[AMPS] Follow Up on the SB-220 Follow UP

[Jon Ogden]

>>I guess it happens to those who think that every bang is a parasitic 
>>oscillation.  For one, with proper suppressor resistors you shouldn't have 
>>an oscillation.  
>
>€  The Challenger o-rings were supposed to be plenty ok in cold weather.  

O-rings in solid rocket boosters has nothing to do with this.  You've still not explained how an amp biased in cut-off can have an oscillation.  If you can explain this to my satisfaction, then fine.  If not, admit that it is possible that other nasty things can happen in an amplifier that is NOT a parasitic oscillation.

I agree that without a proper suppressor circuit, the amp can break into oscillation while being keyed or keyed rapidly as you suggest.  But all common sense and logic tells me that when an active device is biased in the cut-off region, it is just that: cut off.
>
>>I don't think the oscillation changes the value of the 
>>resistors.  
>
>€  A big-bang event happens in a 2-holer..  The suppressor resistors are 
>100 ohm +/- 5% units.  The resistors appear to be in mint condition. .  
>Each resistor is virtually shorted by a few turns of #16 copper wire.  
>The measured resistance of the suppressor resistors was over 400 ohms.   
>I was there.  I made the measurements. 

OK.  You say the resistors "appear to be in mint condition."  That tells me nothing of their value prior to the oscillation.

Resistors in an anode line will get warm.  Warm from some amount of RF flowing thru them in the form or tube output and circulating current.  With tubes with high Cout, this can get large on the upper bands.  The resistors also get warm from thermal effects.  Heat is conducted thru the anode line and convection also warms the resistors due to the fact that the ambient temp in the PA cabinet has raised.  The power handling capability of resistors is directly related to the temperature of the resistors.  There will be a point when the resistor has ZERO dissipation regardless of how many watts is printed on the side.  Resistors running for years at high temperature will also age faster and have a shorter MTTF than a resistor held at 25 C.  Tube amps would fit the high temperature thing.  I believe the resistor failure would be a gradual cascade effect over a long period of time.  Eventually, the resistor will get to the point where it can't handle any power at all and it fails.  It may not be visually noticeable either.

The thing that backs my theory Rich, is that I destroyed all 4 of your "anode fuse suppressor" resistors in my amp on 10M.  Some of you may not know what these are.  They are 3 1 Ohm, 2 Watt resistors all in parallel in parallel with a 1 Ohm 3 watt resistor.  0.25 Ohms at nearly 8 or 9 watts dissipation.  Should be able to handle plenty RF current right?  Wrong.  Pop went the weasle.  Sure they worked for a while but were eventually smoked.  Parasitic?  Hardly.  Rather the resistors got warm from all the RF currents and thermal effects and as a result that 9 Watts dissipation rating dropped to zip.  Bang, they ended.

My belief is that the same thing happens but on a much slower scale in nearly all parasitic suppressor resistors.  Perhaps the old carbon composite resistors where more susceptible to a long term failure mode than modern metal films.  But damage is even less noticeable on a carbon composite.  So the resistor has failed and then the parasitic oscillation happens.  IMHO, not the other way around.

73,

Jon
KE9NA



--------------------------------------------------------------------------
Jon Ogden

jono@enteract.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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