Here is Butler TDB 2150 scheme.

[Staykov]

Here is Butler TDB 2150 scheme.
http://upload8.postimage.org/501713/photo_hosting.html
http://www.butleraudio.com/tdb2150.php
tube heating is 6.3V, non on the scheme..
.. Is it a strange idea to use tubes that way....
What is the benefit for the sound???
Please, post your comments...

[TO-3]

BK uses his tubes in a non-conventional sense. He operates the tubes in the ohmic region using a lower plate voltage which dramatically increases the life of the tube. I will see if I can get him to comment for you.

[djQUAN]

This unique USA patented design reduces both Crossover and Total Harmonic Distortion (THD) figures to LESS than 0.15%. This achievement is almost unheard of in an open-ended, ZERO negative feedback high fidelity amplifier.

differential pair needs feedback right?

[Perry Babin]

The schematic shows global negative feedback but it appears to only be used for DC offset correction.

The differential pair only needs feedback if you want error correction for the signal (that's generally why they're used). Here, there is local feedback that insures low distortion from the VAS. The rest of the circuit consists of followers.

This is one of the few solid state amplifiers that would sound different than the run of the mill amp that uses global negative feedback. I wonder if Richard Clark would lose his $10K bet with this amp.