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.
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