The Variable Voltage Regulator


So you finally received your tube guitar amp, you spent a fortune for it, and you love its sweet tone and all, but. . . You live in a small apartment, with sonically transparent walls, and you want to have happy neighbors. Your dilemma was addressed actually by a lot of amp builders by building attenuators that act just like a speaker load and output just enough power to the speaker that you need. But there’s an easier and more effective way of taming your amp by using a Variable Voltage Regulator (VVR) that will let you turn your amp’s Volume to where you want it, but control the overall amp output to an optimal level.

The VVR is… nothing else but a device that regulates the amount of high voltage (aka B+ in amp geek terms) feeding the amp. You can compare it with a water faucet from where you control the amount of water flow (in our case the amp power). Here I am regulating the overall voltage of a Gabriel V33 head, a class AB cathode biased amp. The VVR comes as a kit and can be purchased from Hall Amplification.

Hall Amplification VVR Manual

The Warning

Please use your common sense. If you do not know what you are doing, then don’t risk your life and try to install this feature yourself since you are dealing with lethal voltages. There are lots of qualified amp builders and technicians that can install this in your amp.

The Video

Here is the video that explains things better than a written blog post:

The Installation

In my case I placed the VVR right after the STANDBY switch, regulating the entire amp. Since it regulates only the high voltage, and NOT the tube filaments (heaters) voltages, it is very efficient since the tubes are operating at their optimal temperature. The kit has a NTE2973 MOSFET (mouser part # 526-NTE2973) that does the voltage regulation, and NEEDS to be mounted to the chassis but NOT touch the chassis. That’s why it is imperative to use the mica insulator between the chassis and the MOSFET just as shown in the video.

The wiring diagram for a cathode biased amp

Scratchy Volumes

You might notice that your guitar or amp volume are all of a sudden scratchy on a lower VVR setting. That is caused by DC leakage on a lower DC setting. This is fixed either by limiting how low your voltage goes — by replacing the two 100k on the VVR kit with higher values (something between 150-200k) or by de-coupling the guitar input and the amp volume with a coupling capacitor. Below is a wiring diagram showing how this tweak can be done. I didn’t need to add a coupling cap to my Phase Inverter since the amp already has it. This is common on most amps that are inspired or built upon the architecture of the 18 watt amp.

Scratchy Volume Tweaks Diagram

The VVR Kit proves to be a great mod to your cathode biased amp to tame its overall power output to your liking. Following are pics of the Gabriel V33 head from the video.





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