[Amps] SB-220 6 meter conversion parasites

[Hardy Landskov]

Hi All,
In my opinion the 2008 article has another glaring mistake, and that is the
plate RF choke. It has way too many turns to be inductive at 6 meters.
Believe me, about 20 turns of #22 wire will get you there but double spaced
wound on a 3/4 inch diameter Teflon rod. 
The problem is when the choke goes capacitive, RF sneaks down the choke and
gets into the power supply. The bypass caps at the bottom of that choke
should be 250 to 500 pF max. That is more than enough. A 1000pF doorknob
bypass at the bottom of the choke is inductive at 50 MHz, just what RF wants
to see to make you an oscillator. It may happen or not but layout starts to
bite you in the butt also if you are not careful. You have to know what you
are doing!

I still have an HP4815A Vector network analyzer and proved all this out for
myself. I wanted to write all this up but the Sunspots were low and I have
had some health issues so it did not get done.

With that all said, the addition of a 0.18uH coil wound on 1/8 inch wire
from the plate to the RF choke is a winner because the plate to grid tube C
is more than the calculated input Pi-Network C for a loaded Q of 12. Thanks
to GM3SEK website to solve that problem. W6WRT is spot on with the coil.

I did this conversion to an SB200 and put in a 3CX800A. The SB200 as
received was a basket case.

I did not recall any measured data from the original article. I have a ton
of pictures from my conversion that have never been published if anyone is
interested.

73 Hardy N7RT




-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Bill Turner
Sent: Sunday, July 19, 2015 1:59 PM
To: amps at contesting.com
Subject: Re: [Amps] SB-220 6 meter conversion parasites

------------ ORIGINAL MESSAGE ------------(may be snipped)

On Sun, 19 Jul 2015 11:03:01 -0600, you wrote:

>
>If I was doing it over, one change I might make is to swap the tune and 
>load capacitors, like the W1QJ King Conversions. That would reduce the 
>lead length from the RF choke to the blocking capacitor.

REPLY:

There is a better way, IMO, to do the conversion although it's too late now
for Steve. But for anyone else considering this, it can be done while still
using the original tune and load capacitors. I did this on my Heath SB-1000
and it works like a charm. The trick is to add a small coil in the anode
lead which creates an L-network to step down the plate impedance to a range
that the existing tune and load caps can handle with reasonable Q.  The
technique is described in any ARRL handbook in the last ten years or so in
the amplifier chapter.  I have pictures of my conversion which I can email
to anyone interested.
Contact me directly at dezrat at outlook.com

73, Bill W6WRT
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