Interesting Blackface Bassman repair

[Rhodesplyr]

A customer of mine brought in a 1966 Fender Bassman for repairs, the usual age-related things: electrolytic capacitors and a few other minor repairs.

It says AB165 on the tube chart, but once inside, I started noticing that things didn't match the schematic and layout. For example, it had adjustable bias, rather than hum balance, definitely factory stock from the oxidation on the solder joints. It turns out to be a hybrid of the AA165 and AB165 circuits. It has the AB165 phase inverter, but lacks the 220k NFB resistors.

How unusual is it to run across Fender amps that came from the factory with elements of two different versions mixed on the same amp?

[Enzo]

Not unusual at all.

[Tom Phillips]

+1
One could write a nice little book about the various Fender oddities that have graced the benches of the forum members.

[Rhodesplyr]

In the end, after some discussion, the owner said he wanted the PI changed to the AA864/AA165 circuit, so I did that, along with all the other 46 year maintenance issues. So it ends up an AB165 with adjustable bias and the older PI circuit. I loved the result, and I'm not even a real guitar player. The owner loves it, too.

For me, personally, Bassmans are some of my favorite Fender amps.