[Amps] Henry 3K-A "gotchas"

[AB5MM]

Message: 8
Date: Sun, 02 Aug 2009 18:09:47 -0700
From: Jim Barber <audioguy at charter.net>
Subject: [Amps] Henry 3K-A 'gotchas'?
To: AMPS <amps at contesting.com>
Message-ID: <4A7638DB.6000005 at 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