Not to bump an old topic but I have recently bought an HD100 mk1. Having the same issues that a lot of owners have, turning the master volume knob just a little bit will make it go from quiet to very loud. Using contact cleaner for this potentiometer made it a bit better but the potentiometer is not very high quality and after years of using it it has some wear, contributing to this problem. I started to remove the potentiometer in order to replace it and thats when I found the root cause of the problem. The original potentiometer is a 100k LINEAR potentiometer! In order to have a more controllable volume at lower/normal levels you need to replace this potentiometer with 100k LOGARITHMIC/audio taper potentiometer. Logarithmic pots are universally used for volume control. Since the master volume pot is directly connected to the analog circuitry of the Bogner tube amp part of the HD100 I dont understand why it originally came with a linear pot. Pure speculation from my side but this could have been an error at the factory. I have read some creative hacks in this tread but I think you are better of by replacing the master volume potentiometer with a good quality 100k log pot. And please be careful with the high voltage inside the amp, the big capacitors keep their charge for a very long time after the amp has been disconnected. You don't want to discharge them with your fingers....
