> 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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