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[Amps] SB220 - 10 Metre input coil adjustment

Subject: [Amps] SB220 - 10 Metre input coil adjustment
From: John Lyles <>
Date: Sat, 1 Jan 2022 22:45:25 -0700
List-post: <>

I can send you email with schematic of my SB220 that I did a lot of mods and work to around 1994. I still have it but it doesn't get lit up these days.

The 10 meter input L was 4 turns on the form, and it had 39 pf in parallel with 22 pf on the input C and 22 pf on the output C (towards the cathodes).

Originally it had 3.0:1 input VSWR at 28.1, dropping to 2.0 at 28.4 and stayed at 2.0 through 28.6 MHz. Then it rose back up to 3.0:1 at 29 MHz and on.

After I modded the input pi network the match was 1.2:1 at 28.0 to 28.5 then dropped to 1.15:1 from 28.6-28.8 and rose to 1.3 at 29 MHz, 1.4 at 29.4 MHz.

This was a significant improvement, and it flattened the power output too. It runs 920 watts at 28.35, 210 mA of grid current and 620 mA of plate current on the high voltage. The modified parasitic suppressor was believed to be reason for the 60% efficiency at that band, more like 69% on the lower bands.

Date: Sat, 1 Jan 2022 14:04:07 -0600
From: "Sam Ferris"<>
Subject: [Amps] SB220 - 10 Metre input coil adjustment
Content-Type: text/plain;       charset="iso-8859-1"

Wonder if anyone can tell me how sensitive/sharp the 10 metre input tuning
coil tuning is on a SB220.  I am repairing an SB220 for a fellow ham and
when I received the amp the central tuning slug for the 10 meter input coil
(part# 40-964) was in a bag of parts ? seems the tuning slug had fallen out
sometime in the past.  It was noted that the 10 and 15 metre input coils
have the same part number so I measured the depth to the outer face of the
slug to the outer edge of the 15 metre coil to get an idea of just where the
10 metre coil should be set initially.  On initial tune up I was not able to
get more that about 450 Watts at 0.5A plate current and 110 mA grid current
on 10 Metres into a 1:1 SWR at 28.600 MHZ (Sweet spot for my antenna).  I
have tried one adjustment moving the coil slug in two turns which proved to
be the wrong way to go.  Have taken another look and decided to return to
the first setting plus about ? turn out ? then try again.  Can anyone advise
just how sharp the tuning on these input coils is?  Would it be best to
devote the time to making a temporary face plate that I can drill an added
access hole through to access the tuning coil from the outside?

The amp functions fine on other bands with reasonable output on a set of
EIMAC tubes.  Other repairs to this amp have included replacement rectifier
board, new parasitic suppressors ? old ones showed significant signs of
heating, replaced broken bandswitch, soldered loose wires, general cleaning
and check of components.




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