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[AMPS] SB-1000 mods?

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Subject: [AMPS] SB-1000 mods?
From: Ian White, G3SEK" < (Ian White, G3SEK)
Date: Sat, 8 Sep 2001 14:10:28 +0100
Thanks, everybody, for all the suggestions about the SB-1000. I'm still
in "repair" mode at present,  but will think carefully about them all.


Johan Nordin wrote:
>>>> Fuses are bad enough because of time delay and internal arcing,
>>>> and resistors make very poor fuses.
>How about inserting 5x10 Ohm 1/4Watt in series as a glitch R
>Would that be a combined glich/fuse??? Good or bad?
Bad... let me explain.

The glitch resistor has two purposes: 1. To absorb most of the energy
from a current surge that might otherwise go into the tube; 2. To set a
limit on the peak current, which protects both the tube and the power

The glitch resistor must be large enough to handle all the stored energy
in the smoothing capacitor, and then hang in there until *something else*
switches off the HV. You may use a fuse in the HV line, but a fuse on its
own is *not* the answer - because it doesn't blow quickly enough. 

A fuse has a reaction time, and while it's heating up it will pass very
high currents... you don't know how high, or for how long. A resistor has
zero reaction time, and you know for sure how it will limit the current
(simple Ohm's law).

So the problems with the idea of a string of small resistors are: 
1. Resistors are very slow fuses, and if you spread the energy between 5
resistors, it will take even longer for one of them to blow; 2. You then
have to open the power supply... make sure the HV is all gone... sweep
out the blasted pieces of resistors... get out the soldering iron... find
some more resistors... you get the idea!

I much prefer a system where a big glitch resistor handles the fast
surge, and a current sensor switches off the mains within a few
milliseconds. It's reliable and there's nothing to replace or repair.

Just one other point: if you have a separate HV supply, the glitch
resistor *must* be in the power supply, not the RF deck. This is because
one of the most likely failures is a short to chassis in the HV cable or
its connectors. If the resistor is in the amp, it's downstream of the
short, and no use at all!

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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