[Amps] [BULK] - Drake L-4B PSU burned resistor

[Steve Katz]

BWH is a molded wirewound device.  Pretty good writeup here:

http://n8sfc.com/bwh.html

For high surge applications, you don't want conventional film resistors;
they fail under normal conditions.

WB2WIK/6

-----Original Message-----
From: Alan Ibbetson [mailto:alan at g3xaq.demon.co.uk]
Sent: Monday, October 31, 2005 1:00 PM
To: amps at contesting.com
Subject: [BULK] - [Amps] Drake L-4B PSU burned resistor


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?

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, so I can buy an equivalent here in the UK?
-- 

Cheers,

Alan G3XAQ
alan at g3xaq.demon.co.uk

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