The 1958-1960 Fender® Narrow Panel Tweed Bassman® circuit, the 5F6a, with 40 Watts and four 10″ Jensen speakers in an open-backed combo cabinet, is considered by many to be the best of the vintage guitar amplifiers, with a much sought-after tone, even today. In the UK, theses amps were hard to come by, and expensive. For this and other reasons, Jim Marshall, Ken Bran, Dudley Craven and Ken Underwood reverse-engineered the 5F6a Bassman to come up with the original Marshall® JTM45 MK I amplifier, introduced in 1963.
Although the circuit is the same, there were a few significant changes that gave the JTM45® a distinctly different sound from the Bassman®. Changes in tubes, transformers, components, and voicing were due to a variety of circumstances, including desired choices and parts availability. Especially, the higher gain stage 1 preamp, the four 12″ Celestion speakers in a closed extension cabinet and more negative feedback dramatically changed the harmonic content and overall sound of by the amplifier.
The Texas Tone 30 and 50 series tube guitar amplifiers carry on the same tradition. I started with the basic 5F61/JTM45 circuit and make numerous changes and modifications to bring the amp up to today’s standards.
The Bassman/JTM had two parallel channels, a “Normal” channel and a “Bright” channel. For starters, I modify the Bright channel to become a “Lead” channel- more gain, less bass, and biased for higher headroom and more asymmetrical distortion to emphasize desired even-order harmonics. This gives the Lead channel more of the “Marshall” sound, but also takes pedals extremely well.
With the Normal channel having more of the traditional Fender 12Ax7 voicing and characteristics, the Texas Tone amp gives the guitarist a choice of more of a Fender type sound or more of a Marshall type sound.
With these amps, jumpering the two channels together with a patch cable became almost standard practice. This jumpering gives a thicker sound, and with both channels passing the same guitar signal, also allows the guitarist to vary the volume of each channel to get just the right blend. To that end, we install a Spit/Jump switch on the input to allow for separate of jumpered input choices.
The Bassman and JTM Presence controls had very different characteristics, as Marshall had much heavier negative feedback in the circuit than Fender did. I split the difference a bit, biased towards heavier, rename the control to “Voicing”, and a NFB switch is optional, to provide light or heavy feedback.
Next, we modify the cathode follower tone stack driver to give more gain, less blocking distortion, and smoother, less fizzy-sounding overdrive, which results in more ‘crunch’. Guitarists love this!
The long-tail phase inverted likewise gets special treatment. Even though Fender used the same basic circuit throughout his amps, some used a 12AT7 and some a 12AX7. What we’ve done with the Texas Tone series is to combine the characteristics of the high-bandwidth phase splitter with those of the higher gain phase inverters, while utilizing a 12AX7 tube. Much as with the re-voiced tone stack driver, guitarists love the re-voiced phase splitter.
Like all Texas Tone amps, the Texas Tone 30 / 50 amps use modern grounding technics, quality components, and power supply filtering to lower the noise floor. High gain amps will inherently have some hiss, and we go to great lengths to reduce hiss and remove annoying hum and buzz. We also use MIL-Spec 600V 150C wiring and NASA soldering techniques (designed for circuits subject to extreme vibration and temperatures.
Check out the Texas Tone 30 and Texas Tone 50 tube guitar amplifiers, available in a variety of configurations – head, 1 x 12, 2 x 12. 4 x10, etc.
Distinctly Different and as versatile as you are- because Your Tone Matters.