Message: 8
Date: Sun, 02 Aug 2009 18:09:47 -0700
From: Jim Barber <audioguy@charter.net>
Subject: [Amps] Henry 3K-A 'gotchas'?
To: AMPS <amps@contesting.com>
Message-ID: <4A7638DB.6000005@charter.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Any fatal flaws or (significant) bad design decisions in the Henry 3K-A?
After squinting at the schematic available at the BAMA site for a while
what I see so far is:
(1) A rather unusual "manual" soft-start circuit
(2) The grid metering is done directly, for some reason...?
(3) Operating bias looks to be established with a zener, but I really
don't understand the combination of RF choke and feedthrough cap that
appears to short the filament transformer center tap to one side of the
filaments... I must be looking at that one wrong!?
Any comments, stories etc?
Thanks,
Jim, N7CXI
Jim,
I purchased a 3K-A from Henry in 1978. The manual step start works superbly,
never giving me a problem. Only a slight hesitation is required as you rotate
the switch from Off through "pre-op" to either CW or SSB. The step-start
resistor
is a rather large heating element used years ago in portable space heaters.
It screws in just like a regular light bulb and is easy to repair if necessary.
The grid metering is both accurate and adjustable. Again, never a problem.
The bias circuit is rather conventional and works at a bias voltage of 10v for
both
CW & SSB. The only difference being the CW position is selecting a different
primary
tap on the plate transformer. The coil and feed through cap is Henrys' attempt
to
keep "rf" off the line back to the filament transformer. They did this to
alleviate
the problem of the "filament transformer talking."
By the way, the power supply w/10 henry choke/resonator cap plus the little
over
21mF makes for one 'stiff' DC supply.
It's a horse, it is built very well, and has never in 31 years given me but one
problem. I think it's more of a tube manufactures problem than the 3K-A's. I
had one of the 3-500Zs melt the solder out of one of the filament pins.
I re-soldered it with silver solder and problem solved.
You'll do well if you purchase the ol-beast, she won't let you down in the
middle
of a RTTY contest.
Good Luck & 73,
Steve AB5MM
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