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[AMPS] Viewstar PT1000

To: <>
Subject: [AMPS] Viewstar PT1000
From: (Rich Measures)
Date: Mon, 23 Mar 98 13:59:18 -0800
>>Good points, Jon.  .  It is my opinion that: at vhf, there is no load 
>>since the hf tank design used here is low pass.
>Actually, at VHF there IS a load.  The load is likely some very high 
>impedance (a lowpass filter will have high SWR and low transmission 
>characteristics in it's cutoff frequency range.).  It is still a load.  
>Even an open circuit is a load.

Agreed, Jon, however, the vhf load is not apparently not low enough to 
control vhf voltage excursions.   

>>  .  RE:  intermittency:  
>>Most of the parasite visitations I have observed occured during anode 
>>current transients, not during A-0 / keydown signal conditions.  To get 
>>one started it seemingly takes the right change of (anode) current or 
>>rapid sequence of changes, on a contest weekend.  .  not.  .  I test for 
>>them with a keyer sending dits at c. 50wpm, with and without QSK.  .  
>>Parasites can apparently occur during transistions between operate-cutoff 
>>as well as cutoff-operate bias states.  I am abundantly aware that upon 
>>first inspection, the possibility of an operate-cutoff change in bias 
>>state parasite sounds Ludicrous, however, if you are interested, I'll 
>>have a go at trying to explain how it might be possible.  
>I would be interested in hearing it.  

ok:    As the cathode bias switch contacts are opening, as cutoff bias is 
being applied,.anode current transistions from c. 100mA per tube (8877s, 
3-500Zs) to zero mA per tube   This current change rings the 
anode-resonant circuit.  If vhf regeneration begins via the anode-cathode 
feedback path, the decreasing anode current begins increasing because the 
cathode becomes driven.  However, the cathode bias switch contacts are 
trying to open as the instant the cathode is trying to draw current.    
If a metal vapour arc occurs across the opening contacts,  the tube does 
not reach cutoff bias for perhaps a few milliseconds, and a full-blown 
parasitic oscillation - a.k.a. "big bang" could occur.  Roughly 5% of the 
seemingly parasitic events I hear about occur in (no signal) bias 
transistion periods.    

>I can see that rapidly 
>transitioning between bias states could cause some ringing in the 
>circuitry which if not properly damped, could cause an oscillatory state. 


> I just don't understand it.  I guess at the transistion point from the 
>active to cutoff region of the tube, distortions are produced.  

Distortions are probably not a factor.  .   Getting from operate to 
cutoff takes time, ringing is taking place, unexplained things happen.  . 
  As I see it, one of the hurdles in understanding an amplifier diagram 
is coming to realize that some of the tuned circuits in the amplifier are 
not on the diagram.  

>I assume 
>that this would be a square wave type distortion similar to what you 
>would get trying to run SSB through a class C amp.  This distortion is 
>the biggest contributor to IM in a class AB amp.  However, since a square 
>wave is the sum of the fundamental plus all harmonics, you definitely 
>have VHF components in there.  

 A small VHF signal is generated whenever any type of current transient 
passes through a VHF-resonant circuit.  Since no signal is typically 
applied during bias transistion, distortion seems unlikely to be a 

>Perhaps of these VHF components are 
>generated fast enough (such as in high speed CW) the time constant of the 
>parasitic dampers might be too slow to sufficiently dampen the VHF 
>signals and since the tube still has lots of gain up there, an 
>oscillatory condition can get started.
I use high speed CW dits as bait for parasites.   However, sometimes a 
single bias transistion appears to have caused one.   
>However, with this guy in question, he wasn't sending 50 wpm CW.  He was 
>just tuning up.  How would that cause the oscillation?  

There are certainly more current transistions when sending 50wpm dits 
than when sending A-0, however, in a 3-500Z, during each 360 deg.  
driving cycle, the anode current changes from 0mA to c. 1200mA.  These 
changes cause some ringing to take place in the anode's vhf resonant 
circuit.  A feedback path exists between the 3-500Z's anode and cathode, 
especially above roughly 80MHz --i.e., the grounded-grid's resonance.  .  

>Could it be the 
>fact that the anode current was varied in just the right way or a certain 
>load and phase angle was seen at VHF that caused it to start.  


>I have 
>difficulty seeing a connection between what we talk about above and this 
>guys problem.  Help me make the connection.
Anode current changes were taking place.  Glitches occurred.  .  It might 
be interesting to high-pot. his Tune C.  
- later, Jon -


R. L. Measures, 805-386-3734, AG6K   

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