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Re: [Amps] FW: SB220 - 10 Metre input coil adjustment

To: "'Sam Ferris'" <>, <>
Subject: Re: [Amps] FW: SB220 - 10 Metre input coil adjustment
From: "Tim Duffy" <>
Date: Sat, 1 Jan 2022 23:01:26 -0500
List-post: <>
So two 3-500Z tubes give about 50 ohms feedpoint Z. W1QJ can correct if need be.

So take OUT the tubes - put IN a 50 ohm non inductive resister there. You know 
where it goes?

Then use the RIGEXPERT and find the match on the input - get it to 1:1 (you can 
do this easily with the core). 

Then you will have over 1 KW on 10 meters output.

Tim K3LR
-----Original Message-----
From: Amps [] On Behalf Of Sam Ferris
Sent: Saturday, January 1, 2022 3:54 PM
Subject: [Amps] FW: SB220 - 10 Metre input coil adjustment

Hello Gudguyham. I am seeing an input SWR of 1.4:1 at 28.005 and it rises to 
about 3:1 at 28.700. On the input side it is above 1.5:1 staring at 28050 MHz.  
No input SWR dip between 28005 and 28050 – just rises all the way.  Have not 
tried beyond 28700.  As far as I know and by the looks of the amp the tank coli 
has never been touched for tuning.  The latest trial yielded about 520W at the 
HV setting, in comparison with the very first setting based on physical depth 
measurements alone.






From: <>  [] 
Sent: Saturday, January 01, 2022 2:45 PM
To: Sam Ferris < <> >; <> 
Subject: Re: [Amps] SB220 - 10 Metre input coil adjustment


What kind of input swr are you seeing as you make these adjustments on the 
input coil?  10 meters is very touchy on these amps.  You also have to take 
readings at a set output power across the band to see if there is a marked 
increase somewhere.  This will tell you if you need to massage the 10 portion 
of the tank coils to bring the “ sweet spot” where you want it.  You should do 
that first off and then fine tune the input because it will make a difference!!

Sent from the all new AOL app for iOS 

On Saturday, January 1, 2022, 3:04 PM, Sam Ferris < 
<> > wrote:

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 ½ 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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