www.sopht.ca
Jeff Duntemann’s 12Volt tube page
The Sopht Amp was proposed on the AX84 site by Stephane. His first post was a preliminary schematic for a low 
volume amp that he requested comment upon. Being very close to the one year point of picking up a soldering iron and having focused on low volume/ low voltage amps, I felt it would be a fine project. I was aware of the “space charge” tubes, but had never tried them. Within days I ordered the tubes and began learning more about their specs. I created a couple of spice models to be used to model the amp before I built it – this something I really enjoy, regardless of its accuracy. Any comment/suggestions to improve the tube model(s) is appreciated.
Some of the things that attracted me to this amp as well as low voltage tube amp design.
- Amp can easily be brought up on batteries during the initial wiring tests (cheap)
- Can use off the shelf power supplies- from wall warts to old PC PSU units (free)
- Low tolerance requirement of the components make it easy to scrounge parts from junk equipment (free)
- Can use low cost 70V line transformer for OT (cheap)
- Something different! (-huh?)
My Design Goals
- Low gain amp with interesting high-mid and treble breakup(as heard on Steph’s samples)
- Try SC tubes-Are they any good?
- Attempt to spice model with higher accuracy
- Build inside a SCSI drive enclosure (free)
- Plan to use amp for close mic’d recording and headphone practice (w/ effects pre and post)
Below is my version of the amp (06/10/04) with rollover, showing voltages. I like the sound, not sure if my variation brings anything drastic to the amp. I added a switch to change the values of R2, R6 and C3 as these appear to change frequency cutoff (shown on the simulation page). Currently I have the RS70v OT connected to reflect 2.4k back to the 12k5, this sounded pretty good to me on the first tests, I haven’t gone back and retried the 600ohm tap which the tube is supposed to like (yet). (09/14/04) went back and tried a few different taps the 689Ohm tap seemed to be a bit louder as well as tighter sounding overall so that is what I’m using now.
