Then the output stage was designed separately. The basic concept here was to get as rich a class AB as possible without sacrificing too much output power and avoiding gm doubling effects.

What Bob wanted was as linear an overall design as possible and avoidance of the "too fat" tube sound typical of many amplifiers. In this amp the objective was achieved.

A small amount of global feedback is added to correct for the natural 3rd harmonic content in all pushpull designs. The result is excellent bandwidth and low output impedance. These were already excellent before the application of feedback, so the feedback's work is easy and not much is needed.

Using the 6U8/6GH8 as an input tube makes the front end performace sharply dependant upon operating point. Therefore the front end tube has adjustable bias. This also permits a range of tonalities as well as usage of a wide variety of tubes of that type. They need not be matched.

Also provided is a bias check point for the output tubes. This amounts to no less than a simple tester for checking 6BX7's for their sum cathode current in order that a matched pair can be put into a channel. Critical matching is not required for this amp however, as each 6BX7 has its own independant self-bias resistor. This lends to better natural DC balance in the output transformers without the need for critically matched tubes.

Tube complement: 5AR4, 4x 6BX7, 2 x 6U8

Here's what Bob had to say about it:

"I spent an hour or so listening to my new amp through the Lowther MLTL's. Hugely deep sound stage and very black background. None of the normal tube fatness, but the SS hint of glassiness is gone. Very good. Bass is very full and impressive. Drives the Lowthers deeper than I remember. I hear stuff in the 20's on the Widor track."

Thanks, Bob.