This blog post will show you how EASY it is to end up with a Fender Bassman Bass Channel that is ‘guitar-usable’.  This is done with only one simple  change to the tone stack, in addition to the capacitor mod that I’ve already gone over.  If you want an even MORE Marshall type tone, you can either make a few changes to the first gain stage’s cathode circuits and/or make some changes to the tone stack.  (One such tone stack mod that I recommend might be to simply change the value of the slope resistor, so that the midrange isn’t so attenuated.)  I will go over this tone stack EASY mod at this time.

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!!!

Or, if you ARE going to modify an AA864 circuit, stick to the bass channel, and make SURE that they are easily reversible.  😉

This is the fifth installment on modifying an AB165 Fender Bassman amp, and the second part to modifying the bass channel.  Since you will be changing the slope resistor and the tone, a test fixture like the one here will help you get the sound you want, but if you utilize this method with testing the mod, please make sure that the circuit is discharged by verifying with your DMM….  (If you look closely, you will still see the cap in the bass channel on the left lifted.) You will also notice the settings that I chose to use, as well as the A/B switch and Power Soak that I used.  😉  Now, let’s discuss a bit about what changing the slope resistor will do.  At 100K, it’s a bit of a ‘gain robber’ and gives you that nice treble ‘fender chime’.  Changing it’s value lower to either 68K or 47K moves the ‘midrange dip’ a bit and will get a little less treble and a bit more bass.  (My AA864 Bassman actually came with the slope resistor of the guitar channel set to 47K, which I moved back to 100K, so that the guitar channel was totally stock.)  If I left the bass channel slope resistor at 100K, I would have two almost identical voiced channels, so I chose to give it a bit more ‘bite’ by lowering its slope resistor to 47K.  (You could even get into this type mod even further by changing the capacitor values in the tone stack.)  I may get into this later, but for now, I think I have my amp sounding the way I like it, simply by two easy mods.  On the left, is Jim’s AB165 amp.  You will notice that I circled in red the bass channel slope resistor, and for your reference, I circled in green, the guitar channel slope resistor (which is at the stock 100K value).  Now, let’s hear a short sound clip and A/B the guitar and bass channels.

Bass and Guitar Channel A/B

 

The above sound clip was recorded using NO pedals, except for the DOD A/B switch.  The amp settings were identical in each channel: Volume at 3.5, Bass and Treble at 5.  Amp output to the Power Soak dialed way down into a Sonic 2X15 Half Stack.  I was using a 2005 Gibson SG set to the treble position, and the volume and tone controls at max.  Your mileage may vary, unless you have removed the negative feedback as detailed in an earlier segment.  (My AA864 circuit has no such feedback, so my sound clip might have more ‘hair’ than yours.)  Hope this helps.  😉

As an afterthought, in case you need to be reminded of how the bass channel sounded before mod, listen to this sound clip.  😉

Bass Channel Unmodified