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12-11-2025, 01:53 PM
Hi Guys
In the quest towards a Power Scale circuit with "ideal" control, I developed some alternative circuits, some similar to the current kits and others that are quite different.
As discussed in the "Power Scale pot sweep" thread, the nominal issues are to avoid having dead spots in the sweep at the ends of the pot rotation, and to have reasonable resolution of control at the quiet end. One would suppose that the ideal solution would be to use an opamp with a tailored response to potentially allow the use of a common llinear pot but achieve a modified log control shape? Opamps are wonderful devices and there are lots of low-power types available. However, they introduce their own problems and would still require interfacing to the high-voltage circuitry. Feedback would definitely be incorporated, and would have to be fancy to achieve the response mentioned, as well as be stable over the entire range of input voltages. The kit afterall has to accommodate the whole range of supply voltages of all the guitar and bass amps on the market.
Of course, it is simple enough to build a discrete high-voltage opamp with flexible input voltage capability, and that is one of the paths investigated. Over a supply voltage range of 200V up to 880V it worked perfectly after some refinements were added. In this design, both NPN and PNP transistors are used and even from the same manufacturer they are not available with the same voltage capabilities. Placing devices in series is a necessity - just like the current kits have - but three devices in cascode for the NPNs and two devices for the PNPs. This is only a minor inconvenience. For precision, all of the resistors are metal-film 1% 600mW types wired in parallel or series to achieve the required net values and to accommodate the heat dissipation at the highest input voltage. All of this adds up to a lot of components, so a "sandwich" board assembly is required, that is two PCBs that plug together and are parallel to each other.
Overall, the high-voltage opamp approach can be made quite small. I laid out the first version using cordwood style resistor installation. The solder pads were too small and too tightly spaced and I could not see myself soldering it, nor the average hobbyist. Someone with fantastic soldering skills and/or a magnifier, and/or maybe someone who does some surface-mount soldering would have no problem. Anyway, I laid out a second then third version to make it more "buildable", which has all the Rs lying down.
One of the amendments added to the circuit allowed "programmable" output voltage at the quiet end of the pot sweep. Four DIP switches allow this selection. There was a4-way rotary switch alternative that turned out to be very low-quality. Overall, the control board began as slightly narrower than the present SV1 board width of 2.3" (59mm) and ended up slightly larger at 2.6" (66mm). The power board changes for SV1 and SV2 applications. The two boards plug together with 0.1" header pins and receptacles, then two bolts with lock nuts to secure them safely.
I found that the heart of the programmable amendment could be added to the present kit circuit and that this greatly enhanced the sweep of the Power Scale pot. The minimum voltage is set to a fixed value and the pot sweep is very good. The board size for the SV1 increased by 0.15" (4mm) width and the SV2 remained at its present width. This keeps everything familiar for techs who have installed a lot of the current kits. These new versions are still called SV1 and SV2 as common designators, but have a smaller designation of SV1-M36 and SV2-M36. The 'M' designates a change to 1M pots for Power Scale and Drive Compensation, and the '36' refers to the 36k resistor in parallel with the PS pot. These boards arrive before the Holidays.
One other amendment to the new kits is the use of 3W resistors in some positions. These are marked on the schematic and the PCB as there are still two positions using 1W of the same value. As always, it is best to sort all the parts before assembly.
Have fun
In the quest towards a Power Scale circuit with "ideal" control, I developed some alternative circuits, some similar to the current kits and others that are quite different.
As discussed in the "Power Scale pot sweep" thread, the nominal issues are to avoid having dead spots in the sweep at the ends of the pot rotation, and to have reasonable resolution of control at the quiet end. One would suppose that the ideal solution would be to use an opamp with a tailored response to potentially allow the use of a common llinear pot but achieve a modified log control shape? Opamps are wonderful devices and there are lots of low-power types available. However, they introduce their own problems and would still require interfacing to the high-voltage circuitry. Feedback would definitely be incorporated, and would have to be fancy to achieve the response mentioned, as well as be stable over the entire range of input voltages. The kit afterall has to accommodate the whole range of supply voltages of all the guitar and bass amps on the market.
Of course, it is simple enough to build a discrete high-voltage opamp with flexible input voltage capability, and that is one of the paths investigated. Over a supply voltage range of 200V up to 880V it worked perfectly after some refinements were added. In this design, both NPN and PNP transistors are used and even from the same manufacturer they are not available with the same voltage capabilities. Placing devices in series is a necessity - just like the current kits have - but three devices in cascode for the NPNs and two devices for the PNPs. This is only a minor inconvenience. For precision, all of the resistors are metal-film 1% 600mW types wired in parallel or series to achieve the required net values and to accommodate the heat dissipation at the highest input voltage. All of this adds up to a lot of components, so a "sandwich" board assembly is required, that is two PCBs that plug together and are parallel to each other.
Overall, the high-voltage opamp approach can be made quite small. I laid out the first version using cordwood style resistor installation. The solder pads were too small and too tightly spaced and I could not see myself soldering it, nor the average hobbyist. Someone with fantastic soldering skills and/or a magnifier, and/or maybe someone who does some surface-mount soldering would have no problem. Anyway, I laid out a second then third version to make it more "buildable", which has all the Rs lying down.
One of the amendments added to the circuit allowed "programmable" output voltage at the quiet end of the pot sweep. Four DIP switches allow this selection. There was a4-way rotary switch alternative that turned out to be very low-quality. Overall, the control board began as slightly narrower than the present SV1 board width of 2.3" (59mm) and ended up slightly larger at 2.6" (66mm). The power board changes for SV1 and SV2 applications. The two boards plug together with 0.1" header pins and receptacles, then two bolts with lock nuts to secure them safely.
I found that the heart of the programmable amendment could be added to the present kit circuit and that this greatly enhanced the sweep of the Power Scale pot. The minimum voltage is set to a fixed value and the pot sweep is very good. The board size for the SV1 increased by 0.15" (4mm) width and the SV2 remained at its present width. This keeps everything familiar for techs who have installed a lot of the current kits. These new versions are still called SV1 and SV2 as common designators, but have a smaller designation of SV1-M36 and SV2-M36. The 'M' designates a change to 1M pots for Power Scale and Drive Compensation, and the '36' refers to the 36k resistor in parallel with the PS pot. These boards arrive before the Holidays.
One other amendment to the new kits is the use of 3W resistors in some positions. These are marked on the schematic and the PCB as there are still two positions using 1W of the same value. As always, it is best to sort all the parts before assembly.
Have fun
