'68 Bassman: AB165 + 5F6-A

Hey folks,

On the chopping block is a '68 Fender Bassman head that I recently picked-up. As bought, it had been modified to the AA864 circuit with all new parts but was terribly hissy and would motorboat. (I wonder who did this handy-work...) So having no original components, I didn't hesitate to rip out the board and make something cool.

The '59 Fender Bassman is a true classic. It served as the model for tons of great guitar amps, most notably the first Marshalls. The '59 used the 5F6-A circuit. While mine won't have shared inputs on the first tube or a tube rectifier, I decided to put the 5F6-A circuit into my '68 Bassman as my Normal Channel.

The '68 Bassman came stock as an AB165 circuit. This circuit has some cool features but suffered from the Fender "improvements." Many people on the net talk about making a hybrid AB165 / AA864 (the circuit used prior to the AB165). I decided to use the hybrid as the model for my Bass Channel.

One cool thing about the AB165 is the added gain stage that really lets the amp sing. While it sounds cool and is very usable, I wanted to keep the 5F6-A circuit as close to the original. So, I combined the two circuits just before the Phase Inverter using a pair of 220k resistors.

The circuit layout would also have to be drastically changed. Thanks to DIY Layout Creator, I was able to design a layout that would work.

Pics: Schematic, Layout, Before Pics, After Pics