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

Sun Jan 2 07:01:49 EST 2022

With solid state radios the 10 meter output varies depending on how sensitive the fold back circuitry is.  High input swr robs valuable drive power and with 12db gain that’s significant loss of drive power.  If your input swr is even 1.5 to 1 you are losing valuable drive power. At any swr level other than that which produces maximum output power, you won’t get full potential power.  Many have reworked the 15 and 10 meter pi-networks to obtain better output.  Rich Measures addressed this years ago and has alternate data on all band pi networks.  In my many years working on these amps I’ve experienced the full gamut of the 10 meter input swr issue.  In some amps with some radios the stock pi input as it came from Heath  may only need some inductor adjustment to satisfy your radio. Some radios require some value changes.  Rich’s page goes into this.  However I have found that providing all the mica values on the input pi networks on 80-20 are correct and the amp was assembled to the manual specs on those bands it is 100% positive to adjust the swr down to 1 to 1 or whatever produces maximum output.  I would not recommend changing anything on those bands unless it’s just a minor slug adjustment. If you can’t flatten those bands you should expect that a mica cap changed value a lot.  On some amps 15 might be terrible no matter what and may require reworking. Remember these were kits and above 21mhz small variations can cause big differences.  Ameritron amps have the finest input circuits of almost any ham amplifier.  Every bands’ input swr is right on from the factory.  Their  amps are very consistent on this because the input circuits are all built on PC boards and this makes no possible way for assembly differences.  If you simply can’t get a good match on 15 and 10 you can check out AG6K for alternate circuits.  Believe me, this can be very frustrating.  Another thing to remember on the SB220, you can massage the few turns on the 10 section on the tank coil to optimize the tank for the portion of the band you want optimized.  I would do this first off!!!!!!  Often times you’ll see that bringing the tank coil to the right inductance will also help the input circuit as well!!!  First do a chart of of output vs frequency to see where you are at, then slightly massage only the 10 meter windings of the coil until the power peaks where you want, then attack the input.  Just a hint, sometimes you can get maximum power output with the input showing wild high input swr that changes with slight load cap changes.  If you are getting lots of power but see this high swr then that reading is a phantom swr reading being caused by some harmonic reflection on the input circuit.  In the ideal world  the input swr the radio sees from the input network should NOT vary as the load control is at slightly different settings.  The swr should be be constant and independent of load cap setting.  Once again it’s up to you if you want to pursue the input circuit  beyond the power level output that satisfies you.  I’ve found that input circuit design is an art and science all in itself.  Off the self circuits may or may not satisfy your amp, especially home brew.  The placement of the input circuit in relation to the cathode, the actual impedance of any connecting coax and the surrounding architecture will effect the results.  Hopefully there is enough leeway in adjustments that these variables can be dealt with.  Remember these few hints as you go along. 10 meters is getting better these days and these amps may need to work to get them right on this band.  Also, if your amp makes good power on 20 meters and much less on 10 it’s really not a tube issue.  In the sb220 it’s most likely input circuit and tank coil adjustments.  3-500 tubes really don’t lose gain at 10 meters!!  Good luck and HNY!

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On Saturday, January 1, 2022, 11:09 PM, MU 4CX250B <4cx250b at miamioh.edu> wrote:

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?
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> 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.
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> 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.
>
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> Thanks
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> Sam
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> 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
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> sometime in the past.  It was noted that the 10 and 15 metre input coils
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> have the same part number so I measured the depth to the outer face of the
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> 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
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> get more that about 450 Watts at 0.5A plate current and 110 mA grid current
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> 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
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> be the wrong way to go.  Have taken another look and decided to return to
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> the first setting plus about ½ turn out – then try again.  Can anyone advise
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> just how sharp the tuning on these input coils is?  Would it be best to
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> devote the time to making a temporary face plate that I can drill an added
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> 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
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> EIMAC tubes.  Other repairs to this amp have included replacement rectifier
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> board, new parasitic suppressors – old ones showed significant signs of
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> heating, replaced broken bandswitch, soldered loose wires, general cleaning
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> and check of components.
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> Thanks
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> Sam
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> VE5SF
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