[Amps] Fault protection

[Robert Bonner]

Well Lets see?

I reach in the file and pull out 1 Drake L-4B manual and start reading,
let's see what it says the resistor is for.

First reference to the .82 Ohm resistor is in the unpacking section each new
amplifier comes with two spare resistors.  (sounds like they expect to use
them as fuses for the HV.

The next reference is in the troubleshooting section and it was made pretty
clear what the device is for.

There is an HV shorting interlock on the amplifier head.  Here's the exact
quote, "In the event of a short circuit, resistor R12 in the power supply
will be destroyed.  This resistor protects the diodes in the power supply
and it should be replaced with an IRC 0.82 Ohm 2 Watt type BWH resistor
Only."

AH HA, I remember when I was 15YO and owned my first L-4 I removed the cover
when I broke a tube.  Lesson of a lifetime, left drive control up and hit
the tune switch.  I then bought a new pair of 3-500Z's.  I remember because
the $60 was quite expensive for me as a kid.

SO there we have it.  They installed a cover interlock to keep me from
killing myself while trying to run the amp before screwing the cover back on
and placed a fuse in the PS (which I promptly jumpered) after blowing it up.

BOB DD

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Tom W8JI
Sent: Thursday, March 22, 2007 4:36 AM
To: Garry; amps at contesting.com
Subject: [Amps] Fault protection

> I have an L-4B and L-7. Both have the same HV supply which 
> uses a 2-watt .82 ohm "fuse" resistor in series in the B+ 
> line. I have often wondered why Drake did this rather than 
> use a real fuse though the primary side of the transformer 
> has breakers. There are 2 50K ohm 50-watt bleeders across 
> the output but no "glitch" resistor. The bleeders cause 
> the supply, which is outboard from the RF deck, to run hot 
> to the touch.

Maybe it explains it in the manual, but I wonder.....

How do you know it was installed as a fuse resistor, and not 
as something else like RF decoupling?

It very well might have been a sacrificial part that was 
poorly thought out, or one that was better than nothing for 
a fuse. But it very well could have been put there with an 
entirely different idea in mind.

Whatever the case, we know it wasn't the correct part for HV 
fault protection. In that case it's more like a reminder 
something happened. :-)

73 Tom 


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