Fender Twin - reducing depth of mid cut [SOLVED]

[daxliniere]

Hey everyone,
I have a Fender "The Twin" and I find that the mid-cut is far too aggressive/deep and so I'd like to reduce it. The section of the schematic is below and I'm thinking that I may just need to add a resistor in the path of the switch, but I'm not sure what kind of value range would be good to start with.
Anyone have any suggestions or advice if my resistor-in-series-with-switch idea isn't adequate?

All the best,
Dax,

UPDATE: Solution in post #9 and explanation in post #7.

[Roe]

see here for different values https://tonestack.yuriturov.com/

[daxliniere]

Thanks Roe, but the 1987-1994 "Red Knob" Twin's EQ circuit isn't included in that tonestack calc.

[WiderGates]

https://tonestack.yuriturov.com/?s=eNpb ... GENAKqjgW0

TheTwin use the "normal" Fender Treble-Mid-Bass Tonestack.

Blue line is MidCut not pulled (C1=250pF)
Green line is MidCut pulled (C2=180pF in series with C3=250pF is C1=100pF)
Red line is MidCut pulled and C2 changed to 470pF (C2=470pF in series with C3=250pF is C1=160pF)

C2 and C3 are the values in TheTwin schem.
C1 is the resulting value in the Tonestack Calculator.

[daxliniere]

Ahh, thank you so much, Dieter (and Roe!!), I wasn't aware that was the case.
I'll give it a whirl now and check back in.

[Stevem]

As with most any Fender 3 knob tone circuit used after 1964 backing down on the amount of bass and treble will provide some level of mid boost.

The louder you play the amp the less bass you need and that helps with the mid’s all on its own.

[daxliniere]

Hey all,
I'm trying to make sense of the TS calulations wrt the Fender 'The Twin' schematic.

The red line doesn't match what I'm hearing when the mid knob is pulled to engage a mid-cut. Sonically, it is quite drastic, but in the graphs it only seems to be about 1dB between red and green traces.

[daxliniere]

A way to potentially 'bypass' this line of investigation might be to jump back a step.

If the mid cut switch is closed when pulled, it would be shorting out and hence bypassing R8 and C2. To obtain a more shallow cut, I believe adding a resistor in series with the switch should reduce the depth of cut.

If the mid cut switch is opened when pulled, it would be putting R8 and C2 into circuit. To obtain a more shallow cut, I believe adding a resistor in parallel with the switch should reduce the depth of cut. (Increasing R8 would achieve the same.)

Does this logic seem sound?

[WiderGates]

The MidCut switch is closed if it is not pulled.
The MidCut switch is open if it is pulled.

Increasing R8=10M will not make any difference.
R8 is only there to eliminate any switching sound.

Blue line is MidCut not pulled.
Green line is MidCut pulled.

At 1kHz green is 5.6db lower as the blue.
At 2kHz green is 5.8db lower as the blue.
At 3kHz green is 4.6db lower as the blue.
At 4kHz green is 3.6db lower as the blue.

Putting the cursor in the graph on the 1kHz line and you can see the values.

The red line is if you change C2 to 470pF and pull the MidCut.

[daxliniere]

Hi Dieter, thanks for your reply.

In the meantime, I have been busy. Can now confirm when mid-cut is disabled (pushed in), the switch contact is closed, shorting over the C2 and R8.

I have tested with a potentiometer across those switch contacts and found a value that I thought sounded best, removed the pot, measured the value (200k) and soldered in a 200k resistor across the switch.

Job done and sounding fab!!

[daxliniere]

FWIW, I would say the the cut was grater than 6dB, I'd guess closer to 12dB, maybe 15dB.

[Smitty]

Try swapping the 10K pot for a 250K audio pot. That way you'll still have access to all the Fender tones.

[Roe]

Try swapping the 10K pot for a 250K audio pot. That way you'll still have access to all the Fender tones.

yes changing the 10k mid pot to 25k or greater gives less mid scoop and more flexibility. an audio pot works well here, e.g. 100ka

[daxliniere]

Hey all,
Thank you, but the problem wasn't the range of the pot, it was how much mids are cut when the mid-cut control is enabled. I have solved it now by putting a 200k resistor across the switch so that when it's open (mid-cut engaged), there is still some signal flowing through that part of the filter circuit, reducing the depth of the midrange cut.

All the best!
Dax.