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[AMPS] Setting the record straight--Dick Ehrhorn

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Subject: [AMPS] Setting the record straight--Dick Ehrhorn
From: (2)
Date: Sat, 21 Jul 2001 11:38:59 -0700
>> Building a stable amplifier that has high gain becomes increasingly
>> difficult the higher ones goes in frequency.  Things such as layout
>> issues, bypassing and genuine parasitics become critical.  When I say
>> "genuine parasitics" I don't mean the kind Rich talks about.  I am talking
>> about the inherent, real world, stray inductances and capacitances in
>> components.
>The real thing that separates amps are the tubes. Tubes used at 
>VHF and UHF typically have almost zero length grid leads because 
>they are brought to a ring or flange. 

/\  A 1-inch [25.4mm] long bar of silver has c. 11nH of inductance.  
Above 100MHz, 11nH is nothing to sneeze at.  

>If you look at tubes that are 
>problematic for VHF stability, they are tubes with long skinny grid 
>(and sometimes anode) leads and long grid structures inside the 
/\   this is generally true.

>Tubes that are problems include  572B's, 811A's, 4-1000A's, etc.
/\  So why do g-g 813s virtually never oscillate?

>Tubes that are almost unconditionally stable when installed 
>correctly include 8877's, 

/\  "almost" is right.  See Figure 24 on my Web site.

> 4CX250's, 

/\  seldom

> 3CX3000's

/\  chortle.  

>  3CX800's, 

/\ both of the 3CX800A7s that I tested had gold-sputtering.

>3CX1200Z7's, etc.
/\  Very low feedback C.

>Some tubes are in the middle, like 3-500Z's and 3CX1200A and D7. 
/\  The 3CX1200A7 has a relatively large amount of feedback-C. (? 0.8 pF)

>If you want an easy to stabilize HF amp, use a compact tube with 
>a grid flange. If you want a tube that is a problem, use a 4-1000A or 
>The problem is virtually always, for the size of tubes we deal with, 
>the lead from the anode...out of the envelope...and to the plate 
>tuning capacitor 

/\  true enough

> and the length of the grid lead from the grid to the 
>grounding point and then through the sheet metal back to the 
>tuning capacitor ground. And no, you can not evaluate that path 
>with a grid dip meter. That is total nonsense, because it is an 
>impedance problem not a resonance or Q problem.

/\  "total nonsense" ?   According to G.E. engineer,  G. W. Fyler,  in 
''Parasites In Transmitters'', Institute of Radio Engineers journal. 
Sept. 1935, resonances support parasites.  Are dipmeters useful for 
spotting resonances?
>VHF amplifiers live happily and are totally stable with anode Q's in 
>the hundreds, because the grid is simply resonant far above the 
>frequency where anode impedance increases. 

/\  A UHF resonant grid - such as the 8877's, 8874's, and 3CX800A7's -  
can undoubtedly support oscillations above the grid's resonant frequency. 

> grid Q isn't even the major issue.

/\  Indeed, Tom.  The major issue is the resonant freq.  of the grid 
>The shorter we make both those leads, the easier the amp is to 
>stabilize. The smaller the impedance of the suppressor can be, and 
>in many cases a suppressor isn't even needed. 

/\  The AL-1500 is the only HF amplifier that has ever made which has no 
VHF suppressor whatsoever.  I have spoken with a number of owners who 
have gone through  three 8877s during the 12-month warranty.  

> That's why no one 
>can offer a formula to calculate the proper size of a suppressor, 
>despite people who "claim" they can.
/\  ... an outstanding non-sequitur.  

>It's a shame this entire subject, and the topic of why tubes fail, has 
>been taken to the low level it is at, with failures caused by tube 
>manufacturing or operational problems blamed on parasitics. 

/\  The subject of gold-sputtering seems to leave Tom sputtering.  
 For a 14-month period in the late 1980s, Eimac had a heat dam problem 
with the 8877.    Mr. Rauch is seemingly still using this as an excuse.  

>But then I guess the reason so many people keep swallowing the bait 
>is it's easier to tell people it's almost always one single problem 
>and to give one magic cure for everything than to give an honest but 
>more complex answer.

/\  I have never claimed such.  However,  the approx. 1400 letters and 
telephone calls I received in the 10-months following the article on 
anode parasites in the October, 1988 issue of *QST* told me that the 
parasitic problem was more pervasive than I thought.  

Cheers, Tom

-  R. L. Measures, 805.386.3734,AG6K,  

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