The Virtues of Power Tube Matching
Some amplifiers don't have enough range on the bias pots to handle the range of tubes so that selection must be made from a particular part of the curve to achieve bias.
In the "good old days," I'm told, the spread of values was much less, allowing reasonable performance with random selection.
Today, however, random selection is certainly not recommended.
Tube Improvements: Emmission Labs
These tubes are full-wave rectifiers of a special kind, and electrically equivalent to the original
Western Electric historical tubes.
The series connection makes the middle of the filaments to be virtually grounded, when using a
filament supply winding with a center tap.
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