Notes for further expansion:
During November 1954, Fender introduced the newly designed 5D6 Bassman amplifier offering four ten inch speakers and was designed utilitizing two rectifier tubes. The 5D6 was a major departure from the earlier 5B6 Fender Bassman model. Designed by Freddie Tavares, longtime R&D man at Fender, the new circuit included two rectifier tubve and became known as the Dual Rectifier Bassman. Instead of the single 15" speaker, four Jensen Alnico P10R speakers were used. The circuit had two innovations: a fixed bias for the power tubes, which increased power in comparison to the earlier cathode bias design, and a cathodyne phase inverter, using half of the 12AX7 tube and allowing a third gain stage on the other half.
The first 4x10 Bassman amplifiers started with a batch of prototypes in November and December 1954, model 5D6. No schematic for the 5D6 circuit has ever been found, but Ken Fox and Frank Roy have created a few from originals, and copies are freely available online.
From FB channel: "Tube Amplifier Repair and Diagnosis" https://www.facebook.com/photo.php?fbid=10213182387928017&set=pcb.1873993402675691&type=3&theater&ifg=1
AP: Uhm OK, if you want to count an amp of which only a couple of dozen were ever made, and only eleven are known to exist today. The 5D6 is more of a unicorn than an amp. I am fairly confidant this isn't a 5D6.
RT3: I own a 5D6-A and a 5E6, why can't it be one of those?
AP: I thought the E6 went back to a single driver. My mistake. You own one of the eleven ~200 5D6-As ever built?
RT3: No, the 5D6 was the 1954 prototype of which only 200 were made. 100 in November of 54 and 100 in December of 54. The 5D6-A was the 1955 model these go up for sale periodically, in fact you can find them on eBay or Gary's vintage guitars for around $10k. The 5E6 is the 1956 model.
AP: My memory was there were about 180 5D6s and abut 200 5D6As. Live and learn.
There appears to have been several different "Bassman" circuit topologies:
- 5B6 (1)
During 1952, the Fender 5B6 Bassman amplifier was introduced as a combo amplifier cabinet that included the amplifier chassis combined with one 15" speaker. The 1952–1954 5B6 Bassman amplifiers had two 6SC7 or 6SL7GT pre-amp tubes, two 5881 power tubes and a single 5U4G rectifier tube. It was designed to generate 26 watts at an 8 ohm impedance load, and offered a cathode-based bias.
From 1952 through the spring of 1954, Fender produced approximately 660 model 5B6 Bassman amplifiers (serial numbers #0001–0660). The earlier cabinets have been called "TV Front" designs, with a front panel that had a rectangular grill cloth with rounded corners and looked much like a television of that era. In 1953 the cabinet designs were changed to the so-called "Wide Panel" design, with a 5 inch wide tweed covered panel above and below a wider swath of grill cloth. Fender ceased production of 5B6 Bassman amplifiers during the spring of 1954.
November 1954 through December 1954 (2)
- 5D6 - Only 173 built. Only 11 still exist. Only 4 are still "vintage".
January 1955 - March 1955 (2)
- 5D6A - around 200 built
March 1955 - 1957
- 5E6 - 2x 5U4 Dual Rectifiers
1957 - 1960 (?)
These all need to be researched and for the information to be posted here...