In this video and at the end of this description I document all of the differences in two original Fender Tweed Bassman amps, 1959 and 1960, as compared to the old schematic. They were relatively untouched when I purchased them so all of the repairs and changes have been done by me. The 1959 was particularly clean and was a great reference point. Recently a major amp builder in Texas was inquiring about particular sections of the circuit because he looks at this 1960 as a good benchmark for the classic Bassman tone. Coincidentally, he opened my eyes to all of the inconsistencies between my amps and the old 5F6-A schematic.
Quote from "Fender, The Sound Heard 'Round the World" - Dick Dale says: " He (Leo Fender) was constantly tweaking the designs, oh God, back and forth, back and forth. Boy, he'd call me up and say, 'Dick, come on down, I got something . . . try this . . . let's see how this works.' So, he always fine-tuned. He never settled. After tinkering in the lab, Leo would take his new idea to a worker wiring chassis in the plant. Without telling anyone else, he would have her change the value of a component, sometimes in the middle of a production run. Forrest White recalled his confusion one day when a befuddled final inspector noticed units not meeting an important specification. White soon discovered the mystery resulted from Leo's creative tampering upstream. Fed up with such mischief and disruptions, White posted a sign mandating that no one make changes in circuits without his written permission."
Here are the differences from the old 5F6-A schematic which have actually been updated in the newer '59 Reissue schematic. The new schematic confirms my assertion that the old schematic was never updated with the changes they made in the factory. I had read somewhere when Fender was working on the design of the Reissue, they went through all of the circuit iterations on file to find the best sounding one and settled on the circuit that is exactly like my 1960 Bassman. I had also read somewhere or saw in a video where they found their favorite Bassman amp and copied the actual circuit in the amp. Either way, they settled on the exact circuit that is in this 1960 Bassman which is so popular on YouTube.
Here are most of the differences and inconsistencies between the real deal and the old schematic:
Heater wire: 1959 is 18AWG solid core wire throughout, 1960 has 20AWG solid core wire throughout
Both amps have 100K slope resistors, schematic shows 56K
Coupling cap going from slope resistor to Treble pot shows .02uF/400v on the schematic but is .1uf/400v on both amps
Schematic shows 1M Audio taper pots on both volumes, both amps have 1M Linear taper pots
Bass pot on the schematic shows 1M Audio: 1960 has 1M Audio, 1959 has 250K Linear.
Middle pot on the schematic is 25K Linear: 1959 has 25K Linear, 1960 has 50K Linear with 56K resistor soldered from right lug to ground.
Presence pot on the schematic is 5K Linear: 1959 has 25K Linear with 4.7K resistor soldered from right lug to ground, 1960 has 50K Linear with 4.7K resistor soldered from right lug to ground.
Tail resistor in phase inverter section is 10K 1 watt on the schematic: 1959 has a 10K 1 watt resistor, 1960 has a 6.8K 1 watt resistor.
Electrolytic Filtering Capacitors:
1959 and 1960:
B+: Two 220uF/350V Kendrick caps in series bridged with two 220K resistors for a final value of 110uF/700V rating
Screens: 40uF/500V Kendrick cap
Phase Inverter: 20uF/500V Sprague Atom
Preamp: 8uF/450V Sprague Atom
B+: Two 20uF/600V caps in parallel for a final value of 40uF/600V rating-600V Sprague Atoms don't fit so everyone uses 500V now
Screens: 20uF/600V cap-use 500V now
Phase Inverter: 20uF/600V cap-use 500V now
Preamp: 8uF/450V cap
'59 Reissue Schematic:
B+: Two 100uF/350V caps in series bridged with two 220K resistors for a final value of 50uF/700V rating
Screens: Two 47uF/350V caps in series bridged with two 220K resistors for a final value of 23.5uF/700v rating
Phase Inverter: 22uF/500V cap
Preamp: Two 22uF/500V caps in parallel for a final value of 44uF/500V rating. Note: Gerald Weber says this should remain at 8uF/450V. Too high will affect tone in the preamp tubes.
This is all I have found so far. Hopefully that is it. I should note that all of the date codes on all of the pots are correct and accurate. A lot of them on the 1959 are 921 or the 21st week of 1959 and the 1960 has 6004 which is the 4th week of 1960. So it looks like these amps were built maybe less than a year apart or around 35 weeks approximately. Serial numbers are BM02734 (1959) & BM03333 (1960).
As soon as I get the 6.8K 1 watt resistor for the 1959, I will try to do another soundcheck to see if we can notice a difference in tone. Honestly, to me the 1959 still sounds good but still everyone always votes for the 1960.