Hi Bob...
I agree completely and absolutely! It's a wonder that any of us post-WW II
teen-age amplifier home brewers survived our ignorance and foolishness. I
got knocked against the wall once while in high school. I was
"test-feeling" a 2200V supply for pp 811's during burn-in to see how warm
the components were. Forgot that the filter choke had surface-mounted
terminals out of my view on the rear of its case. That nasty experience
convinced even a kid of the absolutely essential need for cover interlocks,
HV cover crowbars, and one-hand-in-the-pocket close to HV. Luckily for me I
DID have one hand in a pocket, or there likely wouldn't have been any
Signal/One, ETO or ALPHA.
The point I originally intended to make was that the (VERY DUMB!) pencil
trick illustrated what remarkably high rf voltage can lurk at the anode of
a LIGHTLY-LOADED amp with a low loss tank and very wide tuning cap plate
spacing. With the amp properly loaded the arc was MUCH less impressive.
Actually I guess that experience was with 807's at ~700V -- still foolish
and ignorant but, with great good fortune, never painful. By the time I
graduated to 813's at 2200V the pencil was taped to a plastic rod. There
was still enough capacitance to draw a flaming arc, and with relatively
less risk. With a subsequent move to 4-400A's, the drama of the pencil --
even well insulated -- no longer overwhelmed early glimmers of good
judgment -- or maybe it was just the memory of previous close calls.
A few years ago, an experienced customer and friend down in Texas was
electrocuted while working on a HOME BREW amplifier... His ALPHA wasn't in
any way involved, but it gave me a real start to come across the unexpected
report in QST! I surely wouldn't want to mislead anyone into hurting
him/herself (or worse!) by doing the stupid things some of us did as
"kids".
I STRONGLY urge that NO ONE get closer than a foot of very good insulation
to anything exceeding 120V ... and then only with one hand in a hip pocket.
If you MUST get inside your amplifier, PULL THE PLUG AND WAIT AT LEAST TWO
MINUTES BEFORE REMOVING THE COVER!
Then FIRST THING after opening the cover, short the HV line to chassis two
or three times with an HV-shorting stick, or at least with a long,
well-insulated screwdriver, before touching anything! NEVER TRUST YOUR LIFE
to switches, relays, cover interlocks, crowbars or the like - even with a
well- designed commercial amp, the stakes are just too high to take
chances.
Sorry guys - I know it's been said over and over before - by John Lyles and
others - but this sermon is my penance for mentioning rf and pencils in the
same paragraph. We don't want to lose any friends.
73, Dick W0ID
-----Original Message-----
From: Bob Sutton [SMTP:bob.sutton@xtra.co.nz]
Sent: Tuesday, July 21, 1998 6:10 PM
To: Richard W. Ehrhorn
Cc: amps@contesting.com
Subject: Pencils and fiery inch-long arcs!
Good grief!!! "DON'T TRY THIS AT HOME" is right!!! I know you are
referring to RF voltages, but the cap of an 813 also has the HT voltage of
course. Many years ago a ZL ham died when he used a pencil to tap a
stubborn relay contact in his amplifier that was carrying a very modest
level of HT voltage. The pencil lead and pencil 'body' conducted enough
current through HIS body that HE is no longer current!
Having experienced 900V across my chest from one hand to the other when I
build my first amplifier 22 years ago ... I still have a scar on my little
finger where it brushed the live component and blew a hole in the skin ...
I have always taken the utmost care (and a 'one hand in my pocket'
approach) anywhere near anything that could carry HT voltage.
73 ... Bob, ZL1RS
P.S. Richard. I had an ETO Alpha 77Dx until recently ... it was an
emotional time as I past it onto the new owner!
At 11:06 21/07/98 -0600, you wrote:
>
>As anyone who's ever [FOOLISHLY -- DON'T TRY THIS AT HOME!] used a pencil
>to draw a fiery inch-long arc from the cap of anything bigger than an 813
>can attest!
>
>Dick
>
>-----Original Message-----
>From: Bill Turner, W7TI [SMTP:wrt@eskimo.com]
>Sent: Monday, July 20, 1998 5:17 PM
>To: Jon Ogden
>Cc: Rich Measures; amps@contesting.com
>Subject: Re: [AMPS] Vacuum Cap Rating and Plate Voltage
>
>
>On Thu, 16 Jul 98 14:35:10 -0500, Jon Ogden <jono@enteract.com> wrote:
><snip>
>>The handbook was correct as everyone told me: With a blocking cap the
>>peak RF voltage is equal to the DC plate voltage. It's without a
>>blocking cap that you get the twice the DC voltage at peak. So I
>>remembered incorrectly!
>_______________________________________________________________
>That figure of equal voltage is correct when the output tank is fully
>loaded. With very light loading and lots of drive, the peak RF
>voltage can be MANY times the DC voltage, and that's when the tuning
>cap will arc.
>
>73, Bill W7TI
>
>
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