Hi Trevor,
I just finished working on an SB-220 (after studying the TowerTalk archive
and asking many questions on the list, too) and can offer the following
answers to your questions:
1. Ground the grids directly. First, remove the RFCs and capacitors in the
original circuit. I used 1/4 in wide strips of copper sheet (25 mil
thickness I think) folded around and soldered on each grid socket terminal,
then to new grounded locking solder lugs (like those already installed for
the RFCs and Cs). It is a good idea to clean the chassis under the
grounding lugs, too, for a good connection. After changing the original
SB-220 circuit to directly grounded grids I noticed smoother tuning and a
lower input VSWR on 15m.
2. Parasitic mods are not required, just ground the grids.
3. I don't think you need to modify this circuit if the original components
are good and tuned properly. On mine, input VSWR is under 1.5 on all bands
except 10m (not sure why yet). I would ground the grids first and check
input VSWR on all bands before doing anything else.
4. I don't know. You might look at the values on the Eimac circuit (linked
to 2/3 down this page http://www.radiowrench.com/sonic/) or the Heathkit
circuit and scale the values from 28 MHz to 50 MHz.
5. I'm not sure what you are asking here. The article has some good
information on amplifier operation and things to look out for (bad
components, etc.). I'd be happy to share my experience with you in much
greater detail...just let me know.
73, Chris KA8WFC
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Trevor Pitman
Sent: Saturday, January 13, 2007 5:03 PM
To: Amps@contesting.com
Subject: [Amps] SB-220 Improvements
Hello Group,
I am currently modifying an SB-220 for use on 6 meters by changing the 10
meter section as per the QST article (SB-220 0N 6 METERS) and leaving the
rest of the amp band positions unchanged on HF.
Having read the previous posts on directly grounding the grids, I look for
assistance with the following questions.
1 What is the best solution, directly ground the grids or leave them as
original.
2 Should the grids be left original, what about applying the parasitic mods
to the grids as per the other QST article (IMPROVING THE HEATHKIT
SB-220 AMPLIFIER) and what effect if any does this change have on 6 meter
operation.
3 If the grids are directly grounded, should the input be changed to a L/C
network or left original.
4 If the input is changed to a L/C network what is the coil/cap value for 6
meters.
5 Can anyone advise on the recent January QST article on these changes.
Thanks in advance for assistance
Trevor
VK3VG
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