as
an "insurance payment".
Physically long resistors seem to be key to preventing arc-over, with a
construction that avoids sudden jumps in voltage along the length.
Continuous-film resistors should be best because they guarantee a smooth
voltage profile, but a long wirewound will do it.
G4GCM has wound his own, using something like 26AWG nichrome, space-
wound on a long 0.5-1in former to give 50-80 ohms total.
The other important thing is to make sure that mains power is removed
from the transformer very quickly, to limit the follow-on energy that
can be dumped into the tube.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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