July 27, 2017
I finally got around to building a Supro amp. I recently heard a demo of the new Supro 1624T Dual Tone amp and was impressed so I started gathering info. I took a look at the original 1624T schematic and realized there was only one tone knob. So why was Supro calling it's new 1624 a "Dual Tone"? It didn't take much longer to realize that the new 1624 had some major differences with the original 1624. Things like an extra 12AX7, two tone controls, and linkable channels (probably where the "Dual Tone" name came from) made it obvious that this new amp was not a reissue clone of the original 1624T. But I was intrigued by the 6973 power tubes and also hooked on that demo, so here I go again. Besides, everyone wants to sound like Jimmy, right?
After more research I found that the S-6424 was identical to the 1624T except that it did have two tone controls. One more knob is a good thing, right? So I set about designing a board layout for the S-6424. I used the same ole Fender/Marshall design, tubes across the back, knobs on the front, and board down the middle. This layout has served me well for all my builds and should work well for this Supro too.
I used a Hammond AO-43 amp as the donor for the Supro. As it turns out, the power transformer and output transformer are a very good match for this circuit. Voltages are very close to the original and the OT impedance ratio (7128Ω:8Ω) works very well with the 6973 tubes. I followed the schematic exactly except for the inputs. I used a single input jack with a three position toggle switch to select either channel or both channels. And I used a 1M switch pot for the Intensity control because I could not find a 500K (works just fine). I also replaced the 250Ω cathode resistor on the output tubes with a 470Ω to lower the idle dissipation to 18 watts.
It's a success. This amp turned out pretty good. I'm calling it done. I've played the amp through several different speakers, Celestion, Weber, Jensen. My favorite is the Jensen P12N alnico. Still don't sound like Jimmy (haha). Maybe I need a Tele! The only thing I would do differently is wait until winter time to build it. July in Mobile is not a good time to do this stuff without airconditioning. Here are a few pics...
First a little chassis work...
Now to build the board...
Finally, wiring the chassis...
A few more pics of the completed amp. No cab for this project.
I did this short sound clip using my cell phone. The actual sound is brighter than the recording suggests... test.mp3 (1158KB)
And here's the schematic and layout... supro.pdf (932KB)