Modified AB763 Super Reverb

[TheTinMan]

If you do not frequent the Hoffman forum, you may be interested in my latest build. If you've already seen it, my apologies. This is my fourth baby and she turned out pretty well.

I built this amp for a friend. After playing with my first Princeton Reverb clone, he asked me to clone his son's Blackface Super Reverb. Seems his son wanted his amp back :huh: My friend Frank is in his early '70s and plays in three bands: small classic jazz combo, bluegrass and blues jams. Sometimes his son plays harmonica through the amp at the same time Frank is playing blues.

Frank said he wanted the amp to be "clean and fat". We decided to keep the Vibrato channel virtually stock except for bias vary tremolo (Hoffman's approach). The Normal channel is intended to be like a Brown Vibroverb, including lower plate voltages and that peculiar tapped Treble pot. The Normal channel also has tremolo and reverb. The Bright switch is supposed to be a guitar/harmonica switch with less gain and much bigger cathode bypass caps when the switch is "off". We'll see how that works out in real life.

Specs are:
Mojo cab & chassis
Mercury Magnetics trannies (2, 4 & 8 ohm taps)
Weber 10A125 and 10F150-T (two each, all four 8 ohms)
Accutronics 4AB3C1B
Schematic attached

Tubes:
V1 - Penta Chinese 12AX7 (Normal)
V2 - ANOS GE 12AX7 (Vibrato)
V3 & V6 - NOS Mullard 12AT7 (reverb driver & phase inverter)
V4 - Tung Sol 12AX7 (reverb recovery & gain stage)
V5 - Electrol Harmonics 12AX7 (tremolo)
V7 & V8 - Svetlana "Winged C" 6L6GC
V9 - JJ GZ34

I have an NOS Rayethon 12AX7 that was supposed to be for the V1 spot but it seemed weak and a bit thin. Going through other available 12AX7s, this new Chinese tube just seemed to hit the sweet spot.

You can see the voltages HERE. V3 & V6 are 12AT7s... don't know how to edit that form.

Here's a SLIDESHOW

If you look carefully at the gut shot(s), you can figure out where I improvised along the way. The Normal channel guitar/harp (Bright) switch changed a lot. I also tweaked the Normal tone stack and ordered the wrong value cap - that's why there's a lonely .033uf Orange drop at the left end of the board. After prepping the chassis, I decided to put the screen resistors on the board instead of the tube sockets so they don't look as clean as I'd like. BTW shielding the leads to and from the Reverb pot really seemed to help reduce noise. Also, making sure the leads going from the board to the Speed and Intensity pots away from everything else keeps the tremolo signal only where it's supposed to be. (learned those details through two Princeton Reverb builds)

Special thanks go to Doug Hoffman for his forum and the generous guys over there, including Tubenit who I see over here as well.

Cheers,

Chip

[tubeswell]

That looks very neat chip. How do you make your faceplates? (if you don't mind me asking)

I just finished a (much cruder looking) bassman with reverb and 6G16 trem that I ended up "Hoffmaning" into a 6G9-B trem. What was your impression of the initial 6G16 trem? (I wonder if it sounds better in a 6G16?)

[TheTinMan]

I built the trem circuit exactly as shown on the schematic and didn't change a thing. It's Hoffman's approach with two tweaks for the 6L6s as noted in red. It works great for me.

The front faceplate came from Marsh Amps. It's some kind of relatively thick metal and the letter is white. Not quite as clean as I'd like but it'll do.

Jeannie at BNP Lasers did the backplate for me. She's a sweetheart, but won't do another conversion from one a Visio files again. Apparently it was a giant pain for her. I would work with her again in a heartbeat if I can just figure out how to convert into the right file type for her. The Amplates guy wouldn't put the words "super" and "reverb" together on a plate, despite the different layout and the obvious "Merry Prankster Musical Products" elsewhere on the backplate. No one could look at the backplate and think "it's a Fender!"

Cheers,

Chip