When discussion turns to new amps designs, the phrase, “Why re-invent the wheel?” often comes up in the conversation. I heard it twice at the 2015 4 Amigos Guitar Show in Arlington. This is usually accompanied by a discussion of the iconic, Holy Grail amps of the mid-1950s to mid-1960s, especially the Fender blackface “Reverb-Amp” designs, the Fender tweed Deluxe, and the tweed Bassman, a design which was also the basis for the iconic Marshall JTM45 amp. A well-known website for finding amplifier schematics actually links to the Bassman schematic as the JTM45 schematic.
With these three amps being considered the epitome of tube guitar amp design and implementation, do we really need new or even different amp designs? Do we need to re-invent the wheel?
Leo Fender’s preamp.
Leo Fender’s guitar amplifiers set the standard for guitar amps. Not only are his tweed Deluxe and Bassman amps considered all-time classics, his black-faced Deluxe Reverb and Twin Reverb amps are also classics, the amps by which others are judged. All of these amps, including clones and reissues, are widely used today. The same can be said of the famous Marshall Amp designs.
Although today’s guitarists appreciate those tweed amps for their sweet sustain and creamy, compressed distortion, Mr. Fender didn’t intend for them to be distortion machines. Looking at the evolution from tweed to blackface amps, one can see that Leo Fender was on a quest for more clean headroom, which is what the Twin Reverb is all about – clean and loud.
Throughout his changes in circuit designs, power, voltage, and headroom, one thing remained fairly constant over 15 years – his 1st stage preamp design. Leo certainly didn’t re-invent the wheel. He used stock Western Electric circuits right out of tube manuals, and even paid licensing fees to Ma Bell (AT&T). In fact, the component values for the little-to-no clean headroom tweed Deluxe and the lots-of-clean-headroom Twin, are essentially identical. The 100kΩ load/plate resistor, the 25µF cathode bypass capacitor, and a 1.5kΩ to 1.6kΩ cathode resistor. How does that work?
The only difference between the 1st stage preamp design on a tweed Deluxe and a blackface Deluxe or Twin is that the tweed amps used a lower gain 12AY7 and the blackface amps used a higher gain 12AX7. Oh, and one more thing. Higher DC supply voltage, especially on the Twin. A tweed Pro preamp had a supply voltage of 250V, while the Twin had 410V! All other things being equal, more supply voltage lends to more clean headroom. For example, the 1st 12AX7 in a blackface Deluxe Reverb idles at about -1.3V bias voltage, while in a blackface Twin, this same tube idles at about -2V bias. That extra -0.7 Volts is an indicator of how much more signal the tube can take before it saturates, 1.4 Volts peak-to-peak (-0.7 x 2) – that’s the extra clean headroom.
This higher voltage rule of thumb also applies to the power tubes, by the way. All up to a point, of course. There is such a thing as too much voltage or current, and the key here is “all other things being equal.” When the values of Ra, Ck, and Rk are the same in a tweed Deluxe and a black faced Twin, the supply voltage becomes the big difference.
So, why reinvent the wheel?
Some people don’t want the crunchy distortion of a tweed Deluxe, or the clean and loud of the Twin Reverb. They may not like the higher noise floor of the tweed amps for when they’re recording. There are any number of compromises, cons, and drawbacks to any amp. There are other ways to get clean headroom and smooth distortion characteristics besides higher voltages, including the choice of cathode resistors. Even though the 5F6a tweed Bassman and the JTM45 Marshall share the same circuit diagram, we find that Marshall Amp voltages, and preamp cathode resistor and capacitor values, are quite different than Fender’s, as are the tubes used, and the tone stack component values, all of which help account for the different sound of Marshall amps compared to Fenders.
When I designed the Texas 2:10 special, it was for a friend who played a tweed Bassman clone in small clubs. It was too loud for him; he couldn’t hit that tube amp “sweet spot” at the low volumes required in these clubs, so he resorted to using boost pedals to get the tone he wanted. What did I accomplish with the design? I built an amp with almost no circuit noise that can go from clean to dirty at reasonable volume levels. An amp that can go from the “scooped Mid” feel of blackface clean to a punchy midrange grit, all without being too loud for small clubs or studio use.
It’s not the end all of guitar amps. It’s just really good at what it does. I’m pleased by all the positive reviews I’m getting on it, words such as “amazingly impressive”, “the best amp at the show”, “coolest amps I have ever seen” and “the crisp cleans, and crunchy punch and pop that I’m always looking for.”
Thanks for all the kind words.