[AMPS] Re: Parasitics

[km1h@juno.com]

>
>>It's a VERY simple job to break out a VHF signal source, drive the 
>>output port backwards, and measure RF voltages through the tank. 

I disagree that that is a legitimate test Tom. If as you claim, the Load
C is a VHF short then you will not develop the tank circuit voltages
further back into the amp. 

>>
>>It's only a tad more complex to add a few thousand ohm resistance, 
>>and drive the anode terminal of the cold tube while measuring 
>>voltages in the tank. It would take less time to do this than has 
>>been spent on soapboxs postulating all sorts of nonsense about 
>>extreme VHF voltages causing arcs, and VHF oscillations being load 
>>impedance sensitive.

Been there, done that and they do exist. But at signal generator levels
and modest test equipment the indications can be missed. 

>
>Nonsense?  Was it nonsense that caused this AL-82 bandswitch to arc 
>for 
>no apparent reason?:  
>http://www.vcnet.com/measures/AL82.tbs.JPEG.  
>The bandswitch is interesting to me because the arc was so hot that it 
>
>caused the  (alumina) ceramic to heat so rapidly that it fractured, 
>and, 
>it caused shiny spots on the ceramic surface due to melting.  (switch 
>courtesy of Randy Boyle, KT2D)


The AL-82 is a classic example of a switch eating amp. Perhaps Tom would
care to tell us the number of replacement switches that Ameritron has
supplied, free and for a price.  I hear that lots of switches are sent
free, long after warranty on many of Ameritron's models.

>>
>>I always make this type of measurement (using a sweep generator and 
>>RF detector in a network analyzer) in the normal course of 
>>prototyping new PA's. The voltage (transmission loss) smoothly and 
>>rapidly decreases as the probe is moved through the anode system 
>>towards the tuning cap. By the time the loading cap is reached, 
>>nothing measurable is left. 
>>
>>The anode would have to swing tens of kilovolts at VHF and drive the 
>>system with many amperes of current at the same time to make the ten 
>>meter tap on the bandswitch arc at VHF.

The 10M tap is rarely the failure. 20 and 40M are very common arc points.
Tom has claimed 6KV or so for the modified JV style switch, yet several
arc paths I have examined here are contact to mounting screw....a bit
over 6KV  say what ?? Other paths vaporize two adjacent contacts which
tells me there was extreme voltage in the tank.  Old age and other
excuses dont fly, two of the AL-82 amps I worked on had under 20 hours of
use.

Is the AL-82 a VHF problem...dont know, never measured it. Is the SB-220,
TL-922, SB-230...I believe it is at times.

>
>The measured breakdown potential in a typical SB-220, TL-922, AL-80, 
>AL-82, on and on,  bandswitch is about 6000v.  
>>
>>It's bad enough science to claim a bandswitch or tuning capacitor can 
>
>>arc from a VHF parasitic far removed from the operating frequency, 
>and 
>>worse yet to claim the VHF impedance at the output port has anything 
>>to do with VHF instability.

We agree there Tom.


>>
>>For goodness sakes, common sense tells us that if the VHF energy 
>>could freely pass through the tank, the amplifier would be a harmonic 
>
>>disaster. It would never pass any sort of harmonic level testing. 

Agreed again...but since there is general agreement that the Tune C makes
for a lousy VHF ground path at times then all we have is the
coil/switch/load cap. That 50dB of harmonic attenuation has to go
somewhere on its way to ground thru the Load cap or the antenna port
load.

So Rich  AND Tom, if you two can can the perennial insults and come up
with a plausible explanation I and others are all ears.

73  Carl  KM1H

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