Suggestions on a bass amp tone stack?

[Bob-I]

I built a bass amp based on the Fender Bassman AB165 but using 6550 output. Sounds great, plenty of punch and about 80 watts out of the 2x6550s.

The problem is tone stack effectivness. I used the standard FMV tone stack, 500pF, .1uF and .047uF with a switch to select either a .1 or .047 mid cap, and an addition bass boost with a .1 to ground just like the AB165.

The treble control is the only one very effective. Mids and bass are subtle at best. I'm checking right at the output of the tone stack and I hear very little difference.

I'm looking for suggestions on a tone stack that will be easy to implement and effective.

Thx in advance.

[LOUDthud]

By "mid cap" do you mean the one that goes from the slope resistor to the junction of the mid and bass pots? Try changing that cap to .022uF.

[skeezbo]

Bob-I
I spent some time playing with the Duncan tone stack software: The 500p mid cap gives the treble knob control of more of the high mids than the stock 250p, so the treble control probably seems very effective.

The .1 mid cap moves the range of the bass control way below 100 cycles; it is probably working fine but in a range that your amp (or even your bass) isn't really producing. Also-isn't the bass boost on these amps really just a bass cut in the off positon? That could affect the action of the bass control if the "boost" is not engaged.

The mid controls on these amps don't change the tone that much anyway for bass, in my opinion, even if you use a 25K pot.

After all this is said, though, the stack should still work reasonably well with the .047 mid cap and the bass boost on.

I would try removing the bass boost and switching the .1 mid cap choice for a .02, which would slide the mid dip upward, perhaps increasing the range of the bass control.

I hope this helps,
Skeezbo

[Bob-I]

By "mid cap" do you mean the one that goes from the slope resistor to the junction of the mid and bass pots? Try changing that cap to .022uF.

Yes, that's what I mean. I'll give that a try.

[Bob-I]

Bob-I
I spent some time playing with the Duncan tone stack software: The 500p mid cap gives the treble knob control of more of the high mids than the stock 250p, so the treble control probably seems very effective.

Yes, I had the 250pF in there initially. I may go back to it as this is just too middy sounding.

The .1 mid cap moves the range of the bass control way below 100 cycles; it is probably working fine but in a range that your amp (or even your bass) isn't really producing. Also-isn't the bass boost on these amps really just a bass cut in the off positon? That could affect the action of the bass control if the "boost" is not engaged.

I did the same thing and questioned Fender's wisdom at using that value. I did try a .047 and it seems more effective.

The mid controls on these amps don't change the tone that much anyway for bass, in my opinion, even if you use a 25K pot.

After all this is said, though, the stack should still work reasonably well with the .047 mid cap and the bass boost on.

I would try removing the bass boost and switching the .1 mid cap choice for a .02, which would slide the mid dip upward, perhaps increasing the range of the bass control.

I hope this helps,
Skeezbo

Thx, I'll try that as well. I don't think I really need to remove the bass boost switch, I can just switch it off.

Thx for the suggestions.

[roberto]

Check out the Marshall VBA400 EQ.
I've used it in a bass amp (single 12ax7 in the preamp, TDA1400 Tripath poweramp) and it rocks!

http://www.drtube.com/schematics/marsha ... -60-00.pdf

[Bob-I]

Check out the Marshall VBA400 EQ.
I've used it in a bass amp (single 12ax7 in the preamp, TDA1400 Tripath poweramp) and it rocks!

http://www.drtube.com/schematics/marsha ... -60-00.pdf

Very cool tone stack. I'll need to pickup some 3 position toggles for this one but I think it will be worth the effort.

[roberto]

You can just choose one of the configurations to make it simpler.

[Bob-I]

You can just choose one of the configurations to make it simpler.

Yes, I've been playing with them today. Odd but the mid and bass controls seem to be affecting the same range now. I have the bass boost cap in the ckt as well.

Now I have 250pF, .022uF and .047uF.

[Structo]

Bob I'm sure you know far more than I do about these amps but have you tried modeling the tone stack on the Duncan TSC to see what the changes bring you frequency wise?

[Bob-I]

Bob I'm sure you know far more than I do about these amps but have you tried modeling the tone stack on the Duncan TSC to see what the changes bring you frequency wise?

Yes, I've been doing that since I started this amp about a year ago. Unfortunately I'm not familiar enough with bass response to relate the frequencies to tone. Also, the TSC doen't include the bass boost cap Fender uses. I like the way that cap responds. I don't really hear the boost as much as the change in the fundamental tone of the mids.

The odd thing abot bass is you don't hear the fundamental of the lowest notes, below about a G and with a 5 string bass it's a real challenge.

[roberto]

Maybe you just want more boost, so you can try a baxandall, like ampeg does in its amp.

[Merlinb]

Adding a vairable resistor in series with the slope resistor is useful for bass amps, as it allows you to shift the whole response of the stack up and down. (Larger= shifts down)

[Bob-I]

Adding a vairable resistor in series with the slope resistor is useful for bass amps, as it allows you to shift the whole response of the stack up and down. (Larger= shifts down)

Good thought, but you've got it backwards. Larger shifts up as it directs more signal through the treble cap.

I'll give that a try. Maybe a trimmer at first before I go drilling the face to see if it's worth it.

Thx

[Bob-I]

Maybe you just want more boost, so you can try a baxandall, like ampeg does in its amp.

Hmm... good suggestion. I'd lose the mid control but it's hardly worth anything anyway.

This thread turned out really great. Thx everyone for the suggestions.