SV84 - Super-Versatile Super-Value Power Scaling Kit for Single-Node


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Most guitar amplifiers have separate power supply nodes for the the screen and plate of the output stage. In extremely budgetary amps a single supply node is used to support both of the these tube elements. For these single-node amps, we can simplify the Power Scale circuit and save a bit of space and money. SV84 is designed for these situations.

SV84 provides a single high-voltage regulating path in a compact space. Supply voltages up to 450V are accommodated. Many of these simple amps also lack screen resistors, and surprisingly can eat tubes despite their diminutive power output. SV84 provides a second output via resistor that protects the screen element of the tube.

At the other extreme of performance, an output stage might be wired ultralinear, or in triode mode, or may use actual triode power tubes. These are also single-node output stages that can benefit from SV84's low cost for Power Scaling. Note that screen resistors should be added if they are not present in the UL or wired-triode configurations.

In most of the above amps, adding our QS-LV (Quiet Supply for Low Voltage) will provide super-low-noise performance.

SV84 can also be used to Power Scale preamp stages where one might wish to have control over headroom for a compression or distortion effect.

Multiple Power Scale (PS) pots are readily added and switched in multi-voice amplifiers. The PS pot is ground referenced but sustains up to +60Vdc depending on the supply voltage. Additional PS pots can be paralleled, requiring the change of one resistor on the SV84 board, with the wiper selection synchronised with channel switching. BJTs, mosfets or relays can be used for this function. Our ERK-X (Electronic Relay Kit-X) and QRK (Quad Relay Kit) both provide direct selection of the PS pots.

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.

The SV84 uses small, non-critical pots, allowing easy placement of these components on a crowded front panel. Those supplied are 16mm. These mount on individual PCBs to make wire connections easy.

Adding our VCK (Voltage Clamp Kit) to maintain a constant voltage to the non-scaled circuits helps to maintain tone consistency from loud to quiet, and also provides clean power to those low-level audio circuits. Use the "Whole Clamping" wiring shown in the VCK notes, but position VCK after the main filter cap with VCK feeding SV84 and the feed to the preamp.

In a fixed-biased single-node amp add TBS (Tracking Bias Supply) which requires a low- to medium-impedance bias supply of high output voltage for proper performance. If the stock amp does not have such a bias supply, add the RBX (Raw Bias Auxiliary Supply) to assure a problem-free installation.

PCB dimensions: 2 x 1", 52 x 26mm

The SV84 includes PCB, components, pots, and comprehensive notes with installation instructions + layout and schematic diagrams of the kit. The installer must be able to locate the relevant circuit points within the subject amp.

If your amp is >450V and cathode-biased ONLY, see SV2.

Kit requires assembly. If you would like this kit pre-assembled, add SV-BLT to have the PCB pre-assembled, The assembled boards are bulkier than the loose parts, so shipping is slightly higher.

For more info, see Selecting a Power Scaling Kit.

About London Power

London Power was founded in 1990 by Kevin O'Connor, after two decades of research into innovative audio amplification techniques. Kevin is an audio designer, author and speaker, known for his proprietary methodology called Power Scaling, and for his eleven books on audio subjects. Whether you seek amps, kits or information, you'll find useful answers here on not just the "how" but the "why" of audio amplifier design. Please enjoy the information on this site, and don't hesitate to contact us with questions.

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