[Phil Allison[_4_]]

Scott Dorsey wrote:

Phil Allison

** The THD figures published for a pair of beam pentodes
( like KT66s ) in UL mode are around 2% near full output.
This is much higher than a decent output transformer using
grain oriented steel, except at the very low end of the
frequency range.


Yes, but that is distortion that can almost completely be controlled with
negative feedback, until you get to the last edge of the envelope.



** In order to get a signal usable for NFB, a transformer must first combine the outputs of each tube in a push-pull pair.

When new tube types are tested in the lab, conditions are optimised by using a driver transformer to split the phase for each grid and an oversize output transformer with many primary and secondary tapings. The lab's B+ supply has plenty of capacity, very low hum and adjustable voltages.

DC bias voltage, if needed, is applied to the CT of the secondary of the driver tranny. Plus the test need only be done at 400Hz or 1kHz.

Doing all this eliminates extra distortion or mismatch coming from driver tubes, voltage sag in the PSU or power loss in the output tranny, allowing the best load impedance and bias setting to be found - consistent with the tube's current and dissipation limitations. These numbers are then published..

A tube amp designer has the more difficult task of producing a marketable product using practical size transformers, multiple driver and input tubes plus performs equally well across the whole audio band.



.... Phil