[Amps] SB-220 load capacitor question

[Colin Lamb]

The pi-network is really a transformer.  So, a change in the output capacitance could be based upon a change in the input characteristics as well as the load.  Is the voltage correct?  Are both tubes the same color?  Are any of the turns shorted?

If thee is no logical explanation, it could be an illogical one.  If a friend comes over and you go out of the room, does it still tune the same?  Do you have any confessions you want to share with the group that might cause the amplifier to act in such a manner?  Have you had this effect on amplifiers at club stations?  Have you ever been rejected for a dxpedition, even though you put the money up front?  

You can work backwards and play with the amp offline (unplugged and plate shorted).  Put a resistor equal to the plate impedance from plate to ground, then use a MFJ or Autek (or other) bridge on the output and see where you get 50 ohms.  Is the plate tuning control where it should be?

In the end, if you cannot find the results you expect, you need to keep asking dumber and dumber questions.

Colin K7FM   



-----Original Message-----
From: johna8119-amps at yahoo.com
Sent: Nov 10, 2005 10:15 AM
To: amps at contesting.com
Subject: Re: [Amps] SB-220 load capacitor question

>R Measures wrote:
>On Nov 9, 2005, at 6:09 PM, <johna8119-amps at yahoo.com> wrote:
>
>> I should probably be ashamed to ask this.  I have this SB-220 that I 
>> have been unmodifying, and I
>> have run into a situation that I am wondering about.
>>
>> When I tune this rig up, with excitor power anywhere from 5-100 w, I 
>> have to turn the load
>> capacitor fully clockwise (unmeshed) to maximize power output.  I'm 
>> getting decent power out with
>> it.  Could this be due to the 50 ohm dummy load I'm working into?  Is 
>> it a good thing to have
>> "full" loading?
>
>Two possibilities:  The load R is not 50-ohms -- or the bandswitch has 
>some vapourized contacts due to previous arcing.  This is more likely 
>to occur to the 15m and 10m contacts.
>The load R's ohms can be measured with a DMM.

I had previously checked the dummy load with DMM (47.8 ohms) and with a MFJ 239 SWR meter (50 ohms
and 1:1 swr).  There is no change with switching the bandswitch.  I have polished the contacts
with  a bit of grocery bag paper.  No change.

>Dr. William J. Schmidt, II  K9HZ wrote:
>Sounds like you are using the same piece of bad coax between the amp(s) and 
>the dummy load.  A fully un-meshed loading cap indicates that the output 
>load is not 50 ohms!

I thought so, too, about a month ago, so I made up new (different) cables with RG-8U, and the same
thing happens.

>Robert Dorchuck W6VY wrote:
>Hi John,
>I have four 220's here and none of them load like that.  At low
>drive the load setting is quite low.  My load settings are
>roughly - 75 meters  -- 6 - 7
>          40   "        4 - 5
>          20   "        5 - 6
>          10   "        6 - 7
>
>
>There are very small differences between the amps.
>
>Would seem like something is amiss.  Are the caps fully meshed
>when the knob is set to zero?  Do you have normal grid and
>plate current?

The caps are fully meshed when the knob is turned to zero.  Plate is normal.  Grid current is a
little high at 100 w drive.  I've checked this on all 5 bands, and the load cap setting is the
same (fully unmeshed) for each.


On the amp I am "unmodifying," I have replaced the PC board with the Harbach board.  I have also
replaced the anode wiring circuit with the low-Q circuit recommended by Rich Measures.  The other
220 is pretty stock, though I have replaced the zener with a 7 diode string.  I don't see how
either of these mods could do this.

Thanks
John
w4kv

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