The logic seems sort of sound, except:
If an amplifier is designed to work with 3-500Z tubes, that is it has
the bias set to the proper voltage for clean linear and efficient
operation with 3-500Z, and it has a output network designed to match the
plate impedance of 3-500Z operating at the plate voltage and bias
voltage the amplifier provides to the tubes, how can a tube that is also
a 3-500Z operate differently and be more efficient than a 3-500Z?
Seems like it is not exactly a 3-500Z, but rather a variant of a 3-500Z,
and if it is dissipating less power than a 3-500Z, I would be suspicious
that it actually needs a slightly different bias voltage to operate
properly. If it is dissipating less power, I would suspect that it is
operating more on the Class C side with current flowing in a smaller
portion of the RF cycle. I would wonder about the linearity.
DE N6KB
I
had seen the reviews saying that the SB-220 ran cooler after
installing the new tubes. Reading this casually, I sort of
disregarded it. But when I observed it for myself, I was impressed.
It is beginnng to make sense. The input power was about the same
before the new tubes. If the amp is 60% efficient now, it must be
dissipating about 670 watts. If the additional power I am now getting
out was previously being dissipated, the amp was dissipating 300 -400
watts more before or about a KW. That explains why it is running so
much cooler. If all this logic is sound, the efficiency with the old
tubes was only about 40%. Now that is a good reason to buy new tubes
even though only a dB or two power gain is realized.
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