'75 Fender Bassman Ten Tone Stack

[garytoosweet]

Had one of these on bench today. Final testing everything all good until I plug into Channel 1 and move the Bass knob. Nearly no sound change. I started testing components in the tone stack and couldn't find a fault.

I searched the internet and found many other people with the same amp, same issue and no solution.

Then I realized that the tone stack for this channel was very different from Channel 2, and different from other Fenders I've seen.

It appears it IS working but targeting very low frequency by design. So low it appears to do nearly nothing with a guitar through it, and still isn't very responsive with a bass.

I looked through the Tone Stack Calculator but couldn't find a similar design to this.

I'm thinking I will leave it stock in any case, but still would like to understand what's happening and learn from this.

I'm wondering what is this tone stack design called? What modification would be appropriate to make it more usable in modern day?

[Leo_Gnardo]

What modification would be appropriate to make it more usable in modern day?

A mod I do often on Bassman, Bassman Ten and other 2 channel Fender amps is convert one channel's tone stack (usually the bass channel on bass amps or normal channel on other 2-channel amps) to Fender tweed Bassman 5F6A. Pair of 0.022 uF for mid & bass, 470 or 500 pF treble cap, 56K or 33K contour resistor. Steal the #2 input jack and pop the midrange control in there. For those who want full blues honk use a 25K mid control. So far everyone who's had this done is satisfied. They like the option of another useful selection of tone color.

[Helmholtz]

The bass part corresponds to the bass section of a James/Baxandall stack.
I used the TSC and put in the Fender values in the bass section of a James stack (leaving the default treble section values).

Result shows that the bass control has no effect above 100Hz and almost no effect above 70Hz. This confirm your oberservations.
Interestingly it doesn't cut bass at all, only boosts below 100Hz.

[vintagekiki]

You will get more bass if add 3.3nF - 10nF capacitor in parallel with 330pF capacitor (deep switch blue arrow)

[garytoosweet]

You will get more bass if add 3.3nF - 10nF capacitor in parallel with 330pF capacitor (deep switch blue arrow)

There is actually a good amount of bass already, almost a full/hi-fi sound. The issue is more that there's very little control over the bass eq.

[vintagekiki]

You will get better regulation on bass eq if add 22 - 33k resistor in parallel with 100k resistor (red arrow)

[J M Fahey]

I would ditch that weird/funky tone control and replace it with the classic Fender one shown in the other channel and on 99.9% other Fender amps, for good reason.

Donīt waste time trying to polish a turd.