Short version:

The whole problem stemmed from the fact that the "B" series of tubes is not
the same as the "plain" and "A" series. The parasitic oscillations were
caused by the different bias requirements and by the "fixed" neutralization
of the driver and/or amplifier tube in the Motrac. There was no practical
way to change the circuitry to handle the "B" series tubes. Also, making
such a change would void the "type acceptance" of the units.

In a "practical" sense, it is "OK" to mix 6146 and 6146A tubes since the
primary difference is in the design of the heaters. But, NEVER mix 6146 /
6146A tubes with a 6146B! This is really "asking for trouble".

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