Get the Most out of Your Tube Amp

Due to the nature of tubes being non-linear amplifiers, tube amps tend to have a “sweet spot,” that setting on the volume control where the amp is right at the point of break-up. Pick lightly and it’s clean; bear down hard and it gets that sweet natural tube distortion. Guitarists who play this way roll their guitar volume control down for cleans and back up for distortion.

If your tube amp also has a tube rectifier, then you may have power supply “sag” added, where a musical spike can cause the amplification circuit to draw more power than the power supply has readily available. This phenomenon adds not only tube amp distortion, but also a compression effect as the power supply recovers and starts sending more juice to the amplification tubes.

If your amp is also cathode biased (self-biased) this effect may be enhanced, and the circuit in effects manipulates the tubes performance to self-correct as the music makes more demands on the circuit. All of these things work together to make a guitar amplifier that is dynamic, touch-sensitive, and lively. The amplifier is an extension of your guitar.

Enter the Fender® AB763

Leo Fender’s crowning achievement in Fender® amplifiers was the famous AB763 “blackface” amp circuit dating from July 1963, used in everything from the Deluxe Reverb to the Twin Reverb. If you look at Fender amp development, from woodies to TV, wide panel, narrow panel, blonde, brown, and finally black faced amps, you will see a progress towards more clean headroom*, and the AB763 Fender Twin Reverb Amp was the epitome of loud and clean, so much so that it’s still a favorite sixty years later. The famous Twin Reverb has little if any sag and is so loud and clean that users often play with the volume on 2, and rarely above 4. It defines the Fender sound – full and clear with scooped mids that emphasize the treble and bass. Turning a Twin up loud enough to create distortion usually involves pain and loss of hearing. This makes it an ideal platform for clean pickers, and pedal board users.

The Marshall® model

Due to the popularity of the 1959 40-Watt, 4 x 10″ 5F6A Fender Bassman (narrow panel tweed) amplifier with that wonderful natural tube compression/sag/distortion, and that it was hard to get in the UK, the circuit was reverse engineered and with a few subtle changes became the famous JTM45. The changes were enough that the famous Marshall “drive’ became a thing, and about the same time as the Fender blackface amps were becoming commonplace, the Marshal model 1987 50W “Plexi” Lead amp with the 4 x 12″ cabinet stack became the standard for rock-and-roll, especially when paired with a Gibson Les Paul humbucker equipped guitar. These rigs were not loud and clean like a Twin. They were loud and dirty, and rockers loved them.

The Texas Tone® response

We start with a basic Fender AB763 circuit as used in the 40-Watt Super Reverb, with 2 x 6L6GC power tubes and 4 x 10″ cab. We shift the balance of preamp and power tube amp distortion to favor the output section. In conjunction with this we make the power tube output section looser and livelier. In other words, we make the front-end cleaner and the back end looser. Fender’s optical tremolo is ditched for old school power tube bias modulating tremolo, and the reverb is tamed by making it less bright and prominent. We add a boost circuit the midrange control to take you from Fender scooped mids to all out in-your-face drive.

The result? A Fender Reverb amp that will keep up with the Marshall Lead amp for straight ahead rock and roll, while retaining the Fender “sound” as well as reverb and tremolo.

This brings us back to the question: how to get the most out of your tube amp?

Turn the Volume control up to where you feel and hear the amp move into that sweet spot -at least “4” on the XTM45; “6” is even better. Run the Treble at 4 or 6, and the bass at 2 or 4. A midrange setting of 0 up to 4 gets you the Fender sound. Cranking the Mid control provides up to 18 dB of signal boost and gets you all-out rock-and-roll territory.

Use the guitar volume control, or a volume pedal, to turn the volume down to a reasonable level (“6” on the amp is loud!). This will play clean and bright, and sound like a Fender should. Crank up the volume on your guitar, and all of a sudden, you’re in Marshall territory! If you keep the Mids below 4, then you’re playing a Fender that keeps up with your Marshall buddies.

Check out this short Texas Tone XTM45 Reverb test bench sound clip.

• Volume and Treble on 6
• Mid/Boost at 2
• Bass at 4

We start with a clean harmonic and then roll the volume up on our humbucker equipped PRS. You hear the amp go from clean to distorted on the harmonic. Next, we play a few distorted rock power chords. Finally, we play a little jingle jangle Fender sounding rhythm licks. All this from the same amp without changing any amp settings. Fender to Marshall and back, all driven by the guitarist, with no pedals.

• See Fender Deluxe amp timeline

It’s Only a Model…

I’ve been asked several times if I can create a ‘profile’ of my signature tube guitar amplifiers. Some guitarists use modeling or profiling amps, and they’ve asked me to create a profile, or even asked me if I can let them create one. To answer that, I’ll go back to visit a bit of tube amp history

Tweed Bassman

Many guitarists, especially those who play Fender guitars, if asked if they could have just one amplifier, would choose tweed Fender Bassman. One of the most lauded and influential amps of any sort, for nearly 70 years it has been the go-to for players seeking robust, gutsy, dynamic, gigworthy tones. The most famous and desirable tweed Bassman is the late 1950s 5F6a version, the “amp for all seasons’.

Significant elements of this design were the 12AY7 first stage preamp, the 12AX7 cathode follower tone stack driver and long-tail phase inverter, the pair of 5881 fixed bias output tubes, the 5AR4 tube rectifier, and the four Jensen 10″ speakers in an open backed cabinet. The clean tones were full and rich, and once pushed past ‘clean’, the amp delivered a dynamic, touch sensitive and musically pleasing breakup. This amp was much sought after in its own time and remains so today. Fender re-issued the tweed bassman in the 1990s, many boutique builders offer their own versions, and it’s become a standard model for digital amps.

As Leo Fender was on a decades long quest for more clean headroom, didn’t like changing existing designs, and kept looking forward, in 1964 Fender created an entirely different Bassman, the blonde piggyback that became another classic amp. In the mid 1960s, even with the advent of the blonde and black faced Bassman amps, the ’59 tweed Bassman remained immensely desireable, as it remains today, a Holy Grail amp. As the British rock scene grew, the demand for such an amp was great, and the Fender Bassman was expensive and hard to find.

Enter the JTM45

Jim Marshall, who owned a small music shop in England, also admired the tweed bassman; it was said to be his favorite amp in 1962 when he commisioned Ken Bran; Dudley Craven and Ken Underwood to build a Marshall version, a reverse engineered 5F6a tweed bassman. They did, but didn’t create an exact copy. Although the circuit was the same, they made numerous changes, some due to parts availablity, others deliberate.

The most significant change, according to Ken Bran, was perhaps Dudley Craven adding larger amounts of negative feeback into the Presence control. This gave the JTM45 a diffenent harmonic content and a richer overdrive tone. Another change that gave the Marshall amp more gain was to use a 12AX7 in the first stage preamp. That the JTM45 was a head, with a separate sealed cab with four 12″ speakers, also added to a different sound. The sum of all the changes was to give the JTM45 more distortion that was musically pleasing, what we now call the “Marshall sound”.

Cookbook Recipes

Tube amp design may be likened to cooking recipes, taking basic ingredients and mixing them in such a way as to create a new and/or desirable meal. The basic ingredients of a tube amp are: power supply; input; 1st stage gain; any additional gain stages; tone stack or EQ; phase splitter, powere output; and speakers. These can be changed, rearranged, and modified to make an almost endless array of tube amp choices. For example, the 5F6a Bassman had the tone stack after the 1st stage preamp and DC coupled cathode follower, and right before the phase splitter. Fender blackfaced amps typically had the tone stack in between the the 1st and 2nd preamp stages, earlier in the signal chain.

Figure 1 below is a 5E3 tweed Fender Deluxe schematic, divided into sections. My first amp, the Texas Tone 12, differs greatly from this design, eliminating one of the input stages, changing the voicing of Preamp#1, the EQ secttion, Preamp#2, the Phase Inverter, the Power Stage, and the Power Supply, in other words, everyting. My lush, dynamic, pulsating tremolo, dubbed the Hypnotic Slam Effect, is based on an old mid 1950s Gibson amp design, but here again, I made numerous changes that makes it somegthing completely different.

When I build a 5E3 type amp, I make numerous changes to eliminate the faults of that design, most noteably, too much bass, too much noise (hum and buzz), too little clean headroom, unsafe grounding, and over-biased output tubes. The result still retains the tweed Deluxe tone and sound, just without the bass ‘farting out’, without the excess noise, without the unsafe grounding, and with the ability to play clean louder and without prematurely burning out the 6V6 output tubes.

I built my first Bassman type amp, the Texas Tone 2:10 special, as a mini Bassman, for a guitarist using a tweed Bassman that was just too loud for the small clubs he played. I made numerous changes to enable him to play it wide open and get that dynamic touch sensitive tone without being too loud. I used this also as the basis for the Studio Reverb, and the same philosopy – great tone, small package, no too loud, for the Ahmanson and Lancaster signature amps. The former is Princeton Reverb based and the latter is a mini JCM800, but neither is a clone, but rather totally unique designs. The Texas Tone Traveler is another example, small and lightweight with the ability to play clean or dirty at pretty much any volume from whisper quiet to quite loud.

Not to discriminate, the Texas Tone 30 and 50 are loud amps, inspired by the 5F6a/JTM45 but significantly different enough to be unique designs that are more flexible and have less noise. The Texas Tone Ranger is ‘inspired by’ the Marshall 18W, but has very little in common with it.

Modelling Amps

Digital modelling amps come in several flavors. Some merely sample a real amp and store that as a preset. Some are software models built like cookbook recipes. There are others. The problem with sampling is that a tube amp is both dynamic and non-linear, so you would have to sample it with a variety of tubes, input voltages, speakers, and guitar pickups, but the varieties of those are almost endless, so you really can’t duplicate the real thing. Software models get built using standard designs, so you end up with just another Fender or Marshall amp. Nothing wrong with that, it’s just not what a Texas Tone amp is.

Therin lies the problem. Texas Tone amps are not clones. I can’t take a profiling amp and build a Texas Tone amp, because my tremolo design, my long-tail phase inverter design, and other elements, are not one of their stock building blocks. Were I to take their building blocks of 1st stage preamp, reverb, tone stack, phase inverter, output and power sections, then I would merely be building yet another Princeton Reverb or JTM45, and not a Texas Tone 12, Ahmanson Special, or Texas Tone 30.

I do something different, which is why the Guitar Radio Show called my amps “Distinctly Different” and “the amp Fender should be making”.

Therefore, don’t expect a Texas Tone ‘profile’ for your digital amp, and should you find one, it’s not legit. Let us build you a Distinctly Different Texas Tone amp. After all, you’re distinctly different, and your amp should be too.