[AMPS] 91 information

[Rich Measures]

... a QST...
-  Today, I received a telephone call from a friend in Wisconsin who 
recently purchased (and subsequently returned) an Alpha 91 amplifier.  He 
said that he experienced more than one tube failure in a short time of 
operation.  He asked me if I had heard of any problems with Svetlana 
tubes.  I replied that I had not.  
-  I began asking questions about the 91's circuitry.  The problem seems 
to be that Alpha connected an inductance between the grid-terminting 
resistor and the grid - in order to cancel some the grid's 
capacitive-reactance on the higher frequency bands, thereby improving the 
input SWR, which it does. ..... However, there is a trade-off.   This 
inductor exhibits a high-reactance at the anode circuit's VHF resonance.  
Thus, there is no low-Z shunt path between grid and ground at the anode 
resonance - - so the intrinsic VHF feedback path between the tetrode's 
anode and the grid has a straight-shot at an essentially VHF-open grid.  
-  A possible fix:  
* Connect the grid terminating resistor (GTR) directly to the grid using 
low-L copper foil.
* RF-ground the cold end of the GTR with a triple-bypass using roughly 
100pF, 1000pF and 0.02uF.  Use minimal lead lengths.  This method assures 
acceptable bypassing at 160m to10m AND the lower half of the VHF range.  
* Connect the bias supply to the bypassed end of the GTR.  

IMO, it is good engineering practice to provide a low-Z VHF shunt path to 
ground at the grid of any Class AB1 amplifier -- whether it be 
neutralized or not-neutralized.  Neutralization only works at the 
operating frequency.  It has NO effect at the VHF self-resonance of the 
anode circuit..  Thus, the primary VHF suppressor in Class AB1 amplifiers 
is the GTR.  ......... 'Never leave home without it.'  
Rich---

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


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