What?

It turns out that for me, shopping for an amp feels a lot like shopping for a guitar - which is like shopping for an arm or a knee, a kind of appendage that facilitates expressive and creative movement or work. And it has to fit, sound and feel nearly perfectly right to be of real use. But amps of the kind I'm interested in, are not nearly as plentiful or straightforward to sample as guitars. Getting to actually play my own guitars through three or four or more likely candidates is problematic, especially in the time of Covid-19. 

I instead purchased an inexpensive used modeling amp, the Line 6 DT-25 combo, and finally was able to spend as much time as I needed listening to a lot of different decent representations of classic and modern amps.

The DT-25 has allowed me to try out facsimiles of a wide variety of old and new amplifiers and sounds, and while many of the largely digital versions of those amps make sonically fair representations, they seem to do so only within a narrow range of the modeled amps' actual capabilities and only somewhat capture the modeled amps' vacuum-tube-driven tonal nature and non-linear dynamics (there are of course ongoing arguments online about how well modelers do their job).  YouTube and other sites offer countless clips of vintage and boutique analog amplifiers in use, so the combination of both has allowed me to home in on the general style and design of amplifiers that I think offer what I'm looking for. But as I looked and listened I wondered what actually differentiated one style and design - particularly the ones I like -  from other amps that don't sound as right for me. Cursory investigations into various amp circuit designs and the history and evolution of guitar amps in particular since the 1950's, began to shed light on how and why amps sound and feel the way they do, and answered some of my questions about why I like some more than others. Moreover, it narrowed the field of amps I like most to those based on the 1959 Fender Bassman (aka the 5F6A).

 

It is worth mentioning at this point that once you start reading about the 5F6A, you soon discover that Jim Marshall copied this circuit in nearly every detail when beginning to build amps in the UK in the early 60's. A few modifications were required due to differences in the parts that were available to him and his partners at the time, and it is these seemingly minor, likely unintentional differences and their "knock-on" effects that give the Marshall JTM-45 its own unique set of qualities. 

Some examples cited by Ken Fisher in a draft of an article he wrote (not sure if it was ever published as such, but Michael sent me images of the hand-written pages): several resistor and capacitor values are slightly different between the two designs and reduce or emphasize slightly different guitar frequencies; the JTM chassis - which is an often overlooked electronic component - is aluminum not steel and also affects the signal as it traverses the circuit and/or the magnetic fields of the transformers; the UK-made power transformer and output transformer are different and result in significant differences in the response of the amp to input signals and in the onset of distortion in at least one of the amplifier stages; the amplified signal produced by the speaker is 180 degrees out of phase with the guitar input signal, which affects the "feel" for the player, and how audio feedback is or is not introduced back into the amp through the guitar itself.

I also really like the sound of the JTM, and my original intent was to make a hybrid 5F6A / JTM, but that's not practical given what truly gives each amp its character.  If one wanted both, it would probably be easier and more successful to just build separate true versions of each.

Another aspect of tone in which I have become increasingly enamored in recent years, is the distorted high-gain sound of an amp that uses a succession of cascading voltage amplifiers to shape the audio signal by keeping it right at the limits of what a vacuum tube triode can do. Such designs gained popularity in the later 60's, first as customizations of amps like the JTM or 5F6A, and then as production models made by Mesa Engineering, Marshall, and eventually everyone else. More on that later, but suffice it to say I want my amp to offer some opportunity to add and manage additional gain.