Debug Tycobrahe Octavia - no octave sound

[ItsGiusto]

I've just completed this build of an Octavia:
http://tagboardeffects.blogspot.com/2013/04/tycobrahe-octavia.html

It sounds alright - it's a cool "velcro" fuzz sound. But I'm not getting the octave with the spst toggle switch in either position (though for the record I actually used an spdt) or those ring modulator sounds and cross-modulation undertones. It's nowhere near as strong as my Dunlop JH-OC1 Octavio, even if I crank both the volume and gain knobs on my homemade one, or roll the gain all the way down.

I originally built it with a 2n3906 and two 2n2222s. However, I didn't like that the octave sound wasn't pronounced enough and the fuzz wasn't strong enough, so I replaced them to the 2n5087 and two 2n5088s that tagboardeffects recommends. I think that made it a little stronger, but not nearly enough, and it's still bugging me that there's no octave, even up on the 12th fret with the neck pickup.

I've tested with my DMM to make sure that with the toggle switch in the up position, the vero strips with the striped end of the two diodes are electronically connected, and that when the switch is in the down position, they're not. Yet, the sound of the fuzz itself sounds completely identical in those two positions, even though I've verified that the switch makes a connection difference.

My only guesses are:
1) It's just a weak effect that's difficult to get a sound out of. The Dunlop JH-OC1 has been souped-up to sound better through modern engineering practices, so I'm comparing apples to oranges by comparing the two.
2) I'm using incorrect diodes. I once bought 1n34s a long time ago, but didn't label them. I found diodes that I think were those ones, but they're unmarked, so I'm not too sure. I've just ordered new 1n34s to try to replace the ones in the build and see if that's the problem, but I'm not too confident that's actually the problem.
3) The transformer isn't working well. I used this one: http://smallbear-electronics.mybigcommerce.com/transformer-for-tycho-octavia/
I bought a spare that I could swap in if need be. But is that really likely to be the issue? I'm only suspecting it because I have no clue how to test a transformer to see if it's working correctly.

Any help on this would be appreciated! Thanks!

[thermionix]

One thing that bugs me about Tagboardeffects - no schematic.

I highly doubt you have a bad transformer, but it's not impossible.  You can check it out with an ohmmeter, out of the circuit.  You should have "some" ohms across the secondary (1? 10? 200?).  Infinite resistance between secondary and primary.  On the primary side you should see similar (not same) resistance readings from start to CT, and from finish to CT.

I know some octave effects want matched diodes in the octave section, but I have no experience (ahem) with the Octavia.

[anotherjim]

Use the DMM diode test to compare those diodes. You want closely matched readings between 2 diodes and those readings need to be lower than any silicon diodes to prove they are Ge (Germanium). You want a reading around 0.3v or 300 (mV) depending how your DMM does it. Readings over 0.5v are most likely ordinary silicon diodes. Ge is temperature sensitive so try not to heat them up with your fingers.

Schottky diodes ought to be as good, but some don't like the sharper sound.

[ItsGiusto]

Use the DMM diode test to compare those diodes. You want closely matched readings between 2 diodes and those readings need to be lower than any silicon diodes to prove they are Ge (Germanium). You want a reading around 0.3v or 300 (mV) depending how your DMM does it. Readings over 0.5v are most likely ordinary silicon diodes. Ge is temperature sensitive so try not to heat them up with your fingers.

Schottky diodes ought to be as good, but some don't like the sharper sound.

Thanks so much! Given what you say about how the diodes need to be matched, that very well could be the problem - I didn't try to match them at all. Unfortunately my Dmm doesn't have a good diode test setting - I tried, but I wasn't getting consistent results even with Si diodes. I've been meaning to buy a new Dmm anyway, since I'm currently using the cheapest of the cheap. Do you have any recommendations for what would be a good one to get?

[ItsGiusto]

One thing that bugs me about Tagboardeffects - no schematic.

I highly doubt you have a bad transformer, but it's not impossible.  You can check it out with an ohmmeter, out of the circuit.  You should have "some" ohms across the secondary (1? 10? 200?).  Infinite resistance between secondary and primary.  On the primary side you should see similar (not same) resistance readings from start to CT, and from finish to CT.

I know some octave effects want matched diodes in the octave section, but I have no experience (ahem) with the Octavia.

Thanks! I guess I did run some tests like that on the transformer before I put it in. What you're describing sounds consistent with what I saw.

[jimilee]

Are the diodes from a known good source? There should be a diode setting on the dial of the DMM, I'm confused by your statement.


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[ItsGiusto]

Are the diodes from a known good source? There should be a diode setting on the dial of the DMM, I'm confused by your statement.


Sent from my iPhone using Tapatalk

Nope, I bought them from ebay from some Chinese seller, super cheap, like I buy all my components.

My DMM has a diode test setting, but it wasn't displaying good values - I think, in general, my DMM is miscalibrated. It was giving incorrect FV for some other diodes, and not displaying any FV for these diodes, even though I was successfully able to hook up these diodes in test circuits to make sure they work to some degree like they're supposed to.

I guess in writing this, it does sound like the diodes are the most-likely culprit. I'll definitely try swapping them out for better, better tested ones. Does anyone have a good-quality DMM recommendation for under $70

[ItsGiusto]

I guess one thing that worries me is that even with the volume knob cranked, it doesn't even come to unity gain... Maybe I've screwed up somewhere else in the pedal? Could wrong or mismatched diodes cause that? Maybe if the FV of the diodes were several volts instead of .3 volts, I guess in theory it could cause a volume drop, but I wouldn't have thought that much of a volume drop.

[duck_arse]

not wanting to sound rude, but you did test diode test with the circuit unpowered, didn't you? the meter puts "a voltage" that it knows across the test leads (in resistance settings and continuity and diode settings) and then reads the result, any in-circuit volts will confuse/upset.

[edit :]  can you show some photos of what you have built, parts side and copper side?

[anotherjim]

And if you don't have or don't trust a diode tester - rig one up on a breadboard or hook the bits up with croc-clips.

DC supply  - your 9v pedal power or battery.
A series resistor. 10k to 100k. The resistor is just to limit the current. Too low and it heats the diode and makes the forward voltage read higher. Too high and it makes it more prone to picking up noise and messing the reading.

Connect +9 > resistor > Diode Anode > Cathode > 0v.
Measure voltage across the diode.

The way the pedal works is that it must make the signal voltage out of the transformer higher than the forward voltage of the diodes to get anything worthwhile out. So you might temporarily attach the output volume control to the input side of the transformer to see if there's plenty of signal level going in. It should be horribly loud!

[jimilee]

If you've red Lowes all your joints, and you bought cheap Chinese diodes, it's most likely the culprit. I've experienced it first hand.0ne time while troubleshooting the same circuit, the gain pot melted down. It was kind of exhilarating at first, I'd always heard about it happening.


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[ItsGiusto]

not wanting to sound rude, but you did test diode test with the circuit unpowered, didn't you? the meter puts "a voltage" that it knows across the test leads (in resistance settings and continuity and diode settings) and then reads the result, any in-circuit volts will confuse/upset.

[edit :]  can you show some photos of what you have built, parts side and copper side?

Thanks! I did test the diodes out of the circuit, on their own. They didn't show up on my meter (just showed up as infinite resistance, either way I had the orientation), but like I said I thought the meter might be bad. I tried measuring some other diodes I had around. The 1n4148s seemed right at around 600mv, but I was confused that my 1n5054s (or something) were coming back with way less fv than their datasheets say online (something like 520 mv instead of over 1v in the datasheet). But perhaps the datasheet just had a maximum value instead of average?

I also took apart my Dunlop Octavio and measured the fv of the two diodes in it. Both of them were consistently just above 0.2v, like everyone says the 1n34s should be.

Given all of this information, I think I was just sold some bad 1n34a diodes. I ordered some new ones that hopefully will be reliable. I think I may even put them on a dpdt so I can switch them out with schottky diodes, see how each sound.

I will upload some photos soon, once I have a moment.

[ItsGiusto]

And if you don't have or don't trust a diode tester - rig one up on a breadboard or hook the bits up with croc-clips.

DC supply  - your 9v pedal power or battery.
A series resistor. 10k to 100k. The resistor is just to limit the current. Too low and it heats the diode and makes the forward voltage read higher. Too high and it makes it more prone to picking up noise and messing the reading.

Connect +9 > resistor > Diode Anode > Cathode > 0v.
Measure voltage across the diode.

The way the pedal works is that it must make the signal voltage out of the transformer higher than the forward voltage of the diodes to get anything worthwhile out. So you might temporarily attach the output volume control to the input side of the transformer to see if there's plenty of signal level going in. It should be horribly loud!

I tried testing the diodes in that way and unfortunately got extremely large fv, like over 8v. I'm not sure what went wrong there.

I was messing around with my signal tester before when I was trying out different transistors. I'll try it again right before the transformer to see what the volume is like.

[ItsGiusto]

If you've red Lowes all your joints, and you bought cheap Chinese diodes, it's most likely the culprit. I've experienced it first hand.0ne time while troubleshooting the same circuit, the gain pot melted down. It was kind of exhilarating at first, I'd always heard about it happening.


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What's "red Lowes" mean?
How did you melt a potentiometer?

[Cozybuilder]

Another auto speller gotcha. Try "reflowed"

[duck_arse]

don't hang up on the absolute voltages you might get. they will depend somewhat on the test current. if you get a diode reads 620mV one way, and offscale/OL/nothing the other way, it should be usable as a diode.

[PRR]

> 520 mv instead of over 1v in the datasheet

Over 1V, they reject it.

For many diodes, at *1mA*, the drop is *about* 600mV. (So if it is 1V=1,000mV, it's bad enough to reject.)

Whatever it is, it changes about 60mV for every 10:1 change of current.

At unknown test current, 520mV is a very plausible value. Suggests a fat diode or a low test current.

[ItsGiusto]

Given all of this information, I think I was just sold some bad 1n34a diodes. I ordered some new ones that hopefully will be reliable. I think I may even put them on a dpdt so I can switch them out with schottky diodes, see how each sound.

Actually, is it worth doing this? Do germanium vs schottky diodes sound significantly different? If so, I'd like to have both sounds. If not, I'd rather not drill the extra hole in my enclosure.

[thermionix]

Given all of this information, I think I was just sold some bad 1n34a diodes. I ordered some new ones that hopefully will be reliable. I think I may even put them on a dpdt so I can switch them out with schottky diodes, see how each sound.

Actually, is it worth doing this? Do germanium vs schottky diodes sound significantly different? If so, I'd like to have both sounds. If not, I'd rather not drill the extra hole in my enclosure.

I'm gonna say no.  It's not like clipping diodes where you might get two different flavors.  I think the lowest fv is gonna give you the most pronounced octave effect, so a matched pair of Ge's are your best bet.

[ItsGiusto]

The new 1n34a diodes arrived today, and I selected a pair matched at 276/277mv and put them in the circuit - that made all the difference. The pedal positively howls now! I'm reminded of how my old teacher used to call his octavia clone the "obnoxious box."
I haven't really cranked it yet since it's late here, and I haven't directly A/Bed it against my Dunlop Octavio, but from how I remember it sounding, my build sounds just as good, maybe even better than the Dunlop.

Thanks for all the help!