[AMPS] parasitic fantasia

[Tom Rauch]

Hi Jon,

> This is true.  Very true.  I used to be a firm believer in the nichrome
> theory.  It sounds good on the surface and as one reflector reader put it,
> "The theory sounds correct."  However, an ancient Jewish proverb states in
> effect, "Every story sounds good until another comes and cross examines
> it."  Once I was able to successfully build an amp using a 4-1000A that
> needed no suppressor resistor let alone any nichrome, I realized that
> while Rich's suppressors may work and probably won't cause any harm, are
> they necessary.  I maintain that even for a 4-1000A, they are not. 
> Perhaps some of the other tubes you mentioned, 572B, 811A, etc may benefit
> from nichrome. 

Actually the poorer the tube design, and poorer the layout, the 
more valuable nichrome becomes. That's because the addition of 
loss resistance in the inductor, the primary low frequency path, the 
lower the frequency of the de-Q'ing.

While there are alternatives to nichrome, it is a shotgun approach 
with the only ill effect of de-Q'ing the tank more at ten meters and 
upper HF than other methods.

My main point isn't that nichrome is bad, it just isn't necessary in 
most modern applications. It was important when tubes barely 
operated above 15 MHz, and when most layouts were on wood. 
There was almost no hope of solving problems then, because the 
tubes were operated at or near a frequency where the system 
became unstable.

The only harmful things are claims that removing grid protection is 
a good idea, and nonsense claims that most bandswitch failures 
and tube arcs and failures are caused by parasitics. Nothing could 
be further from the truth.

One acceptable alternative to loss of upper HF efficiency and tank 
Q caused by the shotgun approach of using nichrome is to actually 
resonant the suppressor at the same frequency where the grid is 
resonant. Then a resistor can be placed in the suppressor, to de-Q 
the system. That way the resistor become virtually the entire part 
of the suppressor system impedance at the frequency of instability, 
while having almost no effect at frequencies an octave or more 
away.

Oscillators just don't work like bells, waiting to be struck. They, if 
perturbed any amount and biased into conduction, either oscillate 
or they don't. They have the highest gain at lowest levels, and so 
self-limit with amounts of feedback that are near the edge of 
stability. 

If feedback is severe, they can produce damaging currents or 
voltages (generally not at the same time) but if feedback is severe 
they also oscillate ALL the time and very reliably.

I certainly agree that it is possible for an oscillation, especially one 
at or near the operating frequency, to damage components. But 
feedback enough to cause that would make the stage more than 
an occasional oscillator. I've never seen that happen in a modern 
HF amplifier unless something causes the HF operating 
impedances to soar while the tube is biased on.

That happens in the Yaesu amps with Svetlana 572B's, because 
the tubes (requiring slightly more cutoff bias than other 572 brands) 
remain in conduction while the antenna transfer relay open circuits 
the tank and input circuits. The reason that occurs is threefold. The 
tubes have more quiescent current than normal when biased off, 
the amp has a poor bias scheme, and 572's are right on the edge 
of stability at upper HF when they are NOT neutralized in a 
grounded grid circuit.

Terman covers gas arcs and gettering, and even stability and 
suppression, in his older engineering handbooks. The older Termin 
books are worthwhile reading for those who want facts instead of 
fiction.

The Terman books are a good step beyond systems based on 
tubes that barely made it into the shortwave region that Rich favors, 
and modern tubes. They were written when we could make reliable 
high power tubes that operated beyond medium wave or shortwave 
bands.
  
> Rich always quotes the papers from the 1930s to justify nichrome.  
> Perhaps there is lots we can learn from previous generations.  And 
> perhaps those guys knew quite a bit.  But people once thought the earth
> was flat too.  Technology has also come a long way since the 30's.  Back
> then radio was in infancy and I don't think television was even around let
> alone, computers, VCRs, answering machines, contest keyers, etc.

And tubes had long leads and big elements. Amps had long leads 
and poor layouts, most were breadboard construction on open 
chassis or not even on metal at all. When the amp is a stability 
nightmare, it takes a shotgun to make it behave.

Anyway I've used my years allowance of time in the past few days. 
I'll have to quit this for now. I'll be back next year, for a repeat of the 
same arguments.



  
73, Tom W8JI
w8ji@contesting.com

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