The blocking cap should be in series with the tube anode and the coil of the
Pi network. The RF choke output connects between the tube anode and the
blocking cap. Is that the way it is connected? Also, I am surprised that
the blocking cap is only 1000pf. 2000pf is what my Henry amp has and all my
home brew amps have had at least 2000pf, but I don't think that the
difference between 1000 and 2000pf would cause what you're seeing. I would
check to see what the bias is set to. I am not familiar with the LK800 but
I would think that it is using a 12V Zener in the centertap of the filament
transformer secondary.
Good luck,
Bob W6TR
----- Original Message -----
From: <kk2ed@comcast.net>
To: <amps@contesting.com>
Sent: Thursday, August 21, 2008 1:07 PM
Subject: [Amps] Amp Supply LK800 trouble
> Good afternoon,
> I am still trying to help a friend get his LK800A amp going. Last visit,
> we thought the 1000pf plate blocking doorknob was bad and not passing RF,
> as the amp would draw 1A + of plate current, but no RF was appearing at
> the output. The plate and tune caps seemed to make no difference when
> tuned.
> Well, fast forward, and we replaced the 1000pf doorknob cap. Now, we are
> seeing about 500w make it to the antenna, and the Plate tuning has some
> effect, and max power does in fact equal min plate current (dip), give or
> take a few ma. However, the loading control is having ZERO effect, and
> is affecting the output.
> Close inspection revealed no signes of bad connections or arcing. We
> bypassed the vacuum relay to rule out relay issues, and ohm-ed out the
> tank coils and bandswitch.
> Studying the schematic and checking the components, everything looks good.
> The circuit is quite simple - plate bypass cap from the plate choke,
> tapped coil, load/tune varicaps, then the relay. I'm missing something
> somewhere...
> We also pulled the three tubes, tried a spare set, same results. Also
> tried one at a time, same thing, so I think the tubes aren't an issue.
> Any ideas?
> Thanks
> Eric KE2D
> KE2D
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