The long-delayed reflection:
>
>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.
>
? What is "the frequency of the de-Q'ing"? Wes' measurements showed
that resistance-wire lowered supp. Q from 10MHz to 200Mhz.
>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.
? Surely. Perhaps 1 to 2% loss at 29MHz, which is not much of an
S-unit. Everything has a trade-off.
>
>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.
? the nichrome - wood concession.
>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,
? When the factory's grid-protection shorts during a glitch that results
in (fatal) gold sputtering from the grid, a better mouse trap might be 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.
>
? Amplifier owners often find an extraordinary increase in VHF parasitic
suppressor R after a bandswitch toasting.
>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.
? True. Oscillators require some signal to start them off. Damped-wave
ringing from a tuned circuit is one way.
> 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.
? An oscillation near the HF operating freq. would be delivered to the
HF antenna where it would be radiated. // However, VHF energy has no
place to go due to the low-pass nature of the HF tank.
>........
>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.
? My copy of Terman says nothing about gas gettering when the tube is
not in operation - which is what you contended, Mr. Rauch.
>
>The Terman books are a good step beyond systems based on
>tubes that barely made it into the shortwave region that Rich favors,
? [chortle]
>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.
? I also recommended a book from the 1950s: Dittrich, H. F.; *Tubes
for R. F. Heating*;
N.V. Phillips Gloeilaampenfabrieeken, Eindhoven, the Netherlands. Section
5.8, 'Parasitic Suppression Circuits' begins on page 96.
>> 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
? [chortle]
>> 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.
>
? Mr. Rauch temporarily quit the grate debate after I posted the result
of my telephone conversation with the Eimac/SLC facility's personnel
department.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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