[AMPS] 91 information

Wed, 10 Sep 1997 08:45:27 +0000

> From:          Rich Measures <measures@vc.net>

> ... 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.  

Here we go again. Never saw the problem, never owned the amp, 
never tested the amp. 

> -  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.'  

.....and without even seeing a PA, we now have another "cure" for 
all the owners of 91B's who have lost a tube for whatever reason 
might have happen in one case.

73, Tom W8JI

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