The Fender Bassman Blackface amp is arguably the most sought after tube amp. However, it’s not bass guitar players that covet this amp, but guitar players.
The amp’s circuit underwent three changes in the Blackface era from 1964 through early 1967: AA864, AA165 and AB165. (The AB165 circuit was a product of CBS’ ownership of Fender, and continued to be used in the Silverface era from 1967 through 1970.) The picture above is a 66 Bassman, with matching speaker cabinet. If you are lucky enough to own this pair, you are fortunate indeed, but if you own just the amp, you most certainly own a treasure. This series of blog posts will center on modifications to the
AB165 circuit, which is the most common one out there, and is contained in many Silverface Bassmans like the one pictured to the left. The Silverface era lasted from 1967 until 1977, with the AB165 circuit being used until early 1970. There were a half dozen other circuit variations in the SF amps, but many of these mods can be applied to their circuits as long as you carefully study them. The AB165 circuit contained a couple good points, but more bad points. CBS really got their hands into these circuits, and in a lot of respects ‘sterilized’ them. These mods that I will go over carefully, are for the most part considered upgrades, and do not diminish the value of your amp.
DO NOT MODIFY A RARE LEO FENDER AA864 CIRCUIT!! ALSO, DO NOT ATTEMPT TO WORK ON THESE AMPS UNLESS YOU KNOW WHAT YOU ARE DOING, AS THEY CONTAIN VOLTAGES THAT ARE LETHAL AND WILL KILL YOU!!!
That being said, there are a few things that you most probably should do right away, if your amp is not modified. First, you should replace all the electrolytic filter caps under the doghouse with new electrolytics of the same value. Second, you should REMOVE the ‘Death Cap’, which is wired across the ground switch. Third, you should upgrade the power cord to a three wire power cord with ground. Last, you should probably replace the ‘brown turd’ caps with Sprague ‘orange drops’ of the same values.
This first post in a series of Fender Bassman modifications will center on restoring the horrific mod that CBS did to the bias circuit, turning it into a ‘hum balance’ or ‘bias matching’ circuit. It may have been more important in days gone by when power tubes weren’t ‘matched’ as they are presently, when you purchase new tubes. When the tubes are matched, it’s more important to set the overall tube grid bias than it is to match what should already be matched.
Well, let’s get started. In this picture of a friend’s 1966 Bassman, (Jim Geikler, you should recognize this, heh) you see the area of concern circled in red. You can also see the ‘brown turd’ caps that I spoke of. So, we shall restore this awful attempt at a bias adjust circuit to Leo Fender’s bias circuit. In
this schematic, you can see that I have removed the two 10K resistors attached to both sides of the bias adjust pot, as well as the 220K resistor that provided the ‘fixed’ bias to the grid of the lower power tube. Then, the 220K 
resistor is moved, so that it supplies adjustable bias to BOTH power tube grids through both 220K resistors and the 1500 Ohm grid resistors. This layout shows you how easy this change actually is. Removing two components (the two 10K resistors), and moving
one end of the third component (the 220K resistor). This picture shows the finished product. I have circled the 220K resistor that was moved. I hope that this was easy enough to follow along and you now have a true Leo Fender designed grid bias circuit. (After this mod has been performed, you WILL need to measure and set the bias to the power tubes.) Stay tuned, as our next installment will focus on getting rid of that awful CBS-designed negative feedback loop in the power tubes’ circuit to help reduce the sterility and increase the gain a bit. 😉