[Ian Iveson]

HiTech RedNeck:

In the conventional single ended tube audio output stage,
having a
conventional audio output transformer the ends of whose
primary is wired to
B+ and the output tube plate, and the ends of whose
secondary is wired to
the speaker (don't throw in any complications like
additional circuitry on
either side of the transformer; keep the tube in
conduction at all times,
i.e. class A operation; and provide an input such that the
output stays
within the rated frequency and power range of the speaker
and transformer),
WILL THE INSTANTANEOUS PLATE VOLTAGE EVER RISE ABOVE B+?
Or will it remain
in the range somewhere between B+ and zero volts, with a
midpoint
approximately halfway between the extremes, a midpoint
that depends on the
amplitude of the output signal?

But if it rises above B+, why? And then why don't tube
manuals reflect the
maximum instantaneous permissible plate voltage, but
rather what looks like
maximum quiescent voltage, when they give "absolute
maximum" figures for
plate voltage? (Yes, I know that the data for television
horizontal output
tubes generally give such instantaneous maximum plate
voltage information,
but these tubes cut off conduction in normal operation and
they normally
operate into an underdamped circuit which rings, i.e.
flyback, when the tube
cuts off.)

No experiment is necessary, because you know transformer
coupled SE amps work, and they wouldn't if Va couldn't rise
above HT.

Presumably you are aware that the voltage across a perfect
choke is proportional to the rate of change of current?
That's the same principle that allows a flyback circuit to
generate high voltages. So, a choke-loaded valve stage will
tend to generate whatever voltage is necessary to maintain
constant current through the choke, which consequently
appears to the anode to be a high impedance at audio
frequencies.

I think the common point for logic to fail the unwary is
that a transformer doesn't appear to behave like a choke. As
long as you have an appropriate load on the secondary, then
the primary presents that load, multiplied by the square of
the turns ratio, to the anode. So, if you connect an 8 ohm
resistor to the secondary, and the transformer impedance
ratio is given as 5,000:8, then the anode sees a 5 kohm
resistance. So now what generates the anode voltage when it
rises above HT?

Fortunately the answer is simple. Consider the load
presented by the primary to be that reflected resistance
**in parallel with** the transformer's primary inductance,
which is lots of Henries, usually. At frequencies above low
bass, the inductance passes very little current, and leads
to almost no power loss, but it's still there, doing what
chokes do.

That's why SE amplifiers typically have either no feedback
and sod all low bass, or plenty feedback and distorted bass,
or somewhere in between, because of their relatively low
primary inductance which shunts low F. Of course there is
more to it than that, but it's still a common trait.

Ian