Cygnus X1 wrote:The DR doesn't have a mid control, if that's what you're suggesting.
Sorry I didn't look at the DR schemnatic, assumed it was the same as the TR.
Just had a look at the 67 Blackface DR schematic, I assume this is the same as yours? ... from one side of the bass pot there is a 6.8K resistor to ground, just remove one leg of this resistor from ground and you will get a significant gain boost.
Passive tone control circuits do not and cannot add gain or boost to any signal, they merely attenuate the signal when all controls are in the mid position and when you turn up the bass or treble, it is just attempting to restore the level back toward the normal level, but because there will still be resistive and capacitive components in circuit, connected to ground and in series with the signal path and from a simplistic point of view, acting like a voltage divider, there will still be attenuation of the signal even with all controls at maximum.
Therefore disconnecting the ground connection will reduce the attenuation, the treble pot will still partially in series with the signal path, if you turn the treble pot to maximum, it is no longetr in circuit and this should provide you the maximum amount of gain with this mod. With the ground cionnection lift, the tone controls will no longer act as tone controls, bass will do nothing, treble will affect the volume just a little.
Try it, it is an easy mod and can be reversed if you don't like it. Alternatively if you want significant gain increase you will need more tubes, this is much more serious mod and needs to be designed properly.
There are other mods that can be done to change the gain, but generally they may only squeeze a dB or two here and there and will probably not really get you into that zone of a highly saturated/distorted sound, that maybe you are looking for.
Michael