[Amps] Parasitic Oscillation

[Will Matney]

Rich,

 >The major difference in SB-220s is the particular tubes that happen to 
be plugged in.

I would think so too!

 > >not every wire the same length or in the same place. Really, all 
commercial amps aren't >>either. Plus those tubes in that amp didn't 
look no where new to me so the gain may have >>been lower than fresh tubes


 >However, this is not always the case.  The hottest, squirreliest pair 
of 3-500Zs I ever tested >had a couple of hundred hours on them.

Hmm, one would assume that the more hours on a set of tubes, the gain 
would drop due to the emissions being lower than a new tube. I can't 
recall if the test listed the output power on the SB-220 or not without 
re-reading it. What his conclusions were based solely on was the fact 
that the VHF frequencies were several dB's less than the operating 
frequency with the suppressors in the anode leads. To me, like you had 
done, I would want to see the test less any suppressors. As Marv said 
about Bill Orrs comments, any certain time with the correct conditions, 
a VHF oscillation might happen.

Now, I have some experience with plasma arc cutting machines, the large 
ones mind you. The way the arc is started is the use of a very high 
frequency. They use something similar to a spark gap to generate a very 
high frequency arc. This arc then causes the main conductor at the tip 
to arc to the material. This happens after the oxygen is flushed away 
from the tip and material by a forced jet of CO2, or CO2 with helium 
(shielding & cutting gas). Once this larger arc (cutting arc) is 
established, the high freq generator drops out. Tig welders work by a 
similar process. One must keep in mind that at VHF frequencies, arcing 
is way greater than at High Frequencies. Ignitron tubes used in Flash 
Welders use a high freq put on the control stud which arcs down to a 
mercury pool. This then allows the arc to start between the anode and 
cathode (cathode being the mercury pool).  It would look to me that a 
small VHF generated arc from a possible oscillation would cause a larger 
arc from the power stored in the tank circuit. This could very well burn 
the contacts out on a bandswitch. That's just a theory, not fact. Any 
thoughts on this would be welcome from all.

 > ...Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

Best & 73's

Will Matney