Do you mean instability as in oscillation ? What is the frequency of the
oscillation ?
It shouldn't be that sensitive to a grid resistor value. If it is, you must
have little phase margin, and I would suspect too much phase shift in the tuned
input at freq of oscillation if it is in band.
You should be able to take a decent linear and swing the bandswitch and caps
all over and NEVER have it take off, especially in grounded grid. It is back
to the drawing board if it does.
Is not a SB-200 GG ?
Steve
----- Original Message -----
From: Gudguyham@aol.com
To: amps@contesting.com
Sent: Tuesday, July 17, 2007 3:27:04 PM (GMT-0600) America/Chicago
Subject: [Amps] SB-200 DATA
Just wanted to share some experiences I have had recently with Sb-200
amplifiers. Occasionally we will see posting about instabilities of these amps
on
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
the
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
was
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
amp
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
schedule.
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
TLC
as I have presented above, can be made to make full power output.
Thanks for your time.........73 Lou W1QJ
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