Description
SB1 is the Power Scale solution for 99% of fixed-biased guitar, bass or synth amps
- Precision Power Scale for fixed-bias only
- Precision Power Scale for switchable fixed-bias to cathode-bias
- Full product name: SBX+TBS
- up to 900V plate (Va)
- up to 900V screen (Vs)
- Power range: 100% down to zero
- Loudness range: unusably loud to unusably quiet
- Extends tube life
- PCB size SBX portion: 1.7″ x 1″ 41.7 x 25.4 mm
- PCB size TBS portion: 0.775″ x 0.9″ 19 x 22 mm
- Built height: 1.3″ 33mm
- Controls included: Power Scale, Drive Compensation, Bias
- kit notes include schematic, layout, wiring
SB1 is part of our Precision Power Scale family using the SBX-core main circuitry.
Extreme simplicity. Extreme reliability. Extreme grace in performance.
Produce only as much power as your loudness need requires.
Build time for this kit is one-half-hour. Installation time is extra.
SB1 maintains the shape of the output stage transfer function, which is key to maintaining the same tone from whisper to wail, but it is up to the player to set the panel controls correctly so the rest of the amp behaves as it should. 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
Screen voltage is typically just a few volts less than plate voltage, accommodated by the stock values in SB1. Changing one resistor allows Vs to be far less than Va, as in amplifiers using a voltage-doubler supplier, or where a large resistance ties Vs to Va with a corresponding large voltage difference. SB1 kit notes explain how to calculate this resistor value.
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 for perfect loudness sweep. The stock value is suitable for most amplifiers. The feel of the Power Scale sweep can be altered by adding/removing one resistor (included).
Most stock bias supplies are built almost as an after-thought despite the fact that the bias voltage is the most important voltage in the entire amplifier. SB1 is best supported by RBX Raw Bias Auxiliary supply. See the SB1+RBX bundle page for a small discount. In some amps, RBX-LT can be used instead of RBX.
SB1 uses small pots, allowing easy placement of these components on a crowded front panel. Two mount on the front panel and the other mounts on the tube plane of the chassis. The SBX and TBS modules can be mounted anywhere inside the chassis as both handle only filtered-DC. SBX should be placed where there is airflow over the outside of the chassis – preferably on the tube plane. The TBS module is less critical in its placement. 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. For the pre-assembled kit, London Power has discounted our hourly rate by 20%. London Power is not responsible for any damage caused by misinterpretation of the kit notes
Available configurations:
SB1-COMP: Includes PCB, components, pots, kit notes; assembly required; installation required
SB1-BLT: PCB comes pre-assembled; includes kit notes; installation required
Note that the Built Kit will cover assembly of RBX or RBX-LT if either 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
Questions bout the SV1 and RBX? Join the discussion of these products on The Ultimate Tone Forum!
