Install a Variable Voltage Regulator in Guitar Amplifier
- Jan 11, 2011
- in Guitar Kit Builder - Amplifiers
As guitar players we always want to operate our amplifier at its "sweet spot," that combination of volume, drive and other controls where it has the sound that we crave. Unfortunately that "spot" is often louder than local conditions, such as your home or apartment, will permit. This leads us in search of a solution to get the sweet spot at a lower volume.
One such solution is to lower the high-voltage feeding the tubes. Generally speaking, an amplifier with a lower supply voltage will begin distorting at lower input signals and will have less gain. So we'll get more distortion earlier and at lower volume, which is what we're looking for.
Unfortunately, this doesn't mean that the type or degree of distortion will be exactly the same as at our sweet spot. Lowering the plate voltage means that the tubes are operating on a different "load line", meaning a different region of the operating characteristics of a particular tube design. But in practice, satisfactory results are obtained in amplifiers that contain a variable voltage control for the supply voltage. You can obtain a variable voltage regulator (VVR) mod for your 50-watt or less amplifier from Hall Amplification. They make two models: the VVR3 ($50) is for fixed bias amplifiers and the VVR ($30) is for cathode-biased amps. These units are already assembled and come with instructions for installation. In case you're wondering about the word "bias" mentioned above, this means to set the operating point of the vacuum tube for it's level of operation with no signal applied.
A vacuum tube must be properly biased to operate correctly as an amplifier. In "fixed biasing" we build the power supply circuit to generate a specific negative voltage that is then applied to the grid of the tube, while the cathode is typically grounded. For this reason, the VVR3 device has dual-potentiometers: one to vary the supply voltage and one to vary the fixed bias voltage. Both of these are mounted on the same shaft and adjusted at the same time. In "cathode-biasing" we place a resistor between the cathode and ground, which causes the cathode to be at a voltage of 1 volt or more above ground. We then ground the grid (through a large value resistor) which is at a negative voltage (ground or zero volts) relative to the cathode at a volt or more. The installation of the VVR or VVR3 is relatively simple, but requires suitable space to be found for the potentiometer to be mounted on the chassis and be externally accessible. One component, a three-pin voltage regulator, also needs to be heat-sinked to the chassis for cooling purposes. This means that it is bolted through a hole in the chassis, but a mica-insulator is placed between the device and chassis so that they are not electrically connected. You'll also need to apply some thermal compound to make a good heat transfer connection. Here's an informative and entertaining video on the installation of the VVR: