On Oct 31, 2005, at 12:59 PM, Alan Ibbetson wrote:
> I recently got a Drake L-4B amplifier. I've been using (abusing) it a
> couple of weeks on CW on the high voltage SSB position. Pretty much
> everybody seems to do this. After all, it's only running 2500 volts on
> the anodes and they just glow a dull red when I'm keying.
>
> Yesterday, in the middle of a QSO, the short circuit protection
> resistor
> (R12) in the power supply gave an impressive firework display and
> expired.
>
> First question: has anyone seen this 2 watt component fail under
> "normal" use before? The nominal 0.82 ohm value would have to have
> drifted very high for it to fail at a load current of well under 1 amp.
> Is there an underlying fault that I should go look for?
The first things I would look at is the tubes, testing them for
filament-grid breakdown potential. Next, I would measure the R of the
VHF suppressor resistors. Filament-grid BD potential should be >6000V
and the suppressor R should stray no more than +/–30% from the marked
value
>
> Second question: the manual is very explicit that only an IRC type BWH
> resistor should be fitted. Can someone help with a translation of what
> this specification means,
It means that whoever decided to use a 2W carbon-comp resistor did not
realize that one can Not act as a proper fuse or current limiter in a
2500V circuit. What is needed in this application is a legit glitch R
that can safely discharge the # of Joules (a.k.a. watt-seconds) stored
in the filter C. This requires a been a larger resistor - such as a
10W surge protection type of approximately 10 to 15 ohms.
. Some will argue that since the 0.82-ohm resistor exploded during a
glitch, it stopped the flow of current and was therefore a suitable
fuse. However, since the V-drop across a carbon arc is only a few tens
of V, such a R does virtually nothing in a 2500V circuit to limit
current until after the explosion, which is Not how a fuse should
work.
. It is my opinion that there is a chance that an intermittent
parasitic osc. caused the 0.82 ohm R to explode. You can investigate
this possibility by measuring the dip frequency at the blocking C and
by measuring the resistance of the suppressor resistors.
-- note - as I recall, the dip freq is c. 90MHz, and the dip is sharp
and deep due to the high Q of the stock suppressors.
> so I can buy an equivalent here in the UK?
The question is why would you want to replace the wrong type of part
with the wrong type of part?
cheerz, Alan
> --
>
> Cheers,
>
> Alan G3XAQ
> alan@g3xaq.demon.co.uk
>
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>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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