Hybrid 250+ PCB

[Steve Mavronis]

I was messing around tonight with my gray spec 250 clone PCB layout from 2010/2011 and came up with a new one that lets you make any DOD 250 Overdrive version from all the 'gray specs' to first yellow models, and even MXR Distortion+ by just changing part values. I added spots for 2 extra caps, one in the feedback loop and one from the input going to ground. Those 2 can be omitted to make a '79 era gray spec. Add the input to ground cap and remove the 10uF cap, and change the diodes to make a '77 DOD 250. Change the op-amp and add the feedback cap to make the 80's version yellow 250. Change the part values to make a Distortion+, etc. Make your own hybrid. This is just a rough draft PCB layout version temporarily called the Neoclassic Overdrive Hybrid. After I refine it some more boards will be made for testing. What do you think?

[davepedals]

hey steve.

i've built two grey clones using the original pcb reversed engineered. they sound great, however twice i have attempted to wire it into an enclosure using true bypass and failed both times!  could you please show me a detailed diagram of how you wired yours?  i sure would appreciate it!
thanks!

dave

[Steve Mavronis]

Sure, in this schematics thread (http://www.diystompboxes.com/smfforum/index.php?topic=86932.0) I posted my latest updated wiring diagram. I'll post it here too for you and anyone else. Just follow the off board wiring from the same 4 pads on your PCB. I'll also post a reference illustration of the original 70's DOD layout so you can find the pads correctly:





And in PDF format for printing: http://home.comcast.net/~snmavronis/neo-classic_741_overdrive_schematic.pdf

P.S. If you're not using an on/off LED with your true pypass 3PDT footswitch, then eliminate the entire LED wire run in my schematic.

Also, make sure you identify those jack lugs correctly and use 3/8" inner tooth lock washers where both the stereo input and mono output jacks touch the unpainted bare metal on the 'inside' of the enclosure for good grounding!

Here is the metal open frame type Switchcraft input jack (12B stereo) and output jack (11 mono) lug identification:



Steve

[Steve Mavronis]

I was reading this article about Soft Clipping with this basic soft clip circuit using 4 diodes paired instead of 2:  



So I wondered how that arrangement would sound in a distortion/overdrive pedal? So I modified my previous layout from above to reflect that by looking like this:



The other values may have to be tweaked but is this quad diode mod worth pursuring? I don't have a breadboard yet to test with. I'll have to borrow my fathers next time I see him. I have the urge to build something and don't really need another exact 250 spec clone so a little variation might be nice. Therefore the IC doesn't have to be a 741 as long as a less noisey op-amp substitute with the same pinout is used.

[chi_boy]

I did the same mod on a GGG 250 board, but had a switch to change between 4 Ge's like you show and 2 Si's in the standard configuration.  The volume difference is minimal and for me, the Ge's are preferred.

I have a question though.  I have seen some circuits that show clipping diodes as you have them drawn, and others that don't have the connection between the pairs at the center.  Is there a difference?  I don't think it matters, but I'm curious.

Are you going to have some of those fabbed professionally?

[Steve Mavronis]

I never noticed anyone with the pairs connected in the center but I'm sure it's been done. I thought the soft clipping article schematic configuration looked interesting and I like a smooth rounded clipping waveform for lead work instead of a harsh sounding square wave.

For etching I get my dad to do it for me. He even electroplate tins it for me to protect the copper traces. I could get the ExpressPCB people to fab them for me since for $51 you can get 3 identical default size boards, and I can fit 2 of my size pedal PCB's on each one. So I'd get in effect 6 boards for that price but I don't see the need because I don't build pedals for sale.

[chi_boy]

Sometimes a picture really is worth a thousand words.

I'd like to rephrase my question so that someone might actually be able to understand it.


Is there any performance difference between,

, and

?

[Steve Mavronis]

I asked my dad about that center conenction between the diode pairs and he couldn't see what it does being there vs without it, because each leg conducts one way no matter what. Is there a stability reason or benefit for doing this?

Anyway here I revised part of my vintage grey 250 layout to make it less cluttered in some areas. Plus I didn't like how cramped looking the optional low pass cap pads were so I moved it some. The 2 electrolytic cap component symbols have been revised to represent the shape of tantalum caps because that's what I use.

Is it alright for a ground component connection like that to cross right over a voltage trace?

[kaycee]

Try replacing the cross wire with another diode? I think I've seen that somewhere?

[PRR]

> the connection between the pairs at the center

No electrical difference.

In board layout, old-skool with tape, one 4-hole blob may be faster to lay out.

In tagboard you save a tag by landing all 4 legs on one tag instead of 2-and-2.

> Is it alright for a ground component connection like that to cross right over a voltage trace?

If there is over 1,000V from supply to ground, it may eventually arc through the PCB.

(Meaning.... for 9V/18V work, it's utterly fine. And if you look around, very common.)

However if you move the 100r you lose a jumper and gain the insulating paint on the resistor body.

[Steve Mavronis]

> the connection between the pairs at the center
No electrical difference.

In board layout, old-skool with tape, one 4-hole blob may be faster to lay out.

Thanks on the answer about that 4 diode diagram. Just seemed strange to do like that.

Actually I've been wanting to layout a board old school method with tape. I grew up watching my dad make his projects that way! But time isn't an issue so I don't mind refining my layout if only for asthetic reasons to satisfy me the way it looks. Some people like crossword puzzles to pass the time. I like playing with this.

[Steve Mavronis]

Check out this scale mockup graphic that I made of my PCB with the free program GIMP (www.gimp.org) using a layered combination of photo elements and hand made objects from scratch! This is part of the illustration to help me plan out a better offboard wiring pattern (connecting from the green, black, red, and yellow strain relief wire loops) for neatness and to use as a guide while building.



This part value configuration is for a '79 grey spec 250 Overdrive but anything from a Distortion+ to a 80's DOD yellow 250 could be configured also using the optional part locations that are blank here. But I'll probably try to use those two extra cap spots as lowband filters to see if I can somewhat lower the noise floor to and from whatever op-amp (741, 351, 308, etc.) is used.

Just having fun with graphics.