Dunlop - Octavio

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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alnico
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Post by alnico »

Transistors are BC549B (Q1) and 2N3906 (Q2 & Q3). Transformer is 3.6:1+1. It's not mine, but I might be able to get another look at it sometime if there are questions.
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RnFR
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Post by RnFR »

i've got one of these guys as well. sadly no camera though. glad you were able to get picks up. i can go get mine from the pracice space if there are any other questions.

this pedal actually sounds really good, but you have to have brand new batteries. there's no adapter plug, so i made one out a 9v battery snap. now it's sounds great all the time. the pots sound best when everything is turned all the way up. it'd be cool to be able to squeeze some more gain out of it somehow, and possibly more volume, but it works fine stock.
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kierc
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Post by kierc »

Any files to make one of these? Is the transformer easy to get aswell?

Would love to build one 8)

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Fuzzed
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Post by Fuzzed »

UP UP!

Seriously we have to get more info about this one! :shock:

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JimiB
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Post by JimiB »

Im not getting anything in google for "MCE O-2190T1B"
Allthough it appears to be made by these guys
http://www.mcemagnetics.com/

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puppiesonacid
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Post by puppiesonacid »

its an octavia circuit, like all the others. ggg has a fine layout.

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Alex Frias
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Post by Alex Frias »

So this Dunlop version seems to be a positive ground version of Tycho?

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Post by Sickle »

I've heard nothing but good things about this box.

8)

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Alex Frias
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Post by Alex Frias »

It's very close to what we can see on tonepad/GGG Tycho Octavia schematics, but with some changes.
What I understood by looking to the guts pictures and taking the info about transistors used:
- A positive ground version
- 22uF instead of 33uF electrolitics

We have in mind for the info on Tonepad/GGG that Mouser 42TM022 is an OK transformer for this project, but somebody has some clue about the possible sound changes/enhancing using a more "adequate" one?

Or even the use of a positive ground could affect the resulting sound?

I try to escape from mojo issues, but I admit that this box sounds extremely good to my ears. I built an RM and a Tycho Octavias, but in terms of detail, the Dunlop box is superb and superior to both projects. Tycho is closer, but not the same...

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Post by RnFR »

yeah it does sound awesome. you have to have a fresh battery though, or the goodness kind of fades a bit. i just put a battery snap on a boss adapter.
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Post by Alex Frias »

Comparing schematics of Tychobrahe Octavia collected from the net, I saw as indicated a 220uF for the last transistor colector to ground position capacitor (as it uses 1 NPN and 2 PNP, positive ground), not 100uF as seen on the Tonepad Tetavia.

Anybody knows if reversing the diodes makes any sense when trying to swap the ground polarities in this particular case?

I think I will try the positive ground version with the 220uF capacitor to see what happens...

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Post by noelgrassy »

I'm curious about Alex's diode question too. I'm wondering if the diodes will cease to
function if the polarity is reversed.
Funny about the pedal's desire for "fresh" B+. So many of the older pedals get more
charm to their distortion with a slightly drained battery.
What specific function does the xfmr serve in this circuit? Is it resonance related to
a narrow band of frequencies? Does it have more to do with voltage/current management
in the power supply? I'd like to learn more about the applications of these tiny wire coils. There
are $hitloads of them going for a fraction of their cost at one of my parts suppliers.
The factory markings on most I've seen in surplus stores indicate less than nothing. Would
a portable L meter be effective in determining the suitability of these little xfmrs? They're based
on specific primary to secondary ratios correct? Thanks in advance,

Noel Grassy.
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Post by guiddruid »

I think the circuit driving the transformer is a variable high boost.
The transformer turns the single-ended input into balanced / floating output to allow the simple diode circuit to operate. The drive looks to be reasonably low impedance, and is AC coupled, the load is quite high too, so I doubt the particular design of the transformer will make much difference.
The two diodes just rectify the output. turning them both around would invert the shape of the output waveform, but I doubt it would make any noticeable difference to the sound.
Using a +ve or -ve ground makes no difference to anything other than usability, so long as the output & input are ac coupled... The GGG circuit has an additional AC coupling output capacitor. Not sure what the point of that was, as the original was ac coupled by the transformer anyway... I guess the waveform would tend to bias +ve, which could affect some following circuitry, and the capacitor will get rid of that.

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Post by Greg »

RnFR wrote:yeah it does sound awesome. you have to have a fresh battery though, or the goodness kind of fades a bit. i just put a battery snap on a boss adapter.
True.. my son uses one all the time.
It kills batteries in no time, even when unplugged.. so I wired up a DC jack.
culturejam wrote: We are equal opportunity exposure artists.

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Post by RnFR »

Alex Frias wrote:Comparing schematics of Tychobrahe Octavia collected from the net, I saw as indicated a 220uF for the last transistor colector to ground position capacitor (as it uses 1 NPN and 2 PNP, positive ground), not 100uF as seen on the Tonepad Tetavia.

Anybody knows if reversing the diodes makes any sense when trying to swap the ground polarities in this particular case?

I think I will try the positive ground version with the 220uF capacitor to see what happens...
hey alex-

i'll get on this soon and check it for you. been rather busy the last few days. my unit is already taken apart, so it shouldn't be a problem.
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Post by RnFR »

the gain pot bypass cap is 22uF.
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Post by Alex Frias »

Hmmmmmm...

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Post by controller700 »

I have one since two years now - and I love it to death!!

Great, great pedal! (and the Hendrix Fuzz Face too)

:horsey:

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Post by Alex Frias »

Well, I've just built this project following those directions:

1 - I've used the Tonepad layout and pcb
2 - a Mouser 022 transformer
3 - BC549B (Q1) and 2N3906 (Q2 & Q3). as indicated (thanks to alnico)
3 - 220uF as the Q3 emiter to ground cap
4 - 22uF as the bypass gain cap (thanks to RnFR)
5 - 1N34A as the rectifying diodes non reversed (hope to buy some 1N270 with steve from smallbear and try them)
6 - Positive ground (sharing the same power supply with my Fuzz Face)
7 - So I reversed all electrolitic caps polarities...

My opinion? It sounds really good and very close to what I've heard from some utube vids. I know it's always a matter of taste and a very delicate case of personal perception... But it sounds good and resembles the Dunlop a lot.

Regarding the question about using the diodes reversed... I would like to remember anybody interested in this matter that the schematics I saw in the net of th Prescription COB demonstrates this kind of thing with a negative ground circuit. As I built the project directly on the PCB, I didn't tested it at this time, but soon I think I will...

If someone is breadboarding projects like this one, I recommend try other types of diodes as well, silicon 1N4148 or 1N400X just to evaluate the resulting sound.

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Post by sinner »

RnFR, will you open it up for us and note down all components? PCB is almost finished

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