[Amps] SB220 - 10 Metre input coil adjustment

[John Lyles]

Sam

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"<ve5sf at sasktel.net>
> To:<amps at contesting.com>
> Subject: [Amps] SB220 - 10 Metre input coil adjustment
> Message-ID:<000601d7ff4a$bc92a970$35b7fc50$@sasktel.net>
> 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.
>
>   
>
> Thanks
>
> Sam
>
> VE5SF
>
>   
>
>