The ideal Power Scaling kit for Valve Jr., Blues Jr., Black Heart, GA-5/6, AC-4/6/15/30, all "18W" amps, and all the clones of these.

Most amplifiers designed around EL-84 output tubes have a screen voltage that is 30-40V lower than the plate voltage. SB-84 allows this proportion to be maintained so that tone does not change when dialing the power down, nor is there a dead spot in the sweep of the Power Scale pot as there might be using an SB-2 in this application. The SB-84 is purpose-built for this situation and allows the installer to adjust the exact voltage difference.

There are a few fixed-biased EL-84 amps - usually in the 20W range - and these will require the addition of the TBS Tracking Bias Supply to the SB-84. This will assure proper Power Scale function. Depending on how the raw bias supply is derived, the RBX Raw Bias Auxiliary supply may be required to assure proper function of the TBS.

Supply voltages up to 450Vdc are accommodated.

A "drive compensation" pot is included. This is a non-critical 16mm dual pot that in itself can be used as a master volume.

A 28mm mil-spec PS pot is provided as it is a requirement of this simple circuit design. The mil-spec PS pot is a critical component, so additional PS pots must also be of this type. Automated switching selection of these is limited to relays.

Proper heat sinking is required. In small amps, such as the Valve Jr., the chassis will suffice as the heat sink. In amps over 20W, it is best to add a fan to cool the chassis. An aluminum heat sink is also a good idea on larger amps.

PCB dimensions: 2.6 x 1", 66 x 25mm

Kit includes notes with installation tips + layout and schematic diagrams of the kit.

For more info, see "Selecting a Power Scaling Kit".