1964 AMPEG JET J-12 A/B

This amp is a real classic and one of the great works of engineering done by Ampeg in it’s heyday. It needed a lot of help - All new electrolytic capacitors, all new film coupling caps to replace the paper in oil originals, 3 prong power cord, improved ground scheme, removal of death cap, sanding and cleaning of ground washers, cleaning of pots, new plate load resistors, new cathode resistors, new incandescent lamp, 2 NOS 6SL7 preamp tubes, and re-tensioned tube sockets. It was worth it, as it is easily one of the most character-full amps I have heard.

Tube lineup/topology is - 2 Octal 6SL7 preamp tubes, 2 6V6 power tubes, a 5Y3 rectifier, and bias-modulated tremolo. The Cathode Bias was set to idle at a conservative %86 Class AB Plate Dissipation with a B+ of 380VDC of in order to have a strong Tremolo intensity. With bias-modulated Tremolo, the hotter you set the bias the weaker the effect will become - the phenomena plagues many old amps that were setup incorrectly over the years by techs that don’t understand this balancing act.

This is a transitional model somewhere between the “A” and “B” versions - check out the schematic which shows 7591 power tubes. The tube chart however shows that 7591 was crossed out by hand and 6V6 written in. More quirks like that on this model with in-between values on resistors and caps too. Cool.