... 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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