Neovibe Bulb Bias

[jimgreco87]

Hey guys, I just finished up my Neovibe! Amazing sounding pedal and thanks to a few threads I've found on here, I was able to troubleshoot most of the issues. I have a question about biasing the bulb, I can't seem to set it to a dim orange, which I would think would have the deepest phase sound. With the stock trimpot all the way clockwise, it is lit, but I think its brighter than it should be. Is there any values I could change to make it a little more dim, or is this the brightness I'm looking for? Any input would be appreciated, thanks!

http://imageshack.us/photo/my-images/155/img20111106085741.jpg/

[R.G.]

Wow. Nice board. Did you toner transfer that? If so, good work.

Which version number of the PCB is it? Should be on the printing on the board. There are slight variations from version to version.

[jimgreco87]

It's actually a board from GGG, I'm still pretty new to building pedals, its only my 3rd build! Its v10/07. Sounds great, just trying to see if I could make it sound even better.

[R.G.]

I'll go look at the 10/07 version and see what to change. To more directly answer your question, yes, you can get a better variation of brightness.

Just to get the whole picture, you say you can't make it dim enough. Does it get brighter when you turn the bias the other way? That is, does the bias trimmer work at all?

It's possible that the trimmer has a problem.

[jimgreco87]

Yes it does get brighter when I turn it the other way, just cant seem to set it any lower than its currently set to.

[R.G.]

OK. I'll go look.

[jimgreco87]

Thank you sir, I greatly appreciate it!

[R.G.]

Do this: turn it off, get out your ohmmeter, and measure the values of resistors R47, 48, 49, and 50 and the trimmer pot; then turn it on, turn down the depth control all the way and measure the DC voltages on the collector, base, and emitter of Q13.

It is possible that a part value is off; if they're all correct by actual measurement, then we can modify R48's value to change things a bit.

[jimgreco87]

Numbers fluctuated a lot on some but this is what I measured.

R47 = 35.6k
R48 = 67.3k
R49 = 4.65k
R50 = 34.5k
Trimmer = 214.1

Q13
C = -21.3v
B = -5.8v
E = -5.2v

[R.G.]

Just checking

- is R48 67.3K or 67.3 ohms?  R48 being the one in series with the trimmer.
- Are the voltages really negative? Or is that a meter probe placement thing?


Can you do that again, but this time turn the trimmer all the way the opposite direction?

If we don't find a bug, it may be that for your bulb and transistors you need to change the value of one or more of the resistors, probably R47 or R48.

[jimgreco87]

Woops sorry! R48 is 67.3ohms. As for the negative it was the DMM placement. With the trimmer all the way up it reads 19.2ohms. Let me rewrite this.

R47 = 35.6k
R48 = 67.3
R49 = 4.65k
R50 = 34.5k
Trimmer = 214.1 (dim bulb)
Trimmer = 19.2 (bright bulb)

Q13 (trimpot dim)
C = 21.2v
B = 5.8v
E = 5.2v

Q13 (trimpot bright)
C = 12.77v
B = 2.56v
E = 1.95v

[R.G.]

OK. Those are reasonable, if not expected, results. It's working OK from what I can tell, but there are some oddities.

- it's possible your bulb is a bit unusual; resistance in series with it may help
- you can try increasing R48 from 68 to 150 or even 220. We already know that making the sum of trimmer and R48 be 214+67 = 281 ohms doesn't do anything really bad.
- you can try decreasing R50 from 47K to 33K, or even 22K. This turns the driver more 'off' for a given set of conditions. It's the relative tap point in the R47+R50 resistor chain for the base that determines the fraction of the collector voltage fed back to the base. Lowering the radio gives the emitter bias resistor more to work with.

[R.G.]

Just did a little sim work. Changing the 47K to 33K makes a big difference in the bias range. You get the same effect by paralleling the 47K with 110K to 120K.

[jimgreco87]

Thanks RG It's the standard 12v/25ma radioshack bulb also bought a few extra incase I broke it stripping it to its leads. Might give another bulb a shot. Once I get some spare resistors I'll try those too!

[jimgreco87]

Just threw in a 33k, works perfectly! Made the bias a lot more flexible and was able to get it down right where I wanted it. Thanks for making such a great sounding board and support

[R.G.]

Good deal!

[jimipop]

Hi guys !

I made this neovibe to but the bulb didn't oscillate  ...
The 2n3904 works fine i think.
Of course i tried with different depth and speed position

any idea to resolve this ?

A great thanks for any kind of information !!!

(Sorry for the english, i'm french ...)

[R.G.]

I made this neovibe to but the bulb didn't oscillate  ...
...
any idea to resolve this ?

There have been several people with this problem here in the past. They've all been solved by following the procedure in "Debugging: what to do when it doesn't work".

(Sorry for the english, i'm french ...)

No problem. You'd laugh yourself silly if you heard my French. 

[jimipop]

Thanks R.G. ! I will do my best to mesure the voltages ...

See you next year !

[jimipop]

Hi there !

As i had time and copper board i rebuild a new one with just others new 2n3904 and now ... it works !!!
It was probably a short or a dead something 
Im better builder than debugger hihi !

(made from the neovibe update 8 pages pdf)

Thanks again RG