This is the sixth installment on modifying a vintage Fender Bassman amp.  This will be a continuation of the last segment’s topic of altering the value of the bass channel’s ‘slope’ resistor.  Last time, we took it from the stock 100K value down to 47K.  This time, we will push it even further down to 33K.  This should raise the gain a bit more and give the sound even more bite to it.  Once again, I am posting the schematic and layout so that you can see exactly what we are talking about.  But, please remember to make absolutely SURE that the circuit is totally safe before touching anything.  This link to an earlier mod has a great  video that shows how easy it is to ensure that the filter caps are discharged in a tube amp.  Make SURE to watch the video!!

DO NOT MODIFY A RARE LEO FENDER AA864 CIRCUIT!!  ALSO, DO NOT ATTEMPT TO WORK ON THESE AMPS UNLESS YOU KNOW WHAT YOU ARE DOING, AS THEY CONTAIN VOLTAGES THAT ARE LETHAL AND WILL KILL YOU!!!

First, here’s a clip of the unmodified bass channel.  (100K slope resistor, and the hi-cut cap in the circuit.)

Bass Channel Unmodified

 

From the last segment, this is with the slope resistor changed to 47K, and the hi-cut cap removed from the circuit.

Bass and Guitar Channel 47K Ohm bass slope A/B

 

Finally, this is with the slope resistor changed to 33K and of course the hi-cut cap removed from the circuit.

Bass and Guitar Channel 33K Ohm bass slope A/B

 

Another possible value might be 68K for a reduction in the overall gain, (actually, a slight increase in gain from the stock 100K) and a more treble sound (a bit less treble than stock).  So, you get the picture.  Lowering the slope resistor value will raise the gain, and decrease the treble, and give it more bite.  At any rate, this gives you an idea of how changing that slope resistor will change the overall gain and voicing.  Remember, if you want to play a bass guitar through the bass channel, just flip the deep switch on, which will help to eliminate the higher frequencies.  😉