This may very well be my favorite rare amp. The absolutely beautiful sounding Ampeg Dolphin. Model number 518. I'm becoming a bigger fan of the early Ampeg amps the more I get to see them. Here in NY I see more oddball Ampegs such as this probably cause they were built nearby. Go to Michaels Music in Freeport, Long Island if you want to see and buy some rarities. He's often loaded with great old Ampeg product.
Here's a link to his page: http://www.michaelsmusic.com/
Ok, just click next to the word page! I know it's invisible. I work on old amps that should be in the landfill, not on computers so bear with me!!!
Anyway, I couldn't find a schematic for this amp. The unusual bits are it was originally loaded with the famous Mullard EL-37 tubes. You can sub them out with common 6L6 tubes though. These amps are cathode biased so you don't need to fiddle with that, just drop them in and gig away.
Also the original preamp tube is a 6CG7. This amp had corrections from the factory:
The 6CG7 tube was actually a 12BH7 in this case, and my eyes and experience says it came wired from the factory this way even though it says otherwise on the chart. Cool thing is, I can just drop in whatever 12A tube I like. 12AT7, 12AX7 or 12AU7 which is what I chose and my customer is way happy with his sound. The 12AX7 made this amp buck like a wild horse!
I found a catalog picture of this amp from 1960 and the first preamp tube was listed as a 6SN7. They were obviously experimenting greatly at this point in the company history. The 6SN7 is a low/medium mu dual triode with an amplification factor of 20 just like a 12AU7.
This amp has a very old school way of construction. The preamp and the power amp are separate. This allows for the preamp to have minimal noise entering it from the power supply. The only AC entering the preamp is the filament voltage. You could easily convert that to DC but why bother? When I finished the re-cap this amp became dead quiet!
Shots of the power amp:
And shots of the preamp:
Notice the beautiful old ceramic Jensen speaker! I don't know if it's original to this amp but I don't care either. It's a terrific match! I repaired a hole in the cone. There was one other repair from long ago as well. Sounds heavenly!
One of the fun facts about these amps is the power amp was possibly a copy of an Acrosound high fidelity schematic. I know this cause the first amp I built was this one:
I've built this amp using 6L6 tubes and 2A3 tubes as well. Great circuit. Ampegs have a lovely midrange to them, a bit fuller than a Fender. This I really like. Later Ampeg amps become too mid heavy for me but these early ones are right up there with my favorite Fenders and Gibsons of the day. I hate to use the word 'undervalued' but these amps, well, they aren't quite as sought after, and that's not a bad thing for those who dare to be different! One sold recently on Reverb for $495. My client who bought this paid $80 on Craigslist! Yeah, needed a lot of work but still, that's a lot of wonderful sound for the money!
Here's one last shot of the super cool "Jet Age" styled control panel:
Volume, treble, bass and 'ultra high' for you to enjoy the jet stream with!
While I certainly don't need another amp, if I built anything new I may follow this path. No one else is and I think this is a great amp for a jazz player who likes a little bit of hair to their tone! I'm starting to hear more jazzers play through amps such as the Blues Jr. by Fender and it makes me really happy after nearly 45 years of Polytone and Roland Jazz Chorus blandness. Acoustic instruments such as my roommates oud would absolutely kill through this thing!