This installation of the circuit scales the whole amp and is the most simple implementation of the circuit with excellent results.
There not really much to it. Thats it.
The FIXED BIAS ADDITION.
For those people who want to slide the power of their fixed bias amp, they will need to use this extra circuit in order that the negative bias voltage tracks the lowered B+ voltage.
A dual pot and a P-Channel Mosfet buffer is now implemented to track and vary the -ve bias voltage. too.
The Mosfet must be isolated from the chassis, but it will not get hot so it does NOT need to be mounted against the chassis.
Unfortunately when you add in this circuit it will probably borrow a volt or 2 from your existing negative fixed bias voltage. This will need to be corrected.
If you already have a trimpot to adjust your -ve bias voltage your in business. All that needs to be done is (after installation) Make sure the amp is not having its voltage attenuated (EG its as loud as it can be) Now adjust your fixed bias trim pot for the correct bias for you tubes. Thats it. your done ! You can now Power down the amp and the bias will track the B+.
If you have a network of bias resistors you may need to add your own trimpot so you can make this adjustment. Furthermore you might find you are short on -ve bias voltage. This will require a slight redesign of the final part of you -ve bias circuit. It will only mean the changing of a resistor or 2 to allow a larger -ve bias voltage to be generated - then you can trim it down with your newly installed trimpot !