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

[R.Measures]

On Oct 31, 2005, at 2:04 PM, Steve Katz wrote:

> 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.

The MOF resistors we use in our suppressor retrofit kits are rated by 
Matsushita at 100-ohms, 3W, 70ºC.  In free air, they will dissipate 12W 
for one hour with a resistance change of less than 10%.  At 12W, they 
glow red in a darkened room.
>
> 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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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org