It's a 150W bass amp. Preamp is very similar to ampeg B15 but with middle controls added. First channel has a switch for a passive mid scoop. Second channel has an active middle control where instead of inductors 12AT7 is used as a gyrator.
Power amp is a bit different from the usual stuff seen in guitar and bass amps. Cathode biased class AB power stage is in parallel with a Music Man style class B stage where transistors drive power tube cathodes. 6550, 6L6GC, or EL34 power tubes can be used. I measured 150W output power with 6550 tubes, 120W with 6L6GC's and 150W with EL34's.
The cab is made out of paulownia and the whole thing is very light for a 150W tube amp.
Impressive work! Looks nice, and some interesting solutions there.
I've just built a bass pre amp where one channel is a slightly modified B15. I made a mid shift control in mine, basically a pot instead of the resistor between bass and treble, but I don't get much joy out of that...
Been wondering a bit about that resistor from the bass pot wiper to ground, the 270K in your schematic. I left it out. What does it actually do, soundwise and as to the function of the EQ?
"Hey mister, turn it on, turn it up, turn me loose!"
With the 270k there the frequency response of the circuit is almost flat when both of the pots are in the middle. Without it there's a little bit more bass in the circuit.
Having a pot between the bass and treble pots only kind of works as a mid shift when both of the bass and treble are fully turned up.
It looks like B-52 and Peacemaker have been used for guitar amp names, so I shall name this amp B-36. Thanks for the suggestions.
TUBEDUDE wrote: ↑Wed Dec 26, 2018 2:14 pm
Love the use of toroids. Any fan in the case, or does it use passive cooling?
I wanted to make this amp as light as possible and toroidal transformers weigh a lot less than equivalent laminated core transformers would. No fan in the case. The cathode biased pair is idling at about 20W per tube and the class B pair is idling at 3.5W per tube, so at least at idle it's not super hot. Hopefully it's not going to get too hot in there when playing the amp on full blast for hours.
