[Amps] Parasitic Oscillation

[Will Matney]

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. 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, 
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 I 
would think. I'll bet that those measurements would not have been no 
where near the same with new tubes and the suppressors out of circuit.

Best & 73's

Will Matney


Harold B. Mandel wrote:

 > Will matney raised an issue we have hammered and hammered on this
 > reflector from Day One: Parasitic Oscillations~Phenomenon or
 > Hallucination?
 >
 > Will is mindful of generations of amplifier builders incorporating a
 > suppression
 > network in their circuits. I've seen anode suppressors for over forty
 > years of
 > amateur radio and agree that there might be a reason to include them.
 >
 > Looking to commercial manufacturers,  it seems that if something isn't
 > necessary it usually isn't included. One of my amplifiers came without
 > a High Voltage standby switch but did have anode suppressors.
 >
 > My old SB-220 had suppressors. Would Heathkit have included them
 > if they thought they could get by without them?
 >
 > AG6K invested years of his life and probably a courtly sum of money
 > destroying equipment intentionally to discover attributes of how and why
 > suppressors work. This led to the amendment of suppressor design so
 > that output-Q would be drastically reduced at VHF events where 
unprotected devices would be easily destroyed. Mr. Measures came up with
 > a system of suppression that seems to protect expensive components
 > in a number of ways, but there's no such thing as a free lunch.
 >
 > In building tube amplifiers for HF service I ask myself what am I wanting
 > out of life: Performance, Reliability, Bang-For-Buck?  Am I willing to
 > trade
 > a bit of performance for something else, like safety and reliability,
 > perhaps?
 >
 > There's $4,000 worth of tubes around here and I really don't feel like
 > buying new
 > ones because an event caused a runaway oscillation. While my antenna
 > tuning apparatus is supposedly secure there's always the chance that 
something
 > happens to the 600-odd-foot antenna wire out there, right while I'm 
in the middle of an
 > RTTY
 > transmission. I would much rather buy a handful of small parts and spend
 > a weekend
 > soldering them together than PayPal-ing C&D or someone similar thousands
 > of dollars
 > for another tube.
 > Even if Parasitic Oscillations are humbuggery, the cost of inclusion is
 > but a fraction
 > of the worth of an amplifier output device.
 > Lastly, none of my amplifiers have ever gone into self-oscillation except
 > a particular
 > Australian tetrode GU-78B, and that machine didn't have an AG6K
 > suppressor,
 > nor did it have a neutralization circuit. That amp was the only one out
 > of the eleven
 > seeing service here.
 >
 > Respectfully,
 > Hal Mandel
 > W4HBM
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