Factory low drive is already installed on serial numbers higher than :

PA-76A series, serial 6706; PA-374A serial 7331; PA-78 serial 8663. If the serial number of your amplifier is lower, proceed:

To check if the low drive mod has already been installed, disconnect amp from AC mains power and remove the top cover. Remove the rear tube of a 2-tube amp, or the middle tube of a 3-tube amp (offset to right side of tube deck). By looking down through the air holes at the rear of the tube socket it is possible to see a bank of 2w resistors.

3-tube models: If there is a single row of 470 ohm resistors, the low-drive
mod has been installed. If you see three rows of 220 ohm resistors, it has not.

2-tube models: If there are TWO rows of 220 ohm resistors, the low-drive mod
has been done to the amplifier. If you see three rows of 220 ohm resistors, it
has not. NOTE: One row of the three may have been taken out of the circuit by
clipping the braid connecting it to the other rows, without the resistors actually
being removed. A small dental mirror may help identify the status of the resistor pack.

PA-76A and PA-374A (two tube models) – remove five of the fifteen resistors in the swamping pack underneath the tube deck. Gain figure will increase slightly, perhaps 1 db or so.

PA-76PA/CA and PA-78 (three tube models) – remove original swamping pack from underneath the tube deck and replace it with a pack consisting of five 470-ohm, 2 watt, Allen-Bradley carbon-composition resistors connected in parallel (use carbon-film resistors if carbon-comp are not available). The resistor pack has braid leads on "top", circuit board foil at "bottom". The pack is wired exactly as was the original. Note the seven lug terminal strip. Move the wire jumper connected from ground lug 1 to lug 3 of the strip to connect lugs 1 and 2 instead. The outer braid shield of the miniature input coaxial cable originally connected to lug 3 must also be moved to lug 2. Lugs 1 and 2, when connected together, are both "grounded". All other connections to the terminal strip remain the same. When this modification is completed, the amplifier gain will be increased from 10 to approximately 13db.