“The ST” (aka: Spud Too, Whispering Simpleton, The Simp)
This is my second scratch built amp (07/03). I was Inspired by Bob Stewarts “Spud” to make a simple single tube amp and found a couple BBS threads on AX84 regarding the 6BM8 triode/pentode tube that could make this possible. I used the same board mounted socket design as my first amp. This time I used the nice red board stock. The amp was debugged, with battery power then found a 35V wall wart that I could use for the B+ supply (while still using a lantern battery for the heaters). (06/04) Made a simple PSU out of 2 wall warts and DIN5 connector, moved the amp from the Premier phone base to a VHS cassette box.
One of the nice thing about designing this amp is 6BM8 spice models are available on Duncan’s amp site. I was able to model this amp more accurately than The Whisper which I just faked (Its a miracle that thing works!) This amp uses a $2.50 RS 70V line transformer as well as a Garnet tonestack that I saw on Adams Amps. I think I could tweak the cathode bypass capacitor and Pot value a bit to get a more useful range on the tone control. As it is it just rolls off the treble in the first 1/4 which is OK then acts as a volume knob from there, not so good. I suppose bypassing the pentode cathode could also help a bit, didn’t make much difference in spice though.
Impressions:
This amp has a good feel through my de-tuned 1X12 in my acoustically live living room, slightly quieter than The Whisper but with a little bit crisper sound. Still maybe slightly on the belchy side which to me makes it sound like a bigger amp. The amp has a nice rich growl when combine with “Mr. distortion” (my ROSS distortion box) . At first because of its crispness it wasn’t as useful for my low volume recording – headphone setup caused ear fatigue when played more than an hour or so.
(06/04)Changed input grid resistor, ear fatigue issue now gone, The amp has a rich clean and smooth round/plucky attack texture. With my reverb tank in front the sound can be sweet and clean. Overall volume is still really low. Not really suitable to play in the open air before midnight!
Note: This schematic is still not quite as built, I’m having a hard time finding specimens
…about the wacky symbols