Seymour Duncan SFX-04 Twin Tube Mayhem [traced]

[jhergonz]

First of all, sorry for the bad grammar.

I had a Seymour Duncan SFX-04 Twin Tube Mayhem for repair, and the owner let me trace the pedal.

I didn't include the Power section, because I think it is the same from the SFX-03. Except for the power section of the ICs which is powered by regulated +/-12v.

I believe I traced the circuit properly, but feel free to recheck if you have the same unit with you.

Edit:

I forgot to include the path of the heater. +12v connected to pin six of V2, pin 3 of V2 connected to Pin 6 of V1, pin 3 of V1 to GND2

[ppluis0]

Hi jhergonz,

Can you confirm the value of R8 connected at pin 5 and 7 of the second tube ?

[jhergonz]

Hi jhergonz,

Can you confirm the value of R8 connected at pin 5 and 7 of the second tube ?

It should be 100k, my bad.

[VivMeLol]

What do D1 and D2 do ?

[ppluis0]

What do D1 and D2 do ?

Hi VivMeLol,

Those diodes are to protect the input of U2.2 as the output signal of the second tube stage can be so high that damage that op amp.

Cheers,
Jose

[jhergonz]

What do D1 and D2 do ?

Those are overvoltage protection for the opamps.

Tubes has high output that may cause damage to the opamps. That is why that opamps must be protected.

[Manfred]

I made a rough math, R41 should be about 100kOhms and not 5.6kOhms.
I revised the schematic regarding the visibility.
I did not change the component values, that has time until all values are confirmed.

[Manfred]

I ran a circuit simulation of the first stage.
I must take back my statement, am surprised the first stage also works with a collector resistance of 5.6 kiloohm, the stage gain is 5.

[Manfred]

I ran a second circuit simulation of the entire semiconductor input stage with C27 in parallel with R2 as the load, the value of R41 was 5.6 Kiloohms.
I found the following gains depending on the value of VR1.1 at an input signal of 1 volt peak to peak @ 1 kHz.
VR1.1 = 0 Ohm -> gain ~6, VR1.1 = 250 Kiloohm -> gain ~44

[ddpawel]

I made a rough math, R41 should be about 100kOhms and not 5.6kOhms.
I revised the schematic regarding the visibility.
I did not change the component values, that has time until all values are confirmed.
Seymour Duncan SFX-04 Twin Tube Mayhem SW.jpg

C30 in mid control should be equal to 3.9nF. This will give mid frequency around 600Hz and Q around 2.5.

[jhergonz]

I made a rough math, R41 should be about 100kOhms and not 5.6kOhms.
I revised the schematic regarding the visibility.
I did not change the component values, that has time until all values are confirmed.
Seymour Duncan SFX-04 Twin Tube Mayhem SW.jpg

C30 in mid control should be equal to 3.9nF. This will give mid frequency around 600Hz and Q around 2.5.

i send the unit back to the owner so I can't check.

[ddpawel]

I made a rough math, R41 should be about 100kOhms and not 5.6kOhms.
I revised the schematic regarding the visibility.
I did not change the component values, that has time until all values are confirmed.
Seymour Duncan SFX-04 Twin Tube Mayhem SW.jpg

C30 in mid control should be equal to 3.9nF. This will give mid frequency around 600Hz and Q around 2.5.

i send the unit back to the owner so I can't check.

You can't check, but we can calculate resonant frequencies f of gyrators.
For 39nF f is approximately equal to 1/(2 Pi sqrt(100000 Ohm * 2700 Ohm *(18 + 22)nF *(2.7 + 39)nF))~=237 Hz.
For 3.9nF f is approximately equal to=1/(2 Pi sqrt(100000 Ohm * 2700 Ohm *(18 + 22)nF *(2.7 + 3.9)nF))~=596 Hz.

[Manfred]

There is an online calculator for:
http://tools.griffineffects.com/gyrator.php

[Manfred]

In both tube stages the resistance values recommended in the table are applied, only the operating voltage is 150 volts here.

I didn't include the Power section, because I think it is the same from the SFX-03. Except for the power section of the ICs which is powered by regulated +/-12v.

Are you sure with the specified 230 volts, because the power section was not included?

[jhergonz]

I measured it with my multi meter, it is somewhere around 230v, not exact but close.

I already sent the unit to the owner, so I can't measure it again.

But I didn't measure the resistor value with my multimeter, I just read the resistors' band.

I maybe wrong.

[jhergonz]

I made a rough math, R41 should be about 100kOhms and not 5.6kOhms.
I revised the schematic regarding the visibility.
I did not change the component values, that has time until all values are confirmed.
Seymour Duncan SFX-04 Twin Tube Mayhem SW.jpg

C30 in mid control should be equal to 3.9nF. This will give mid frequency around 600Hz and Q around 2.5.

i send the unit back to the owner so I can't check.

You can't check, but we can calculate resonant frequencies f of gyrators.
For 39nF f is approximately equal to 1/(2 Pi sqrt(100000 Ohm * 2700 Ohm *(18 + 22)nF *(2.7 + 39)nF))~=237 Hz.
For 3.9nF f is approximately equal to=1/(2 Pi sqrt(100000 Ohm * 2700 Ohm *(18 + 22)nF *(2.7 + 3.9)nF))~=596 Hz.

maybe you're right.