[Amps] Parasitic Oscillation

[Jan Erik Holm]

Will,

This is interesting. I have had flashovers in the tank circuit
in my 4-1000A amp being more then 1 inch long.
What you write below could explain it.

73 Jim SM2EKM
-----------------

Will Matney wrote:

> 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