An Ultralinear Guitar Amp
copyright 2001 by Scott Swartz, all rights reserved
This page will describe an amp design that utilizes an output stage arrangement known as ultralinear that originated in early 50's tube hifi equipment. Why, you ask, should you be interested in a tube hifi concept? Does smoother distortion interest you? First, the engineering details, yes I know a lot of you are already scanning down the page to get to the schematics.
Ultralinear refers to supplying the screens of the pentode power tubes from taps on the output transformer primary, typically at about 40% of the half winding. This causes the screen voltage to move in concert with the plate voltage, but at a reduced magnitude. This creates a mode of operation which is essentially a compromise between pentode operation, where the screen voltage is held constant, and triode connection where the plate and screen are tied together and move together. The advantages of pentode (higher power output) and triode (better damping, lower order harmonics) are also compromised, with UL having only a little less power than pentode and a little less damping than triode.
For a guitar amp, extremely low damping isn't required anyway, so UL effectively trades off a little power for lower order harmonic distortion.
If you have experimented with operating a pentode guitar amp output stage without feedback and liked the smooth transition to distortion but found the bass to be too "woofy", the UL output stage will address that problem.
Preamp and Phase Inverter
I used the standard Bassman style preamp, with some slight tweaking of values. The interesting thing is I used octal tubes, like 12SL7, etc. The octal tubes definitely sound different than the more normal 12AX7 9 pin tubes, tending to give a bigger, smoother tone.
The phase inverter is the standard long tailed pair as used on most guitar amps, but implemented with an octal 6SC7. Note that there is no feedback from the output stage, as stated above the UL connection tightens the bass enough that feedback is not required.
See the Schematics
These are scans of hand drawn schematics, but what do want for free?
Page 1 - Power Supply
Page 2 - Preamp
Page 3 - Phase Inverter and Output Stage