Tube Circuits

Tubes, Tone and Topologies










A.L.I.V.E. <--


It's . . . A.L.I.V.E !

Early amps from the 1930s, 40s, and 50s were typically low power by today's standards, and generally placed a volume control, and an optional tone control between two sequential preamp stages.

Since vacuum tubes have  a finite range of compliance, as you turned the amp’s volume up, the tubes began to clip, that is, the peaks of the waveforms became flattened and compressed. This effect occurred in the preamp, the phase inverter, the power tubes, and also, in the output transformer and the speaker.

This distortion became an integral part of the early blues sound. The problem was, if you wanted to play loud and clean, you were out of luck (the amps lacked headroom), or conversely, you were similarly limited if you wanted to play dirty and quiet.

In the 1960s, amplifier technicians  had the idea of putting a second volume control just before the phase inverter. This allowed you to overload the preamp stages, but then cut the volume down to a reasonable level before feeding it forward.

This worked well enough, however, it quickly became obvious that the overloading of the remaining components contributed significantly to the real overdriven sound. Using amaster volume simply didn’t have the same tonal effect. Moreover, since the master volume is typically wired as a static load from the signal path to ground, with the wiper tapping the signal at various stages of attenuation, the overall tone of an amp with a master volume differs to some degree from one without, since there is always an additional path from signal to ground.

I approach this goal through my Active-Load Integrated Volume Extenuator (ALIVE) [Patent Pending].

Attenuators have been around for several years, aimed at bleeding off some output before it can reach the speaker. Theis permits the overloading of the preamp, phase inverter (where present) power amp, and output transformers, for a more natural  and complex overdrive.

Early attempts to achieve this with high-current resistors, rheostats, etc., were unsatisfactory. A speaker exhibits a complex, reactive impedance, as opposed to the simple linear resistance of a power resistor. The synergy between the various oscillatory interactions (inductance, capacitance, resistance, etc.) all combine to produce the tone in an overdriven amp.

Today, there are several reactive attenuators available, and some companies are even incorporating them into their amplifiers. However, in every case, the attenuation circuitry is essentially an add-on at the end of the signal chain.

ALIVE  differs from other designs in the following respects:

The overall amplifier remains a simple design, with no master volume to bleed tone. My preamps are voiced for maximum clean headroom and broad tonal response. This basic topology is tightly integrated with an active attenuation circuit that delivers a rich, full-spectrum overdrive sound on demand.

Unlike competing designs, the ALIVE attenuator is an integral component in the overall design, allowing it to be switched in an out via a footswitch, like a common overdrive channel.  

The user interface is kept simple and intuitive through the use of dual concentric stacked controls, that is, each knob has an inner and outer ring. The inner (center) control sets the corresponding parameter employed when the ALIVE circuit is disengaged (clean), while the outer ring controls the parameter when the ALIVE circuit is engaged (overdrive).

Specifics vary from model to model and from parameter to parameter. For example, in the case of simple controls, such as volume, tone, reverb, etc., the corresponding inner and outer knobs adjust the settings for clean/overdrive configurations. In models with more complex controls (multi-band tone stacks, for example) the same approach is followed: one control, two settings, but a completely separate board is used in order to minimize the proliferation of relays in the circuit.

The ALIVE controller also contains a central logic circuit and control bus, to which other critical controls are linked. A representative example is the preamp boost circuit used in the Rock Troll (and other) models:

The center toggle, a 3-positon switch, selects the source of control for the function: In the left position (panel), the corresponding circuit is engaged/disengaged by leftmost toggle, which in this case, is either 'on' or 'off'. In the middle position ('fs'), the function is controlled by an individual button on the included foot controller. In the right position ('link'), the function is tied to the operation of the ALIVE switch. In this position, the two-position switch to the right ('logic') is activated. When set to 'true', the function will turn on when ALIVE is engaged, and off when it is disengaged. Conversely, when set to 'false', the opposite logic prevails: the function turns off when ALIVE is engaged, and on when ALIVE is disengaged.

In the case of dual-setting functions, such as reverb, the leftmost button is an 'ON-ON' switch, which toggles between the two presets.

The result is a convenient, flexible, and highly intuitive system for sculpting two distinct sounds and efficiently switching between them with a single button.

The key components of this design are:

The end result is that, unlike other approaches that try to reconfigure the overall circuitry in numerous disparate ways, my amps deliver the intrinsic tone of a single circuit, in two of its intrinsic states. At the extremes, this translates to the ability to immediately switch from a clean tone, to that of the fully overdriven amp (minus, of course, speaker distortion), but in practice, the use of a continuous Rheostat (as opposed to the less costly multi-position switches used by many attenuators), allows a continuous range of nuanced tones.