1969 Fender Bassman

The Fender Bassman was one of the very first designs that CBS put their hands on after their buyout of Leo Fender’s company. There are a lot of different schematic designs for the Bassman, maybe more so than any other Fender model. This is the first Silver-Panel model the AC568, which for 1968 and 1969 comes in the “Drip-Edge” aesthetics you see here. This is a transitional model, the tube chart still identifies it as the previous revision. The AC568 Bassman, like it shows in the other article I did on the AC568 Fender Twin Reverb, has a number of bizarre and undesirable changes to the circuit that many people (including this customer) prefer to have modded back to a previous model. This amp has an extremely odd mix of cathode and fixed bias with a power transformer putting out a significantly higher B+ voltage. This was not a great design and resulted in many returns and problems for Fender - they also just don’t sound as good as the other versions made before. The AC568 has a darker and more compressed tone that does not hold up well at high volumes. It by no means sounds bad as is, but it isn’t great either. The customer wanted to change this back to the previous revision model AB165 but with the standard AB763 adjustable bias control. Many people like to convert the AB165 itself back to AA864 specs because they are more valuable, and because the myth developed that the AA864 sounds much better. I will say that they certainly sound different but not better in my opinion. The earlier AA864 is cleaner and sounds more Black-Panel Fender-like, while the AB165 that we have converted to here has more mid-range, grind, and growl than any other 60’s or 70’s Fender Bassman. So I think it is a great pick especially when paired with the Trainwreck Style Type-3 Master Volume we also installed. If you are confused with all the model numbers thrown out here then join the club - even I have a hard time remembering all the revisions and what happened in each! Notable users include: Noel Gallagher, Mike McCready, Jack White, and Nick O’Malley.

This amp came in mostly original condition with the exception of someone having changed the coupling caps on the phase inverter plates to GE branded units from the 80’s. It needed a full restoration including a cap job, new power/misc. resistors, cleaning, and new tubes - along with the AB165 conversion and master volume addition.

Amp was completely restored and modded to AB165 specs. All filter/bias/bypass caps were replaced with upgraded voltage and temperature rated F+T/MOD brand electrolytic’s. New 2 Watt Metal Oxide power dropping resistors were installed for better reliability and lower noise floor. All plate resistors were replaced with 2 Watt Reduced Mass Metal Film type units for preventative maintenance and reliability. The cathode resistors for V4 were replaced with 2 Watt Metal Oxide/3 Watt Cement type units for preventative maintenance - these resistors are common failure points in most Fender designs due to being under-rated in wattage. All screen resistors were replaced with 3 Watt Cement 470 ohm type units for same reasoning. New vintage spec correct Carbon Comp 1/2 Watt resistors were installed for the bias and NFB circuits in the power amp section for the modification. All resistor values were chosen to make amp compliant with AB165 values. 1 ohm resistors were installed on pin 8 of the power tubes to provide an easy way to check the bias and was a convenient replacement for the cathode bias resistors previously in place. Two .022uf 500v SOZO brand film capacitors were used to mod phase inverter to AB165 spec and keep the tone as close as possible to the original "Blue Molded" coupling caps. A Trainwreck Style Type-3 Master Volume circuit was put in to replace the "Ground" switch, per customer request. A new 3AG Fuse was installed as the one present was the wrong value.

The Bias Supply was given my standard modification to be an Adjustable Fixed Bias control rather then the original Balance control - this allows the power tubes to be set to idle at the correct current/voltage.

The original tubes did not all test good and some were replaced. V1-V3 = RCA 7025, V4 = JAN/PHILLIPS 12AT7, V5-V6 = TAD 6L6WGC-STR Matched Pair. These tubes were picked for best tone, and reliability. The 6L6GC’s Fixed Bias was set to %62 Class AB Plate Dissipation with a B+ of 445VDC. The tube sockets were all treated with De-Oxit to remove/prevent corrosion. The pots were sprayed out and the amp was cleaned inside and out. Amp sounds great, with a lot of overdrive available, especially for a Fender.