[AMPS] FAILURE OF PARASITIC RESISTORS IN AL-811X AMPLIFIER

[measures]

>
>> Jon, you stated previously that you were in the resistor business.
>> Why don't you provide a mini lecture on the inductance in helical wound
>> "metal film" resistors of the type that are probably going to be
>> encountered in the 5-10 Watt class on RF suppressors? Here in the third
>> world, even 1/4 Watt "metal film" resistors turn out to be helical wound
>> jobs, readily proven by burning the paint off them and inspecting the
>> remains... Surely if the function of a resistor is to emulate R only, then
>> inherent inductance must be kept low, and carbon composition types are the
>> preferred option?
>
>Or you can put a series capacitor in series with the R and cancel 
>the reactance at the frequency of instability. 
>
An interesting idea, Tom.  
A 100-ohm, 3w Matsushita MOF resistor has c. 11nH.  At 100MHz, this 
amounts to 7-ohms of reactance.  At 100MHz, roughly 200pF would be needed 
to cancel 7-ohms of Xl.  .  However, the typical 200pF doorknob cap. has 
approx. the same amount of L.  .  I don't know of anyone who has tried 
this approach.  
>> 
>> Another thing that worries me about this parasitics debate:
>> 1. If certain tubes have a tendency to take off at low VHF freqs, ~110
>> MHz, what about 28 x 4 = 112 MHz, 21 x 5 = 105 MHz, 14 x 8 = 112 MHz, etc?
>> The point is, if there is a tendency to oscillation out of band in a
>> linear, a sniff of unwanted harmonics would drive it over the edge. 
>
>Oscillators don't work that way. They either have enough feedback 
>to overcome loss and they oscillate, or they don't. 
>
amen.  Harmonics have nothing to do with vhf parasitics.  

>Now you can vary the loss or gain by adjusting controls or 
>voltages. You might have a loose connection that, when you 
>smack the thing, a connection breaks and the oscillation starts.

Any current transient in the anode's VHF-resonant circuit, produces 
damped-wave ringing.  I have heard dozens of reports where a big-bang was 
heard when the ZSAC switched on or switched off -- with no RF being 
applied.  
>
>You can have heat related gain or loss changes, but once 
>regeneration exceeds loss the thing oscillates. If loss exceeds 
>regeneration, it won't.
>
>"Banging" it with a harmonic is certainly not any requirement to 
>start an oscillation, especially since a typical HF PA has a shorter 
>than 360 degree conduction angle and is thus rich in harmonics all 
>by itself even with pure sinewave drive.
>
>
>
>> This could explain why there are thousands of unmodified, stock SB 220's out
>> there, which have never hiccupped for years - the drive has low harmonics.
>
>No, they don't "hiccup" because they don't have gassy tube, or 
>because the builder tunes them correctly, or because the exciter 
>doesn't have 3x normal power in leading-edge transients that 
>overdrive the PA and cause excessive RF voltages in the tank.

SOS

>Sometimes a bad antenna connection will trigger an arc.
>
True.  Always mix something that is true with shenanigans.  

>One thing we can be sure of, the LAST thing that will trigger an arc 
>is a harmonic or any frequency far above the operating frequency.
>
agreed

>If we have an ounce of knowledge about components and networks 
>we know the impedance looking across the plate capacitor is less 
>on the second harmonic and higher, and so any voltages developed 
>there will be less as frequency is increased...not more.
>
What about resonances in the Tune-C?  As I recall, you measured a 
resonance in the AL-80 Tune-C that was c. 170MHz.  

>By an overwhelming amount the largest problem with arcing occurs 
>because of fundamental frequency voltages, not higher frequencies. 

When a Tune-C intermittently arcs how can one determine what the 
frequency of the arc was?
>........
Cheers, Tom.  Welcome back to AMPS.   Things were semi-boring during your 
absense.  

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