How many players reading this own or have played through a 5F6-A 4×10 Fender Bassman? A real vintage one or a high quality hand-wired clone not the current production Fender reissue. There is some magic in the 5F6-A circuit and even though it was designed as a bass amp it totally rocks for guitar! I for one love this design and do not think of this as a bass amp at all!
There are so many features and attributes that makes this a great guitar amp vs a great bass amp. First of all having 4-10″ speakers delivers a nice clear tone with plenty of bottom end for the guitar, but not too much. It seems that the bass frequencies start to roll off around 90 Hz on the Bassman. By coincidence the low E on an electric guitar is around 82 Hz which seems like a total match to me. The low E on a bass guitar is around 40 Hz which would have those vintage Jensen P10R’s flapping and eventually fail.
The tube rectifier is another plus for electric guitar players because tube rectification can add some “sag” (also called compression) which makes the Bassman more dynamic and expressive. Diode rectification would probably serve a true bass amp better than a vacuum tube. You can also swap rectifier tubes to fine tune the amount of sag. My personal favorite is the 5V4GB rather than the stock GZ34. The 5V4GB gives the amp more sag and a little less headroom so it will break up at lower volumes.
While we are tube swapping let’s talk about V1 (the preamp tube all the way to the right looking at the back of the amp). The V1 tube can alter the sound of your amp and can also be swapped for more fine tuning fun. The stock preamp tube from the factory according to the tube chart is a 12AY7 which has a gain factor of 45. You can use a 5751 with a gain factor of 70 for increased gain. The sound and characteristics of these two tubes are similar with the 5751 having a bit more girth to it. I like to use a 12AX7 with a gain factor of 100 which obviously adds more gain and a pronounced midrange. To complicate things even further each brand of tube will have a different effect on the sound and response of your amp as well.
Let’s take a look at the output transformer. This was a costly and unique design for a guitar amplifier back in the 1950s. First of all it has a 2 ohm secondary which is an unusual output impedance. Why did Leo wire the 4, 8 ohm 10″ speakers in parallel for 2 ohms and not series-parallel for 8 ohms? 8 ohm transformers were much more common and probably cheaper. My theory is that the underpowered Jensen speakers of the time had a high failure rate as bands played louder. Leo may have thought that if a series-parallel arrangement was used then if one speaker failed it would silence the one it’s connected to thus only having 2 out of 4×10’s functional, plus doubling the speaker load to 16 ohms. This 2 ohm output transformer is also an interleaved design. Volumes could be written on transformers so we will not delve deeper into this at the moment. We’ll leave that to Sergio over at Mercury Magnetics.
Finally the 5F6-A Bassman along with the 5F8-A Twin were the first Fender amplifiers to have a full tone stack featuring Treble, Bass and Middle controls which was very unique for a 1950s guitar amp. This cathode follower tone stack became an archetypal model for countless amplifier manufacturers to follow including Jim Marshalls first amp designs!
In the end all the ingredients that go into the 5F6-A Bassman amp yields a wonderful, dare I say near perfect, guitar amplifier. It is loud enough for almost any stage, used with pedals can be extremely versatile, most every guitar sounds wonderful through it and the classic vintage design is easy on the eyes. If you have the opportunity to try one of these golden era gems be prepared to enjoy a classic tone machine. You’ll be glad that you did.