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

[MU 4CX250B]

I agree with Tim. Also, seems to me the grid current is too low. I’m
going by memory, but for two 3-500Zs, shouldn’t it be above 200mA?
Possibly the drive is too low?
73,
Jim w8zr

Sent from my iPhone

> On Jan 1, 2022, at 9:01 PM, Tim Duffy <k3lr at k3lr.com> wrote:
>
> 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.
>
> 73
> Tim K3LR
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Sam Ferris
> Sent: Saturday, January 1, 2022 3:54 PM
> To: amps at contesting.com
> 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.
>
>
>
> Thanks
>
> Sam
>
>
>
>
>
> From: gudguyham at aol.com <mailto:gudguyham at aol.com>  [mailto:gudguyham at aol.com]
> Sent: Saturday, January 01, 2022 2:45 PM
> To: Sam Ferris <ve5sf at sasktel.net <mailto:ve5sf at sasktel.net> >; amps at contesting.com <mailto:amps at contesting.com>
> 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 <https://apps.apple.com/us/app/aol-news-email-weather-video/id646100661>
>
> On Saturday, January 1, 2022, 3:04 PM, Sam Ferris <ve5sf at sasktel.net <mailto:ve5sf at sasktel.net> > 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
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> 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
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> 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?
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> 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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> Thanks
>
>
>
> Sam
>
>
>
> VE5SF
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