[Amps] Parasitic Oscillation

[R.Measures]

On Nov 3, 2004, at 11:02 PM, Jan Erik Holm wrote:

> Will,
>
> This is interesting. I have had flashovers in the tank circuit
> in my 4-1000A amp being more then 1 inch long.

Wow, Jim, that's got to be >26,000V.

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