Reverse Engineering the Grey DOD 250 Overdrive

[Steve Mavronis]

I'm going to try to make a replica clone of the original grey DOD Preamp Overdrive 250, including reverse engineering the original PCB layout design.

I've been playing around with the DIY Layout Creator (beta) for the first time today and after a few hours came up with this PCB layout based on photographic research of the grey 250:



Don't take this image as accurate or build off of it yet. It was just part of a trial and error exercise to help me get used to the DIYLC program, which I think is pretty nice. I know the traces should be on the other side of the PCB, but right now I'm just trying to map out what I see looking at various photos I've collected from around the net. I'm sure some of the trace path connections like underneath the IC chip may be wrong (I tried to cross reference other people's schematics and layout variations) and the diodes may be reversed in my PCB layout since I see them mounted opposite directions in some photographs, which puzzles me.

I'm wondering if those with better grey DOD 250 overdrive knowledge than me could point out any mistakes? Also, on some photographs there is an extra resistor near the bottom of the large curved trace that goes up to pin 7 of the IC. Why does that appear on some grey 250's and not others?

[Speeddemon]

Looks good, aside from 2 things:
1) the resistor right of the opamp in your pic reads 54.7k. That needs to be 470k.
2) the original guts that I've seen, have a 50nF ceramic cap instead of 47nF, and are without the 100 Ohm resistor.

Seeing that the 100 Ohm resistor goes directly to the +9V connection in your picture, its use is probably to 'denoise' any powersupply, right?

[Steve Mavronis]

Looks good, aside from 2 things:
1) the resistor right of the opamp in your pic reads 54.7k. That needs to be 470k.
2) the original guts that I've seen, have a 50nF ceramic cap instead of 47nF, and are without the 100 Ohm resistor.

Seeing that the 100 Ohm resistor goes directly to the +9V connection in your picture, its use is probably to 'denoise' any powersupply, right?

The resistor off of op-amp pin 2 is 4.7K. You are seeing a labeling see-thru effect from the 500K pot underneath. Also I guess you mean uF (micro-farad) for the caps instead of nF (nano-farad). The 100 ohm resistor is a puzzle. I know most gray 250's are without it, yet I have some photos from later gray 250's with it, as well as the first early yellows which are essentially the same circuit in a different colored box. I'm not sure what it does. If it added to 1/2 of the 22K voltage divider resistor pair, it doesn't really add any offset bias (22.01K vs 22K) to speak of at all.

BTW, I have another thread (http://www.diystompboxes.com/smfforum/index.php?topic=82633.0) documenting the evolution (literally) of my clone pedal project where I decided on a new original layout of my own design but with the same gray spec part values and all the part-to-part interconnections are preserved from the original DOD layout yet looks different and unique. As long as that 100 ohm resistor is the only difference, for me it works basically as a convienent jumper to hop over a trace in order to route to the V+ pad in my own PCB clone design layout

[Speeddemon]

The resistor off of op-amp pin 2 is 4.7K. You are seeing a labeling bleedthru from the 500K pot underneath.

Oops, my bad. 4.7k indeed. But the 500k bleedthru had me off.   

Also I guess you mean uF (micro-farad) for the caps instead of nF (nano-farad).

50nF *is* 0.05uF, right? (or in this case 0.047uF, since the 50nF ones are very hard to come by).

The 100 ohm resistor is a puzzle. I know most gray 250's are without it, yet I have some photos from later gray 250's with it, as well as the first early yellows which are essentially the same circuit in a different colored box. I'm not sure what it does. If it added to 1/2 of the 22K voltage divider resistor pair, it doesn't really add any offset bias (22.01K vs 22K) to speak of at all.

BTW, I have another thread documenting the evolution (literally) of my clone pedal project where I decided on a new original layout of my own design but with the same gray spec part values and all the part-to-part interconnections are preserved from the original DOD layout yet looks different and unique.

Interesting! I made one or 2 greybox clones a few years ago; I acquired an early 80's yellow box version, which has the 100 Ohm resistor added, along with the 25pF cap on the IC (in this case a TL081CP), input cap is a mere 1nF (0.001uF). To me the greybox version does sound better. I still keep a heavilly modded 250 RI on my board.

[Steve Mavronis]

With my research I've been collecting reference photos of the gray 250's and first yellow 250's. Some pictures are from Google image searches and some are from monitoring ebay every day for new auctions. Here are 2 photos of the same gray pedal with that 100 ohm resistor at the PCB bottom right corner. Compare the top end gray paint wear patterns on the bottom edge to confirm they are the same pedal. The pictures were taken with different backgrounds:





I have no way to date the pedal unless the person who photographed it states it or if the dated pot codes are visible. Maybe the vintage RCA LM741CN chip is a clue?

[Speeddemon]

With my research I've been collecting reference photos of the gray 250's and first yellow 250's.

Same here, but all the pics I have (9 different ones) have the EXACT same layout, with the only variable (like in your picture), that the 47nF and the 10nF (input cap) caps are either film or ceramic. But they all have 7 resistors, so without the 100 Ohm one.

[Steve Mavronis]

I think in later gray 250's DOD replaced all ceramic caps with film. Maybe someone can confirm this. I have to go through my notes of forum discussions. I captured some info from a German forum (google translated) where they were really getting into the part differences and changes including 741 op-amp brands. But as far as having that 100 ohm resistor or not isn't going to affect the output sound at all. The op-amp and especially the cap values have the biggest effect on that.

Here are my research notes captured from web forums on the history of the 250:

The DOD 250 preamp/overdrive pedal was first introduced in 1973 (yellow re-issue ad)

From this gutshot I can tell you that it is most likely a 1979 unit, at least from the last weeks of 1978. Only during this period they used the 741´s from RCA in the grey 250.

Anyway, the feedback resistor was in all units from 1976 to 1979: 1M.
The bias resistor was in 1976 470k, in 77/early 78 510k, the rest of 78 and 79 it was again 470k. With a freeware graphic programm maybe you will be able to recognize the yellow and violet ring in the gutshot.

BTW, do you have a 741 from RCA ? I think this will have more influence on the sound than the stupid bias resistor - especially when the input coupling cap has a tolerance of 20%, like in this case.

The late grey is identical to the early yellow 250. The early grey and yellow have no Ra and no R8 (R8 only on the newer 250 from China).

Here are the differences in the later DOD250 vs. The earlier '77 DOD250:

Larger cap across the diodes, .002 instead of .001 = less buzz and a smoother clipping, makes sense.

Larger cap on the  output 20uf instead of 1uf. Can't see why this would make much of a change with a guitar though...anyone know?

Larger cap to ground  at the input .0015 or .0022 instead of .001 = a slight high cut at the input (circuit amplifies the highs slightly less.
Some had no cap to ground, some had a small input cap which would make a sort of treble booster distoertion. (malmsteen unit)

No cap 10uf at the voltage divider. I do not know what NOT having a cap here does. Most every FX circuit has one Anyone know?
Smaller bias reisistors in the voltage divider 22k/22k and 470K as oppossed to 1M/1M and 1M.

My conclusions so far:

The Voltage divider and cap. Not sure what the cap is going other than supplying a resovoir of current for the bias voltage.
There is no cap there in the '77 OD250 version so maybe the bias shifts a little more or is a little unstable. Good, bad???
Larger resistors has a lower current draw, makes sence. Effect on sound? not sure.

The ouput cap is 22uf in the '77 version verses 1uf in the later units. I can't see how this would make any difference.

Everything else makes scence and I know what it does...and it's outlined in the linked article above.
SO it would seem that the main difference in the tone of the '77 version of the OD250 is the extra capacitance at the input and output caps to ground which = a smoother sound as more high end it dumped to ground.
Obviously capacitor material type, diode choice and chip # makes a difference but the caps seem to be the main tone difference between the 250 variants.

Joe Hart's website (www.rabbathrecordings.com). It pertains to the DOD 250 and what each component does and possible mods.http://rabbathrecordings.com/DOD250.htm

And now, onto what the parts of the circuit do and what modifications you can make to the circuit. Just remember that changing one thing will likely change more than one aspect of the resulting tone because of the interaction of the different elements in the circuit. So, feel free to play around, but always listen to what the circuit sounds like after each change.

C1 is the Input Cap. This sets the tone going into the circuit. A smaller value cuts more bass out of your signal and a larger value lets more bass in. So, a .001uF capacitor would create a more high-endy pedal because it's one-tenth the value of the original capacitor. Likewise, a .022uF capacitor would create a bass-ier sounding pedal. A lot of people like to socket this component and then plug in different capacitors and see what they like for their individual guitar and amp setup.

R1 help set the Input Impedance and filters out radio frequencies (RF is bad). This is best left alone. Things will work better using the 10K.

C2 is a Filter Cap for the DC current (battery). Not much happens playing around with 10uF value, but I've seen values ranging from 1uF to 100uF - so, whatever.

R2 and R3 are Voltage Dividers. This means that they take the 9V and feed 4.5V to the Op-Amp. Again, nothing much happens playing with these values, except that the values need to be the same. You can use values from 10K to 1M. Lower values use up more battery power, then higher values. But I like sticking with what the schematic calls for.

R4 sets the bias for the Op-Amp. This is best left as is.

R5 is the Feedback Resistor. This works with R6 and R7 to set the gain level of the Op-Amp. If we look at R6 and R7 as one resistor, and we know that R7 is variable from 0 to 500K, then the two resistors together are variable from 4.7K to 504.7K. Now the way the gain is calculated, you divide R5 by the sum of R6 and R7. So, if R7 is all the way to 0, it would be 1,000,000/4,700 which gives us a gain of 212.77. So, the guitar signal will be amplified over 200 times. If R7 is turned to 500K, the gain would be 1,000,000/504,700. This is a gain of 1.98 - just under 2 times. I like to play with the value of R6 to adjust gain. A lower value will give more gain, and a higher value will give less gain. You can also play with the value of the pot (or even the feedback resistor), just keep in mind the calculations for gain.

C3 also affects the tone of the circuit. A larger value here will cut highs, and a lower value will cut bass. Mess around with this component.

IC1 can be any number of Op-Amps. And they all sound slightly different. And when I say slightly, I mean SLIGHTLY. There is a lot of debate over the differences (and even if there are any differences). I can hear differences if I do a side-by-side comparison, but it is very subtle. And playing with caps and resistors has a much, much bigger effect on the sound of the pedal. So, don't get too wrapped up in the Op-Amp. But on the plus side, you can use tons of different ones and still end up with a cool pedal.

C4 is the Coupling Capacitor. It blocks DC current from continuing onto the output. I have seen values from 2.2uF to 22uF used.

R8 - I think this sets the output impedance so the pedal will play well with others. If there is something funky with the impedances, pedals start to act strangely.

C5 helps cut some high end out of the signal because creating distortion also creates harsh higher frequencies. This helps "smooth out" the distortion. Feel free to play with this capacitor.

R9 is the Volume Pot. Not much happens here with different values (you could use anything from 20K to 1M). A higher value pot will allow a little more volume, but this pedal should have more than enough volume for you anyway.

And the diodes. These are fun to fool around with. What is happening here (in the circuit), is that the Op-Amp boosts the signal, then the diodes clip the tops of the waveform off, and this creates distortion. Again, there is a wealth of information on the internet about this, so feel free to research this. There are a few common types that are used and they all sound a little different. Different diodes clip at different voltages, so a lower clipping threshold will create more distortion but a lower overall volume, and diodes with a higher clipping threshold will give less distortion, but more volume. The types of diodes generally used are (in order from lower clipping threshold to higher): germanium diodes, silicon diodes, 3mm LED's, 5mm LED's. Feel free to experiment. Socketing these diodes may be a good idea. Also, the diodes are set up in the original circuit to produce symmetrical clipping. This means that the signal is clipped the same amount on both sides of the waveform. Many people like asymmetrical clipping better. This can be achieved by mismatching the diodes. Either use two diodes with different clipping thresholds or add another diodes in series with one of the original diodes. This creates one side with twice the clipping threshold (this side will clip less). I have become quite fond of using a 6 position rotary switch. This way you can switch between 6 different sets of diodes. I can get a LOT of different distortion tones with this setup.

You can also toss stuff into the feedback loop of the op-amp. If you add two back-to-back diodes (one going one way and the other going the other way) between the output of the op-amp and the inverting input of the op-amp, you will get even more fuzziness. You can put diodes in the feedback loop, and take the original diodes out, and you will get a softer distortion (more of an overdrive like a Tube Screamer pedal).

Also, you can put a capacitor in the feedback loop. This can help smooth out some of the high end in the distortion. Try capacitors in the 20pF to 100pF range. This is a subtle change, but you may like it.

After I noticed how well these gray OD-sounding 250 I got me some info and pictures and this board treader something more structured and analyzed (Shems fitting).

The part on E-bay was probably one of the first 250, about 1977 or so and still at the Dist + ajar. Whether the polycondensors can not have n 2.2 I have to say some have 1n, and a few other 1.5 n no 1n the entrance. The 47n and 10n are ceramic types.

I am yet a very good photo of the board in '78 received Goats (the series of allegedly Malmsteen plays (if he's not back in the hospital is located, and his gout is treated in the fingers). From '78 has there is nothing changed about a year, the first yellow 250 are also identical, then so was only the bad-sounding 250-Waste with the LF355 IC, it has so far not much has changed.

Some details of the model of '78:
The diodes are 1N914 (4148 sound harder and not so good), the capacitors are high quality metallized polytypes and the capacitors are also excellent, the capacitor at the diode is a 1.5 n. The IC is a uA741C (Texas Instruments).

The two gray 250, I then derived according to some of the experiement Tweedy (who has transformed into my Blackface Tweed, hence the name). If the diode switches to VREF, then the sound is somewhat harsh and dynamic (Alternative 1). Alternative 2 has additionally an Tonepoti.

For the bias divider, I 1% 0.6 W metal film used R for a symmetric near-perfect pitch, which were 5% in the CC too much (when the divider is not nearly balanced, then it sounds like squishy and not longer as "gaudy". The smooth sounding electrolytics also much more pleasant than the simple cheap with a rough slide.
The metallized Polycaps have 5% tolerance, which are ceramic hours

For an authentic sound of the 741c in all three ODs necessary (the can actually everywhere) to acquire, the 1458 has a shade less mids, some harder levels, has a little more gain and possibly slightly higher noise when Gain on max. The 741c but spread quite strong, I bought 10 and the 3 best are selected by ear.

The capacitor at the diode is 10k with a filter, which is still somewhat influenced by the diodes, since they must operate by ear still finetuning, the bass ought not muding (15n) is a good starting point.

In the '77-model of the Goats in the input capacitor also affects the sound, so I've socketed the 1-2,2 s well, the CAP also erst mal. The 10n and 47n are not critical.

The three circuits sound a little different, MMN outstanding when the IC is saturated, then the momentum is impressive, almost like a tube. Using volume and tone potentiometer in the guitar you can still fine tuning of all shades, the circuits that respond very well. My old TS-808, however, just sounds cheap, because that can not keep up.

From '78-Goats, there is a sound file on Tonefrenzy or something like that is very realistic because there is also a newer yellow by the kicker.

This Rev-log pots are very useful when high-gain, or OD driving when you create a 250 with only a slight crunch, or use the thing for fattening up or boosting phase, then a linear potentiometer ranges completely.

When I have the gain potentiometer can not reserve about 80k to the end (with my Stratocaster Vintage), about 120k, with an LP, the special characteristic curves of the C-pots so they will not exploit.

The lower section of the 500k gain is doing nothing at all until about 200k will change the tone barely audible, a 250k MMN is more suitable because you can then fine tuning of.

For a little more mid-thrust, you can use instead of 47n also 56n in the RC combination. If the part in the bass sounds a bit simplistic can be reduced to 8.2 n the 10n, which often helps.

For the volume potentiometer, you can also use a log, is on with but always just before the attack, including care of accordingly.

The 77 does not have a 10uF electrolytic capacitor, and for good reason which would produce only an undifferentiated bass.

Some 77 have a capacitor in the input, others do not stop.
(can also be used by IC manufacturers linked, 1 - 1.5 n, one can try out the clay is then centered and somewhat soft, the little C on the diode has much more effect on the sound. I have the capacitor in the input not installed, the tone seemed to me that the bass too soft.

10uF Standard was the first in the version with 1N914 diodes.
In the 78 version, I do not vewende the 1n in the entrance, hangs on the diode, a 1.5 s, hold as in MR. Malmsteen and it sounds pretty good.

VREF is half the supply voltage, is subtracted from the combination of the two 22k resistors.

The LM is very good and low noise, which is why I prefer even the uA741 before, from the sound are both the same.

A good alternative is the National 1458P, with open ports in the second half, which has fallen by at clean high boost very well at high bias not so much because I prefer the LM741.

The circuit of 250 is indeed totally crude and with a bit of finetuning we can get one thing (zurechttrimmen to the personal taste then you should then solder the parts, which versions, or alligator clips to create enough pressure, because the sound is a little thinner in the heights).

The gray pedal was supposed to pull over and give it a little bit output. Times Check to see if 500k is installed after the gain potentiometer and a 4.7 k R, this resistance is greater than dragging it at much less.

[Brymus]

Thats cool another link in the above article to Joe's website > Rabbath recordings.

[Electron Tornado]

Have you seen this video comparison of a Gray and Yellow?

http://www.youtube.com/watch?v=lc8wjq2TjhA

[Steve Mavronis]

Have you seen this video comparison of a Gray and Yellow?

Yeah I've seen that video. A while ago I posted a question there (I'm Yngwie100Percent on YouTube) asking what years are the gray and yellow but he never answered. The date codes on the pots would give the best info to go by.

[Speeddemon]

Hey Steve,

did you see this auction?
http://cgi.ebay.com/DOD-Preamp-250-with-LM741CN-chip-gray-70s-vintage_W0QQitemZ320513329246QQcmdZViewItemQQptZGuitar_Accessories?hash=item4aa015485e

$425!!! 
Anyway, as you see, the coupling electrolytic is 4.7uF (so far I haven't seen 22uF's in any grey 250 yet, nor in yellow ones), and no 100 Ohm either. It looks like the 100 Ohm resistor is only added in the later 'grey years', right before they switched to the yellow ones.

And from that big copied German text I can decipher that they too recommended 500k lineair gain pots.

[Steve Mavronis]

So I guess my clone would be of a final version 1979 gray 250 then.

[Speeddemon]

So I guess my clone would be of a final version 1979 gray 250 then.

Yep. And since the 100 Ohm doesn't change the tone, it's not a big issue. Maybe only for purists, who are looking for an early greybox.

But if anyone can shed any light how GGG's '77 Grey-schematic looks quite different from what Steve, Joe and I (amongst others) have deciphered...?
It seems to me as if they were going for a certain sound that they had in mind, when thinking of the illusive grey version, instead of actually checking the original PCB's.
Their version has higher value caps for more high-end roll-off and added smoothness, that just wasn't there.
Steve, I've looked through a handful more of ended Ebay auctions (and print-screened the gut-shots for my lil' Greybox library  ) and all of them follow Joe's schematic; 4.7uF, 10uF, 10nF, 47nF, 1nF, and the usual resistors....that is, 7 of them, not 8. 
Maybe you can make the 100 Ohm resistor switchable internally, and check it with a powersupply and max. gain and volume if it matters. Label the switch '77/'79 and you're good to go. 

[Steve Mavronis]

So I guess my clone would be of a final version 1979 gray 250 then.

Yep. And since the 100 Ohm doesn't change the tone, it's not a big issue. Maybe only for purists, who are looking for an early greybox. Maybe you can make the 100 Ohm resistor switchable internally, and check it with a powersupply and max. gain and volume if it matters. Label the switch '77/'79 and you're good to go.  

Not that big a deal. If DOD added it I suppose they had their reasons to justify the 100 ohm mod as an improvement tweak. It serves it's purpose for me because it jumps over a trace that otherwise would need an entire PCB layout redo! Here is my latest custom original PCB version:

[skrunk]

great info, thanks!
I've been following your grey spec build steve as I built one recently.

Anyway, as you see, the coupling electrolytic is 4.7uF (so far I haven't seen 22uF's in any grey 250 yet, nor in yellow ones)

I used this vero by Ulysses which had a 22uf elektro. So the 47uf is more common?
Where might the 22uf have come from?
Mine sounds pretty good (used a LM741CN and 1N4003 diodes) but I'm gonna build another with some different values, I'll try the 47uf in there.

[Speeddemon]

Skrunk, 4.7uF!!! Not 47uF.

[skrunk]

oops, oh right, 4.7uf 

[Toney]

Steve, it seems you have done quite extensive research on this baby...
DOD had quite a few versions of the grey 250, as it evolved. Perhaps some of the variations were simply a result of the parts in stock as it was being manufactured.
I was having a bit of a read and it looks like JHS held a lot of information on these. If you read though this post (below) the '77 is considered "the one" by quite a few people.

[/url]http://www.diystompboxes.com/smfforum/index.php?topic=46548.0[/url]

One of the earliest incarnations, and seemingly well liked included these parts:

From JHS' post:
   
   Re: Gut shot of 80's OD-250 on Ebay.
« Reply #8 on: June 28, 2006, 05:53:25 PM »
   Reply with quote
The circuits of the '77 and '78 are nearly identical, so every  Dist+ or 250 board and layout can be used for a build.

'77 details:
=========

No 10uF cap in the BIAS-divider

I-cap is 10n ceramic type
I-cap to ground is 2,2n green Mylar

47n cap is ceramic type
4,7uF is 22uF
cap parallel to the diodes is 2,2n green Mylar

diodes are 1N4001

gain pot is 500k lin
vol pot is 100k lin

1M, 470k, 2x10k like on the normal OD-250

Not sure what your goals are with this, but it's such a simple circuit that I guess I'll build a few versions and pick my favorite.  I still really enjoy an Dist+ I build a few years back with a film 1uf cap and 1n4148s.

What are your thoughts on the above variations - have you seen  any versions such as these?

[Steve Mavronis]

Here are various 250 layouts/configurations from the Gallery here to study:



The 250 clone I'm working on pretty much matches the "Grey Specs" configuration above. C3=0.050uF vs 0.047uF ~ the same value. Whether or not it had a reverse log gain pot doesn't affect the tone, it affects the useful adjustment of the control across the whole 500K range instead of just a part of it. I'd rather have the whole rotation to fine tune the gain (reverse log) and not just the last 1/3 of it (linear taper) being usable.

[Speeddemon]

O.k., maybe I'm missing something, but does anyone have any gutshots from a grey '77 then? 1N4001's? Never seen them in the DODs. The 2nd 1M resistor instead of 470k, never seen it either, not to mention the added cap near the input, the 2nF near the diodes, the 22uF coupling electrolytic...it's all fine and dandy, but *I* never saw a grey box with those parts, period. 

Somehow I think more and more people are after a perceived, EXPECTED TONE of a certain 'mythical' circuit, instead of actually copying the circuit itself.