neovibe troubleshooting - not working correctly

[larsthestrat]

It's working! Had some spare-time and changed Q11/Q12 for new ones and now it's oscillating! Lamp works fine now.

I do have so issues whem strumming the lower E, it kinds of distort a little when I do this. It doesn't change when turning potmeters or trimpot (TR1).
Any ideas?

[R.G.]

Hey! Good work! That would be the next place to go anyway.

There have been some issues with low gain in Q11/Q12 not letting the oscillator start. You hit the right one!

Distortion on loud notes can be a problem, due to the design. The limitation is mostly in the first three transistors, which make a kind of dual-output 'opamp' circuit. Even though it's running from 15V (in the updated, regulated power supply design in the Neovibe anyway), the signal only has a limited swing on the collector and emitter of Q3. This is because the signal from Q3's outputs (!) share the 15V, so each can have no more than 7.5V to swing in a perfect situation. In reality, neither the collector nor the emitter can actually swing a full 7.5V peak to peak. So the Q1-Q3 circuit may have slightly less signal swing than a 9V single ended circuit if biased perfectly. If it's not perfectly biased - and nothing is ever truly perfect - then the swings available can be less. There could be other issues, but this one exists even if it's working fine.

Your notes on the potentiometers not changing this is correct - none of the pots have any effect on what causes it to distort.

So - what kind of guitar and pickups are you driving it with? Or is it being driven from the output of another pedal?

[larsthestrat]

Thanks!

I did use a stratocaster with no boost or anything in front. I just soldered a 220K before R1 (or R2, between footswitch and input resistor) 10 minutes ago to change the input a little bit and now the distortion on the low notes is gone. But then I have trouble getting it at unity gain (not enough output woth 100K pot) so maybe changing the volumepotmeter to lets say 220K is going to solve that one? Or will the distorted signal then return?

[larsthestrat]

could you check on my lamp and the flashing, is this correct?

http://www.youtube.com/watch?v=BurFmJvblpI&feature=youtu.be

[larsthestrat]

anyone?

[lonewolf]

you could add a 250k trimmer or pot on the input then adjust to eliminate clipping and If you set up the cancel switch instead of true bypass that should help with the volume drop and since it turns off the bulb it should last longer...the bias offset trimmer mod helps dial in the right  swish sound and my effect didnt sound right until I used a c100k dual pot...you have to use your ears to tweak the bulb to get that sound..

[larsthestrat]

I will make a soundrecording to let you help judge me.

[alfafalfa]

That's the way to go. And listen to Steve Boutte's clips,  to me they are a real standard .

Just google them .

[larsthestrat]

I did some mods to help the pedal sound 'better'. Changed C6, C8, C11, C14, C17, C18 to 10uF for some more depth, 2.2k on R42/R43 to increase speed (like that one!)

[larsthestrat]

I'v read that you could use mps-a13 darlington on q10, q11/q12,q13. Does this have any effect on the sound?

[lonewolf]

using darlingtons will not make it sound any better..it may keep the bulb driver trans from getting too hot ...and it may not oscillate as well...roll the dice..I would leave it alone...(I changed to mpsa-13 but switched back to 2n3904's)

[larsthestrat]

oke, thanks. i will be building another one, hoping to find some better parts. For example I used this LDR's
http://www.banzaimusic.com/banzaisearch.php?q=17332&btnG=Search&site=banzai_music&client=banzai_music&proxystylesheet=banzai_music&output=xml_no_dtd&getfields=productimage&filter=0&start=0

Are these better for example?

http://www.banzaimusic.com/Photo-Cell-132k-3-0M.html

or these?

http://www.banzaimusic.com/Photo-Cell-132k-3M.html

[larsthestrat]

got the correct ones. Final question, is it possible to swap the following caps form more depth (10uF instead of 1uF). Building another one...

c6, c7,c8,c10,c11,c13,c14, c17, c18

[lonewolf]

I would say no..although if you use sockets you could swap.leave them the stock values in my opinion.I would replace R 35-36 with a 250k trimmer..then you can adjust the output mix for the strongest effect..and  a 250k 20 turn trimmer for the bias offset with a 2n3904 darlington driver for the bulb

[larsthestrat]

sorry I was referring to C6, C17 and C18.

I have used 10uF on the caps and that worked fine.

[larsthestrat]

oke, changed them works fine, somewhat more depth I guess. If I would like to let the lamp be brighter on top speed en depth what resistor do I need to change. I'm already using a 500ohm trimmer. Could you please reply?

[R.G.]

It's probably not as simple as changing one resistor to get the lamp to be brighter on top speed and depth.

The lamp, being an incandescent filament, has some thermal time delay as a result of its mass. At slow speeds, it can get to full brightness on voltage peaks because it has time to heat up. As the speed increases, even if the LFO driving voltage/current is the same, it can't heat up to full brightness before the LFO drive starts decreasing again. So as the speed gets faster, the lamp light becomes more and more of an average of the driving voltage. This is why incandescent lamps in houses running on 50 or 60Hz don't flicker - they don't have time to change brightness between cycles of the AC power, and only glow an average of the driving voltage.

In a way, the thermal delay of the filament is acting like a low pass filter.

You might be able to get some improvement by using a different lamp bulb. Probably smaller is better as long as it doesn't burn out quickly.

[larsthestrat]

thanks, your explanantion helps a lot in understanding what is happening.

I will order a 12V (now I do have a 18V). Just swapped out the 68 ohm and used the trimpot to decrease the ohmage to bout 30 ohms, that seemed to help a little bit. I also changed R50 to 100K and 330K but that didn't have any effect.