[Amps] Parasitic Oscillation

[R.Measures]

On Nov 3, 2004, at 5:40 PM, Will Matney wrote:

> Hal,
>
> Marv made a very good point earlier when he mentioned about Bill Orr 
> being associated with Eimac and said "there is no such thing as an 
> amplifier that does not have parasitics!  He made it clear that at 
> some frequency, under some mix of power, drive, settings etc., any amp 
> can be made to oscillate."
>
> I think I would more tend to believe someone with Eimac who actually 
> makes and tests the tubes than anyone else. From personal experience I 
> have seen it happen due to long lead lengths in some crudely designed 
> amps. Actually the crude part came from the builder being too tight to 
> build it the correct way.
>
> A person would think the SB-220 bad but do you remember that 10 or 12 
> tube 3-500Z box on eBay for sale with all the problems seen? I would 
> think it would have been a nightmare.
>
> Tom Rauch's webpage with the SB-220 amp shows testing one with the 
> parasitic suppressors *still connected*. Keep in mind that this is the 
> only amp that was tested and he states he doesn't have time to test 
> more. Now I would think one would want to do the test with NO 
> suppressors installed to see what happens.

Will -- I did no a suppressors experiment with my SB-220 on 40m.  The 
amplifier was not stable.
-  In my experiences, "not enough time" usually has nothing to do with 
the availability thereof..

> By testing with them installed, only shows that one amp was working 
> correctly with its parasitic suppressors in circuit! Plus, not every 
> SB-220 is the same,

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

> 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.

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