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[AMPS] 3-500Z emission saturation

To: <amps@contesting.com>
Subject: [AMPS] 3-500Z emission saturation
From: 2@vc.net (2)
Date: Wed, 10 Oct 2001 05:21:58 -0700
>
>> I did not think, or say, it was  I don't know why you brought Orwell
>> into it.. You asked if I had read it- I said yes. If you insist on
>> asking silly questions, at least take the answer at face value. > >// 
>>  3-500Z manufacturers say 400mA.  In my experiences, a fresh 3-500Z
>> begins to saturate c. 420mA.  However, one amplifier "expert"says
>> tuning >up at 550mA is fb and he claims to have a letter from Eimac
>> saying such.  > He has not produced the letter.  >My guess is that
>> this letter is unsliced bologna.  
>> 
>> Like the references (quotes) I have asked you about a couple of times
>> recently?
>
>Anyone who claims a 3-500Z starts to "saturate" at "c. 420 mA" 
>clearly has no idea what he is saying, or is intentionally giving 
>misinformation.
>
//  420mA was the average current measured.   The corresponding peak 
anode current would have been c. 1.25a.  

>Thoriated tungsten tubes typically begin to show emission limits at 
>50-100 mA per watt at 1900 degrees C, 

//   In Care and Feeding ..., Eimac states that the temp is c. 1820ºK.   
, 

>which places a 15 ampere 
>5 volt filament at 3.75-7.5 amperes for the beginning point of 
>emission limiting. (source Vacuum Tube Electron Emission, 
>Electronic Designers Handbook 2nd edition section 9-9). 
>
//  According to the 3-500Z constant current curves, peak currents above 
1.2 amperes are progressively non-linear.  

>The actual fully saturated emission of 3-500Z, where no more 
>current can be draw no matter what the applied voltages are, is in 
>the order of 11 amperes.

// Agreed, however, this has nothing to do with linear operation.  
>
>You can easily understand the "game" when you read step 13 of 
>the AL80A manual which is either ignored or misquoted. Step 13 
>says "In CW operation drive power is reduced until plate current is 
>400mA or less."
>
//  Surely.  The rub is that a tube that is tuned in its not-linear 
region is hardly going to be linear when it not readjusted for operation 
in the linear region.  

>1.) On one hand, claims are made by a certain person a parasitic 
>can cause enough anode current to "bend filament wires from 
>magnetic fields". 
>
//  See photograph of bent 3-500Z filament helices on p.15 of "Parasitics 
Revisited" in the September. 1990 *QST*.  

>2.) On the other hand, the same person claims he "starts to see 
>current saturation at 420 mA".
>
//  This was in an SB-1000 with a low-hrs. tube.  Mr. Rauch has not yet 
published the alleged Eimac letter authorizing 500+mA for the 3-500Z.

>GOOD tuning instructions tell a person to over-couple the PA 
>(which reduces gain, reduces peak tank voltages, and improves 
>linearity).
>
//  However, a well tuned up 3-500Z is already c. 8db cleaner than the 
avg. transceiver, so no one is likely to notice a couple of db 
improvement in amplifier IMD.  

>POOR tuning instructions tell a person to load the PA only to the 
>expected maximum average current, which virtually ensures 
>excessive tank voltages and non-linearity on peaks.

To: <amps@contesting.com>
//  Date: 8 Nov 1996 05:45:01 GMT
 ...
(Tom Rauch):
>"And then I wrote the RF tank voltage can exceed the dc supply voltage by
> several times when the PA is unloaded. ,,,"

In a SB-220, Anode supply potential is c. 2900VDC.  3 x 2900V= 8700V. 
Normal tank potential at the Tune-C is c. 2700V-pk.  During a worst case 
scenario, this potential is c.  3600V-pk.   At sea-level, the Tune-C's 
breakdown potential is c.3700v.   

>73, Tom W8JI
>W8JI@contesting.com 
>
cheers

-  R. L. Measures, 805.386.3734, AG6K, www.vcnet.com/measures.  
end


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