Description
SB2 is now a COMPLETE Power Scale solution for 99% of cathode-biased guitar, bass or synth amps
- Precision Power Scale for cathode-bias only
- Full product name: SBX+VCK
- includes Voltage Clamp
- up to 500V plate (Va)
- up to 500V screen (Vs)
- Power range: 100% down to zero
- Loudness range: unusably loud to unusably quiet
- Produce only the power needed
- Extends tube life
- adjustable Voltage Clamp <200Vmax in/out difference
- PCB size SBX portion: 1.7: x 1″ 41.7 x 25.4mm
- PCB size VCK portion: 1″ 1.4″ 25.4 x 34.3 mm
- Built height: 1.3″ 33mm
- Controls included: Power Scale, Drive Compensation
- kit notes include schematic, layout and wiring
Vox amps with tremolo also require TREM
If your amp has bias-modulated tremolo, our TREM Tremolo Level Tracker is also required.
MusicMan amps with the hybrid output stage also use SV2 for Power Scaling.
SB2 is part of our Precision Power Scale family using the SBX-core main circuitry.
Extreme simplicity. Extreme reliability. Extreme performance with grace.
Produce only as much power as is needed.
SB2 maintains the shape of the output stage transfer curve and thus maintains the same tone over the complete loudness range. The interaction between the tubes and the speaker is not effected, as would be the case with ALL load attenuators despite modern sophistication. For a desired loudness level, only the amount of power required will be produced by the amp, extending tube life as well as improving overall amplifier reliability. Power range extends to zero, so you can decide how quiet “quiet” is.
With the Power Scale and Drive Compensation controls there are four operating modes for the amp, one of which is Power Scaling. If a single-knob solution is desired, see this thread on our forum: https://theultimatetone.com/Thread-Automatic-Drive-Compensation
Screen voltage is typically just a few volts less than plate voltage, accommodated by the stock values in SB2. Changing one resistor allows Vs to be far less than Va, in amplifiers using a voltage-doubler supplier, or where a large resistance ties Vs to Va with a corresponding large voltage difference. A few Vox models are like this. SB2 kit notes explain how to calculate this resistor value.
When power is dialed down, the supply voltage will rise. The included VCK Voltage Clamp will maintain a constant voltage to the non-scaled circuits to help maintain tone consistency from loud to quiet, and also provides clean power to those low-level audio circuits. EVERY production cathode-biased amp requires both circuit sections for a proper Power Scale solution, so SB2 is a complete Power Scale solution for all cathode-biased amps without bias-modulated tremolo.
Some cathode-biased Fender and Vox amplifiers have grid-modulated tremolo and will require SB2+TREM. See the bundle page for this offering at a small discount.
What is Audible Transition? Because the Power Scale control range extends to zero, there may be a point at the quiet end of the Power Scale pot sweep where the sound disappears before the pot is fully counter-clockwise. The Audible Transition point on the pot sweep can be adjusted by changing one resistor value. The stock value is suitable for most amplifiers. The feel of the Power Scale sweep can also be adjusted by changing one resistor – the most useful value is included and can be installed or left out to suit the player.
A Drive Compensation pot is provided. Using DriveComp and Power Scale together maintains a given tone, while setting them differently provides four different operating modes for the amp. If the amp has a master volume that regulates ALL sounds of the amp, then the MV can perform the DriveComp function. Depending on the amplifier’s circuit, there is a single-knob solution as described in our forum: https://theultimatetone.com/Thread-Automatic-Drive-Compensation
SB2 uses small pots, allowing easy placement of these components on a crowded front panel. Those supplied mount on individual PCBs to make wire connections easy. The SBX and VCK modules mount inside the chassis, with SBX situated where there is air flow over the outside of that area. Both modules carry filtered-DC and their location is otherwise not critical noise-wise. Kit notes include schematic, expanded board layout, and wiring instructions. Components use precise values and should be sorted prior to assembly. The connection points within the amplifier are common to all tube power amps and are shown schematically.
Available configurations:
SB2-COMP: Includes PCB, components, pots, kit notes; assembly required; installation required
SB2-BLT: PCB comes pre-assembled; includes kit notes; installation required
Note that the Built Kit will cover assembly of TREM as well, if TREM is on your order.
For more information about Power Scaling Kits, see also:
Who can install Power Scaling for you?
Licensed Power Scaling Builders and Installers
