This Week on the Breadboard: The Yankee ScrewDriver Deluxe Turbo

[Chuck D. Bones]

I should point out that the max gain in Turbo mode depends on Q4's transconductance and Q3's output impedance. The output impedance of Ge transistors varies widely. I had good success with the 104NU71 and 2N1308. MP38A & AC127 will most likely have less gain due to their lower output impedance. YMMV. My scope is OtR at the moment, so I can't measure any other trannys.

 

[cooder]

It's been awhile in the planning and now finally come together.
First impression: really great, a ton of gain available and then there's turbo, still fairly low noise even on higher gain settings.
I dig the gainier sounds on this and the tone stack plus quite efficient mid boost offer tons to dscover, I have just barely scratched the surface.
Great design, dig it, thanks yet again so much!
The UV print looks much better in real life, the photos don't really do it justice.

 

[Chuck D. Bones]

Here's the latest, hopefully final, mod. Getting the maximum gain in TURBO mode used to require a very special JFET for Q4. No longer. Changing Q4 to a MOSFET means no selecting, no auditioning, no paying thru the nose for out of production parts. R8 was removed from the Bias Trim circuit and moved to the source lead of Q4. R8 increases Q4's output impedance, which maximizes the gain in TURBO mode. All that gain caused a stability issue with the 1st stage on the breadboard, so I added R19 & C16 to filter Q1's power. I threw in C17 for good measure. I added C18 to break the DC path from Q2-B to Q2-C, which improves Q2's the bias point. Lastly, I added R20. One of my power sources did not like powering this pedal. Regulated power supplies, especially switching converters, have a tough time remaining stable for all loads. This pedal had the wrong combination of current draw and load capacitance for that particular power supply. R20 fixes all that by reducing the effect of C15 on the power source's stability. The voltage drop in R20 is minimal. I'm going to start including R20 in all of my designs going forward.

 

[Brett]

Here's the latest, hopefully final, mod...

Does this mean it's on the breadboard again?

 

[Chuck D. Bones]

Yes. It also means I had to buy more proto-boards and machine up more brackets to keep up with all of my projects. Please don't use the Reply button indiscriminately. Nothing useful comes from requoting the previous post in its entirety.

 

[Brett]

My bad, I edited the post to remove the entire message immediately after posting.

Did you have any luck finding what may have been causing the significant volume jump with Turbo engaged?

 

[Chuck D. Bones]

If the FUZZ and MIDBOOST are set low enough that the signal is not clipped, then increasing the gain by engaging TURBO mode should cause a large volume boost. On my breadboard, if FUZZ is set high enough to clip the signal, then engaging TURBO mode makes it a little louder, not significantly louder.

 

[cooder]

What he said. I found the same with the circuit, the jump is more drastic when fuzz and midboost are lower.

 

[Chuck D. Bones]

Brett,
If that's not acceptable, then PM me and we'll work your case there.

 

[szukalski]

I finally verified my layout. This is quite a versatile pedal, good with Strat or SG. Definitely a volume boost on turbo mode

 

[Chuck D. Bones]

No oscillation with everything cranked?

Yeah, between the MIDBOOST and the BASS control, this should accommodate any kind of pickup.

 

[szukalski]

No oscillation, and pretty quiet considering the gain and single coils I was testing with. Does something in the layout look like it'd oscillate? I try to keep the traces going into amplifying components as short as possible.

None of the J112s were in the range needed for Q1, but a MMBF4393 worked well with a slight adjustment of R18.

The only change I will try is swapping the VOLUME pot taper. I barely get past 10 o'clock at best!

 

[Chuck D. Bones]

Putting the TREBLE pot right next to Q2 could cause problems. The circuit has a lot of gain and it's hard to keep input and output away from each other given the constrains of size, pot locations and having all of the I/O in close proximity at the bottom of the board. I'd have swapped BASS & TREBLE to get BASS closer to Q1/Q2 and TREBLE closer to the VOLUME pot.

What taper are you considering for the VOLUME pot? A-taper spreads out the bottom end. If you're not going to use the top end of the VOLUME pot's range, then insert a resistor between VOLUME pin 3 and TREBLE pin 2. Start with 100K and go up from there.

Whatever JFET gets the desired drain current, that's the one you want. Part numbers are of secondary concern.

 

[szukalski]

I'll try the resistor with the VOLUME first, that'll probably do the trick for my needs.

I guess I lucked out this time with oscillation. I wanted to try and have the pot layout in the "traditional" manner.

 

[Chuck D. Bones]

Form follows function in my book. I've had the tone control layout discussion with Cooder too.

 

[jwin615]

Form follows function in my book. I've had the tone control layout discussion with Cooder too.

There comes a time in every builders young adulthood where it's time to sit down and have the control layout talk with Uncle Chuck.

 

[Erik S]

Back in the late 19th century, The North Brothers Mfg Co in Philadelphia began manufacturing & selling a ratcheting spiral screwdriver under the Yankee brand name. It can be operated like a standard screwdriver, or if the handle is pumped in & out it becomes an "automatic" screwdriver.

Here's Robert DeNiro using one in the movie Brazil.

I still use mine pretty regularly.

They also featured in “The Blues Brothers” and in “Ghost Dog: The Way of the Samurai”

 

[Chuck D. Bones]

Robert DeNiro's character Harry Tuttle wielded one in "Brazil."