[Amps] SB-220 load capacitor question

[johna8119-amps@yahoo.com]

>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