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[Amps] SB-200 DATA

Subject: [Amps] SB-200 DATA
Date: Tue, 17 Jul 2007 16:27:04 EDT
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Just wanted to share some experiences I have had recently with Sb-200  
amplifiers. Occasionally we will see posting about instabilities of these amps  
higher bands 15 and 10 meters, however I have seen one ham complain about it  
on 40 meters.  9 out of 10 times the culprit(s) are the 33 ohm 1 watt grid  
resistors.  I personally had a stability problem with one, and upon  checking 
2 grid resistors, one was 42 ohms and one was 48 ohms.  Not  only are they 
way off from 33 ohms, but it is equally as important that they are  both equal 
in value.  So, by having a situation where one is 42 and one 48,  it is a 
compound problem.  Upon changing them with new GOLD banded  resistors at the 
perfect 33 ohms, the amp then became stable as a rock!   Another factor that is 
discussed is 120 volts vs 240 volts.  Today I had a  unit that I tested that 
just purchased at a hamfest.  Brought it up on  a variac to reform the caps and 
them plugged it in direct.  I took power  readings on 80, 40, and 20 meters.  
80 meters with 100 watts drive gave me  600 watts
40 meters gave me 600 watts and 20 meters gave me only 400 watts.   Shut the 
amp down and checked the grid resistors. They read 37 ohms and 45  ohms.  Both 
not near 33 ohms and both not the same value, a compounded  problem.  I 
replaced them with a new set of gold banded resistors and I  also switched the 
to 220 volts.  Went back to check the amp again on 80  40 and 20.  The new 
power levels were....
80 = 725 watts,  40 = 725 watts and 20 = 610 watts.  It seems to  me that 
many of these Sb-200 amps that may have the original silver banded grid  
resistors that are 30 years plus old, need to be changed as a maintenance  
Also I think there is much better performance when used on a  good 220 volt 
line instead of 120 volts.  The figure don't lie.
These amps are very popular and I can't believe how many of them are out  
there.  E-bay is loaded with several every week and there is usually a few  at 
every hamfest.  They can be gotten at a reasonable price, and with  a little 
as I have presented above, can be made to make full power  output.
Thanks for your time.........73 Lou W1QJ

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