'66 Fender Bassman Repair
December 2020
Here is a classic vintage Fender Bassman that came into the shop. This is a 'Blackface' version, and based on the power transformer code (1966), and the chassis serial number, it looks like it was built in 1966.
'66 Fender Bassman - Front
'66 Fender Bassman - Back
The tube chart indicates that this has the AB165 circuit, and the back plate tells us it is a CBS model (CBS models are labeled "Fender Musical Instruments", where as the pre CBS actual Fender models are labeled "Fender Electric Instruments").
'66 Fender Bassman - Tube Chart
'66 Fender Bassman - Back Panel Label
We tested the tubes, most of which look possibly original. At least the power tubes were labeled as Fender, and the preamp tubes labels were completely worn off. The phase inverter tube was a replacement. They actually all tested OK in the tube tester. Being 54 years old, we decided to pull the chassis and look for any obvious signs of burns or blown caps before powering it up. The layout of the circuit seems to verify that this is an AB165 circuit (Fender had a tendency to change the circuit, but continue to use old tube chart labels until they were used up before printing new ones).
'66 Fender Bassman - Chassis
The AB165 circuit had a slightly odd bias setup for the output tubes. Instead of the more common bias adjustment pot, which adjusted both of the tubes at the same time, one tube had a fixed bias setting via a resistor bridge, and the other tube's bias was actually adjustable. With this setup, the bias of the second tube could be adjusted to match the bias of the first tube, even if the tubes were not matched. However, the tube with the fixed bias could not be adjusted. Most Fender amps with adjustable bias adjust them both, and since it is common these days to use matched output tubes, this is considered usually more desirable. This circuit is easily modified to be setup as either version.
'66 Fender Bassman - Showing Bias Balance circuit, and Feedback circuit
The other controversial circuit detail that is not very common on the older Fenders is the negative feedback circuit between the output tubes and the phase inverter. This can also be fairly easily modded to remove this feature if desired.
It also looks like the output stage was modified by adding a 1 ohm resistor between the output tube cathodes and ground. More commonly on Fenders, the cathodes are tied directly to ground. This mod was probably added to allow more accurate measurements of each tubes bias setting, based on plate current, rather than grid bias voltage. One of the 1 ohm resistors, however, was created using 4 resistors in parallel, which is a bit messy, so we may clean that up or remove the mod. We use a device that plugs in between the tube and the socket to accurately measure the plate current, so this mod isn`t really neccessary.
At some point the power supply filter caps were replaced, along with a few cathode bypass caps, but other than that, everything looks pretty original.