This circuit achieves the goal of being able to play a tube amp dimed with THAT tone, but at lower volume levels.
It works extremely well and there are people who use this technique that think it is the best way to attenuate the natrual sound of a dimed tube amp. It is a matter of opinion, but there is no denying that it works really well.
It is to be noted that the speaker plays a part in the dynamic tone when driven really loudly and we cannot emulate that..nobody can.
It is also really cheap compared to the alternatives. Coming in at around $12 for the Cathode bias version and another $10 for the Fixed Bias version.
This circuit is derived from old Power supply designs with Mosfets.
Kevin O'Connor paved the way using the technique with amplifiers with his own designed circuit on how to scale down the power output of tube amplifiers.
His circuits can be found in his book The Ultimate Tone Vol 4. It is very interesting and enlightening read.
Installation procedure:
This circuit is very simple to install by anyone with some skills in electronics/amp building or modding.
There are only a couple of points of note.
Firstly it must be stated that there are voltages in Tube amplifiers that can kill.
You must remove the amplifiers plug from its socket so it is completely disconnected from any source of electricity.
You MUST discharge high capacitor voltages before you attempt any electronic work inside the amplifier.
If you do not know how to do this I do not think you should attempt this modification.
You can build your layout on any type of format..PCB or PTP.
The Power Mosfet used in the Cathode Bias circuit (N-channel High voltage one) Must have a heatsink or be bolted to the chassis (but it must still be electrically isolated from the chassis-there are plastic spacers used for this) in order to dissipate the heat generated.
I the cathode Bias version you can place the circuit between the output of your rectifier (post standby and HT fuse)and the first Filter capacitor.
CONTINUED: