Andy Marshman kindly let me have the following notes which described how to liven up an unused triode section in the Fender Blues Deluxe in order to give more gain. Main features are as follows:
Here are Andy's notes - please read them carefully, and please don't attack your amp unless you are 100% sure of what you are doing!
Disclaimer: Prior to starting, disconnect power cord and ensure all filter capacitors are fully discharged. Use the services of a trained technician if you are unsure about any portion of this modification. This modification will void your warranty. Study and understand the entire procedure before starting. Modifying a printed circuit board amplifier is more difficult and unconventional than an eyelet point to point board. Some unusual connection techniques are required, e.g. soldering a wire to the bottom of a tube socket connection. The author assumes no responsibility or liability for equipment damage or personal harm from individuals working on their own amplifier.
Note: by step 5, you will have to remove the chassis from the cabinet. Optionally, it can be removed right at the beginning.
NB cut the PCB trace, right at the Drive pot that jumpers its wiper to the ground end. Leave the trace from ground intact going to the end of the pot. THIS STEP REQUIRES YOU TO REMOVE THE ENTIRE CHASSIS FROM THE CABINET.
At this time, solder a twisted pair of wires to the pot wiper and the top end of the pot. Again, about 16" long to start with. This will soon be trimmed shorter.
Solder the wire from the wiper of the Drive pot to pin 7 of V2B (grid).
Located the power supply filter capacitor C46. Follow the PCB trace over to R18. Tack solder a flying lead to this point. This will be the +ve supply to the new tube stage. Located one of the chassis mounting screws and secure a solder lug under the screw with a flying lead (ground of course) for connection to the Auxilliary Board.
The Auxilliary Circuit Board
This is and added board that can be a breadboard, one with copper tracks on one side, or an eyelet board. It's mounted with standoffs so it clears the main PCB and allows the back cover to be installed and maintain clearance.
** High voltage DC will be connected to this board in the completed circuit. Observe spacing clearances so that arcing does not occur between any adjacent components or copper tracks (if they exist on your board).
To mount the board, I used existing chassis screws on the main board; I removed them and replaced them with longer ones to hold up the Auxilliary Board. I also used plastic covers (from small Banana plugs) to cover the screws and act as a spacer as well. I was able to use one screw at each of the two diagonal corners.
Examine the diagrams, and note the location of the filter capacitor UNDER this board. This allows installation of the extra circuitry in a compact fashion, allowing clearance to the back cover of the amp. This back cover has a metallic foil on it for shielding. I added a piece of thin cardboard over that shielding in the vicinity of the Auxilliary Board to prevent high voltage jumping to the metal.
The layout sketch of the Auxilliary Board shows a typical component layout. This isn't critical. You may use your own point-to-point wiring techniques. Observe clearances for power supply voltages present on this circuit board.
All resistors are 1/2W 1%, the filter cap is rated at 500VDC, other caps are 400VDC.
Rear view of amplifier showing approximate
Connext new circuit to existing power supply at point "X"
This modification adds tons of gain to the original circuit. You won't want to turn the Master all the way unless you're in a stadium. However, if you must have more gain than this provides, lower the values of resistors on the extra gain stage down from 1 meg, 2 meg to as low as 470k ohms. There is no capacitor across the cathode resistor for the same reason; adding one would only increase the stage gain. But again, experiment if you wish.
Fender designed a great amp. This mod just allows you to get a wide range of tones from subtle overdrive to high gain tube distortion when in the Drive mode.
Feedback from people who have done this mod indicatesthat the 47uF cap and 10K resistor are NOT ESSENTIAL and may be omitted. That means you don't need to build an auxiliary board, you may wire components directly to sockets and/or run lead wires from point to point to build the circuit.
Any questions or comments on the above can be emailed to me at email@example.com
All trademarks acknowledged