Tone Stack Help... pot value

[diddymix]

Hi folks!

I was hoping someone with some good knowledge of tone stack circuits could help me with a dilemma I have on my amp.. I'm not an experienced electronics tech so please bear with me.. I have upgraded and tweaked my amplifier just changing things and listening and so far have really good results, and have revoiced my amp following some circuit changes Ive read up myself. The thing is now is I've got the amp sounding great and how I like it apart from the tone stack balance... My amplifier seems to have a funny tone stack where there is only a treble and mid cap... but no bass cap. I think the 'mid' cap actually works interactively with the bass pot value and affects its frequency range also..
I've included the schematic but what Ive done is replaced the 22nf mid cap to a 10nf... and now the amplifier with my cab sounds really alive and juicy in the mid range like its opened it up, its really impressive and I want to keep that the same.... however it seems to have moved the bass range up in frequency as well, which is annoying because now its lacking in low end... the bass control now seems to be lower mids.. I was hoping there may be a way to keep this cap value as the highs and mids are just right.... but maybe change the B200K (VR4) pot value, so that the 10nf cap works with it to keep the bass lower in frequency... could this be possible?? I don't understand the maths with eq circuits like this so please if anyone could chime in with some knowledge it would be most appreciated! Hopefully if this is possible I could keep the mid cap the same but lower the bass which currently has moved up in frequency which I want to keep lower down the range...
Ive included a schematic hope this helps..

Thanks so much for your time! Dave

[pdf64]

I think that C74 is the bass cap for the left hand tone stack that's fed from the input stage plate.
It's effectively in series with the mid cap, so the effective mid cap value is somewhat lower than C103.

[diddymix]

Ok thanks! Really though? To me it just looks like a coupling cap thats not in the tone stack.. In this circuit is this affecting the bass pot frequency range??

[tubeswell]

My amplifier seems to have a funny tone stack where there is only a treble and mid cap... but no bass cap. I think the 'mid' cap actually works interactively with the bass pot value and affects its frequency range also.

If you have an iPhone or iPad, there's a great app called Tonestack Kit. https://apps.apple.com/us/app/tonestack ... 1455438010

Free trial version and (for a few bucks) a full version.

Free:
AB763 (Duncan's "Fender" circuit)
Plexi (Duncan's "Marshall" circuit)
DR103 (Duncan's "Hiwatt" circuit)
E-Series (Same as in TSC)
James (Same as in TSC)
Top Boost (Duncan's "Vox" circuit)
Tilt (Same as in TSC)
Bone Ray (Developed by Merlin Blencowe, "The Valve Wizard")

Full version adds:
60TC (Carlsbro 60TC)
6G7 (Fender Bandmaster)
Baxandall
Guitar Mate (Traynor Guitar Mate)
Voigt
18W Normal (Marshall 18W normal channel)
18W Tremolo (Marshall 18W tremolo channel)
5F2A (Fender Princeton)
6162 (Gretsch 6162 Twin Reverb)
Bridged-T (Bridged-T mid control, Gibson-style)
Bridged-T Alt (Bridged-T mid control, Framus-style)
Moonlight

Both versions you play around with resistor/pot and cap values and see the bode plots.

I've included the schematic but what Ive done is replaced the 22nf mid cap to a 10nf... and now the amplifier with my cab sounds really alive and juicy in the mid range like its opened it up, its really impressive and I want to keep that the same.... however it seems to have moved the bass range up in frequency as well, which is annoying because now its lacking in low end... the bass control now seems to be lower mids.. I was hoping there may be a way to keep this cap value as the highs and mids are just right.... but maybe change the B200K (VR4) pot value, so that the 10nf cap works with it to keep the bass lower in frequency... could this be possible??

The FMV tone stack is a series of interactive filters, so if you change one part, it'll affect everything else. Now that' you've changed the mid cap value, the easiest things to think about changing are the slope resistor (R130 on your schematic) and/or the bass cap (C106). (Assuming that you changed the mid cap (C107) - If you changed C106 thinking it was the mid cap, that may explain your result). If you did change C107 to 10nF, and you want more bottom end, you could try increasing the bass cap to 47nF. If you want less mid scoop, then decrease the slope resistor to 22k.

[diddymix]

My amplifier seems to have a funny tone stack where there is only a treble and mid cap... but no bass cap. I think the 'mid' cap actually works interactively with the bass pot value and affects its frequency range also.

If you have an iPhone or iPad, there's a great app called Tonestack Kit. https://apps.apple.com/us/app/tonestack ... 1455438010

Free trial version and (for a few bucks) a full version.

Free:
AB763 (Duncan's "Fender" circuit)
Plexi (Duncan's "Marshall" circuit)
DR103 (Duncan's "Hiwatt" circuit)
E-Series (Same as in TSC)
James (Same as in TSC)
Top Boost (Duncan's "Vox" circuit)
Tilt (Same as in TSC)
Bone Ray (Developed by Merlin Blencowe, "The Valve Wizard")

Full version adds:
60TC (Carlsbro 60TC)
6G7 (Fender Bandmaster)
Baxandall
Guitar Mate (Traynor Guitar Mate)
Voigt
18W Normal (Marshall 18W normal channel)
18W Tremolo (Marshall 18W tremolo channel)
5F2A (Fender Princeton)
6162 (Gretsch 6162 Twin Reverb)
Bridged-T (Bridged-T mid control, Gibson-style)
Bridged-T Alt (Bridged-T mid control, Framus-style)
Moonlight

Both versions you play around with resistor/pot and cap values and see the bode plots.

I've included the schematic but what Ive done is replaced the 22nf mid cap to a 10nf... and now the amplifier with my cab sounds really alive and juicy in the mid range like its opened it up, its really impressive and I want to keep that the same.... however it seems to have moved the bass range up in frequency as well, which is annoying because now its lacking in low end... the bass control now seems to be lower mids.. I was hoping there may be a way to keep this cap value as the highs and mids are just right.... but maybe change the B200K (VR4) pot value, so that the 10nf cap works with it to keep the bass lower in frequency... could this be possible??

The FMV tone stack is a series of interactive filters, so if you change one part, it'll affect everything else. Now that' you've changed the mid cap value, the easiest things to think about changing are the slope resistor (R130 on your schematic) and/or the bass cap (C106). (Assuming that you changed the mid cap (C107) - If you changed C106 thinking it was the mid cap, that may explain your result). If you did change C107 to 10nF, and you want more bottom end, you could try increasing the bass cap to 47nF. If you want less mid scoop, then decrease the slope resistor to 22k.

Thats great thanks for the link Tubeswell! With regard to the tips I think you are looking at the wrong tonestack.. that one is for the OD channel. If you look to V1 and follow V1A plate output....the clean channel tonestack that follows is what I am tweaking.. as you can see there is no bass cap (to my eyes) only a mid (C103) and treble cap.. it then has the pots but I have a feeling that the bass is affected from the mid cap itself with the bass pot VR4. Its an unusual tonestack but I think the VOX ones may be simelar.. do you think I may be able to shift the bass range down in frequency with a different pot value? Thanks for your input!

[Tony Bones]

What I see is a Fender tone stack with the bass cap replaced with a wire. Tone Stack Calculator won't let you put a wire there; there needs to be a cap. But, you can simulate a wire with a HUGE cap. In the image below I set the cap to 1F which is 1000000uF.

I used the Windows version of Duncan's Tone Stack Calculator. Download from: http://www.duncanamps.com/tsc/ DSL401.zip contains the TSC file.

[diddymix]

What I see is a Fender tone stack with the bass cap replaced with a wire. Tone Stack Calculator won't let you put a wire there; there needs to be a cap. But, you can simulate a wire with a HUGE cap. In the image below I set the cap to 1F which is 1000000uF.

I used the Windows version of Duncan's Tone Stack Calculator. Download from: http://www.duncanamps.com/tsc/ DSL401.zip contains the TSC file.

Aha yes i see what you mean actually.. thanks! So if thats correct.. would I be right in saying I can literally just add a cap n for the bass between the slope resistor (R118) and my bass pot (VR4)?? I would have thought provided I wire it up right this would effectively make it like a fender stack and I could shift the bass frequency... is this possible?

[Tony Bones]

For sure you can cut the trace on your amp and add the cap. I have no familiarity with your amp, so I can't offer an specific advice. Maybe with some well lit, high res photos...

You can also download TSC and play around changing the existing values to see what happens.

[pdf64]

Adding caps / cutting pcb tracks is unnecessary; this arrangement of the Fender tonestack has been around as long as I have, eg https://el34world.com/charts/Schematics ... ii_50w.pdf

Ok thanks! Really though? To me it just looks like a coupling cap thats not in the tone stack.. In this circuit is this affecting the bass pot frequency range??

Yes, really, C74 is the bass cap, yes its value affects the bass control range. You can plug its value into TSC and the result will be valid.
In a series circuit, component order doesn't matter.
With a regular Fender BF tonestack, notice how the treble cap also looks just like a coupling cap?

In regard of the bass control, the BF tone stack is just a frequency selective potential divider. The top part is formed by the series combination of source resistance, slope resistor and bass cap impedance. The bottom half is formed by the series combination of the bass control resistance and the mid control resistance.
For the bass control, it makes no difference which way around the slope resistor and the bass cap are.
For the mid control, I think that the mid cap actual value is now formed by the series combination of C74 and C103. Find an online calculator to work that out for you and plug that into TSC.

Trivially, the treble cap actual value will also be formed by the series combination of C74 and, in this case, C98. But the difference of that to the c98 value will be insignificant, so just plug C98's value into TSC.

[diddymix]

Adding caps / cutting pcb tracks is unnecessary; this arrangement of the Fender tonestack has been around as long as I have, eg https://el34world.com/charts/Schematics ... ii_50w.pdf

Ok thanks! Really though? To me it just looks like a coupling cap thats not in the tone stack.. In this circuit is this affecting the bass pot frequency range??

Yes, really, C74 is the bass cap, yes its value affects the bass control range. You can plug its value into TSC and the result will be valid.
In a series circuit, component order doesn't matter.
With a regular Fender BF tonestack, notice how the treble cap also looks just like a coupling cap?

In regard of the bass control, the BF tone stack is just a frequency selective potential divider. The top part is formed by the series combination of source resistance, slope resistor and bass cap impedance. The bottom half is formed by the series combination of the bass control resistance and the mid control resistance.
For the bass control, it makes no difference which way around the slope resistor and the bass cap are.
For the mid control, I think that the mid cap actual value is now formed by the series combination of C74 and C103. Find an online calculator to work that out for you and plug that into TSC.

Trivially, the treble cap actual value will also be formed by the series combination of C74 and, in this case, C98. But the difference of that to the c98 value will be insignificant, so just plug C98's value into TSC.

Brilliant! Thanks so much I understand now.. it makes sense too because I replaced C74 with a 0.047 and noticed that the bass control seemed to operate more in the upper bass/low mic area.. I think then I can just change that back to a 0.1uf and get that low end back! Thanks for the information !!