This Week on the Breadboard: Super 65 - cb mods

[Chuck D. Bones]

Phill suggested I check out this circuit. It's a Fender preamp emulator. I built it per the schematic you can find on the Internets, but found the tone to be less than satisfying. One of the things that goes wrong when emulating a tube amp with FETs is headroom. Unless you hit a tube amp with a boost or OD pedal, the first stage runs clean because it has at least 100V of headroom. No so with a FET running on 9V. Even 18V doesn't necessarily get you there. But... why not throttle the gain of the first stage so that we can make it run clean if we want? There will still be plenty of signal to overdrive the second stage. Here's what I ended up with:

Black knob: VOLUME - Green knob: GAIN - WHITE knob: TREBLE - Red knob: BASS



Plays nice with Humbuckers, P-90s and single coil pickups. This is a low-to-medium gain pedal. Will make some dirt and sustain, but not a huge amount. It's mainly for making some low-order harmonics, some EQ and boost. Because the LEDs limit the swing on Q2's drain, we can set the drain voltage lower than we usually would to get the best tone. J113 works very well here. So does MPF4393. A variety of JFETs will work, but make sure the Vp is above 1.5V because we don't want to overdrive Q2's gate. The first stage gain varies from 6dB to 30dB at 1KHz. The bass drops off a bit at higher GAIN settings, which keeps Q1 from getting muddy. The BASS knob can more than make up for any bass lost in C1. The second stage gain is 26dB at 1KHz. The tone stack loses up to 24dB depending on freq and knob settings. Maximum output is a little over 2Vp-p. Try it, you might like it!

 

[lrgaraujo]

Nice one, as always, Chuck!
I have a couple of questions, if you don't mind:
1) Resistance values in the tone stack are smaller than we typically see in tube and jfet design, is it because of the lower impedance of mosfets?
2) other than 1) (assuming my reasoning is right) is there a reason for a mosfet in the first stage? I know you generally prefer jfet inputs for lower noise

 

[Chuck D. Bones]

Thank you. I try.

1) Yes. The output impedance of a 12AX7A in a typical tube amp is around 30K to 40K. Q1's drain load (R5) is about 10x smaller than that. Therefore, all of the tone stack impedances are scaled down approx 10x. The value of C3 is not important, as long as it is large enough. Its main purpose is to block DC.

2) you're right, I do prefer JFETs for their lower noise. Since the gain of this circuit is fairly low, we can tolerate having a MOSFET there. I could have made Q1 a JFET, but decided it wasn't necessary. No hiss in my breadboard and Phill assures me that his Super 65 pedal is "...quiet as a church mouse..."

Notice that I omitted the usual zener to protect the MOSFET gate against ESD. Zeners can be leaky, and leakage makes noise and can upset the bias. I'm thinking of a way to protect the MOSFET without a zener. Once I have it worked out, I'll report back.

 

[lrgaraujo]

Nice! Thanks for taking the time to answer!

 

[music6000]

Here It Is:

 

[music6000]

I used A MPF102 in Q2 on my build which is equivalent to a J113.

 

[Chuck D. Bones]

They are not equivalent. MPF102 has a very wide Vp range; some MPF102 will have Vp that is too high to work in this circuit. J113 has a higher transconductance (gain). In either case, we would need to test & select the JFET.

 

[juice886]

I really like that J113 and BS170 are used here, hence no problem with finding and buying transistors. Thanks, Chuck!
btw, what is power supply here, 9V or 18V?

 

[Chuck D. Bones]

I really like that J113 and BS170 are used here, hence no problem with finding and buying transistors.

I figured everyone would appreciate that.

btw, what is power supply here, 9V or 18V?

I run mine on 9V. You could try 18V, but you'll need to decrease R4 to get Q1-D up around 9V when GAIN is dimed if you want to take advantage of the increased headroom. Tune Q2's bias by ear.

I see from your profile that you're in Kyiv. Know that the American People stand behind Ukraine, even if some cowardly politicians here don't.

 

[music6000]

They are not equivalent. MPF102 has a very wide Vp range; some MPF102 will have Vp that is too high to work in this circuit. J113 has a higher transconductance (gain). In either case, we would need to test & select the JFET.

I originally had a 2n5457 & the pedal did not sound right & when I cross referenced similar jfet's, the MPF102 was suggested & that's why I tried it.
It was Great right out of the box , then I fine tuned it with my temporary trimmer til it sounded perfect, I removed the pot, measured it & put in a fixed value resistor.
As I said, this sounds Great & there is no background noise at all even at full gain.
Mine was built years ago using my In House Eyelet board design.

 

[Chuck D. Bones]

I didn't like the sound of the 2N5457 either. Vp is too low and the gate was getting forward-biased during part of the waveform. Makes for some nasty harmonics.

Thanks again, Phill, for pointing this circuit out to me.

 

[Chuck D. Bones]

I ended up installing zener gate protection for Q1 after all. Found some that are low leakage, just a few nA. I'm done making changes.
The LEDs flicker while playing. One could use super-bright LEDs and mount one or both on the front panel. As you might expect, the GAIN pot scratches when turned. It's normal. The more I play it, the more I like it.

 

[juice886]

I see from your profile that you're in Kyiv. Know that the American People stand behind Ukraine, even if some cowardly politicians here don't.

Thank you for the support! Yeah, difficult times here. Making stompboxes helps staying sane though.

 

[jesuscrisp]

So it's basically a Peppermill with a tonestack between the 2 stages...
I'd actually be tempted to take this even further by fixing the input stage at just high enough gain to offset the volume suckage of the tonestack, then add another adjustable gain FET stage to drive the last JFET.

 

[Chuck D. Bones]

I see the resemblance to the Peppermill. Have you checked out the ROG Britannia?

 

[jesuscrisp]

Certainly looks weird and interesting... Mostly interesting for the fact that I haven't seen this many clipping diodes in a "FET-based" dirt pedal yet. Then again, I'm not usually a big fan of cascaded FET based pedals for anything between light grit and saggy high gain stoner/doom stuff so maybe the diodes help with the gain structure?

Drifting off-topic here though.

 

[Chuck D. Bones]

Certainly looks weird and interesting... Mostly interesting for the fact that I haven't seen this many clipping diodes in a "FET-based" dirt pedal yet. Then again, I'm not usually a big fan of cascaded FET based pedals for anything between light grit and saggy high gain stoner/doom stuff so maybe the diodes help with the gain structure?

Drifting off-topic here though.

https://forum.pedalpcb.com/threads/this-week-on-the-breadboard-rog-britannia.11109

 

[giovanni]

Do the two RC cascading circuits on the output (C7, C8 on the second schematic) make a second order low pass filter? Would first order not work?

 

[lrgaraujo]

Do the two RC cascading circuits on the output (C7, C8 on the second schematic) make a second order low pass filter? Would first order not work?

In RunoffGroove designs (which I believe were the inspiration for this one) they usually use a second order lpf at the output to simulate (broadly speaking) the frequency response of a speaker. It's very common in jfet amps-in-a-box (right now I can think of wampler, eqd and rog designs that use this)

 

[giovanni]

Interesting. So how does that affect the tone in practice in this circuit?