biasing the Neovibe lamp driver

[powerplayj]

I maxed out the trimmer and it seems I should probably have a bit more glow (about half brightness according to Geo) when the depth pot is minimized.  Should I increase the resistor (68ohms) in series with the trimmer or should I change the base resistor (100k) to get the desired intensity?  If I need to change the base resistor, would I need to increase or decrease the value?

http://www.generalguitargadgets.com/diagrams/neovibe.pdf

Thanks,

Jason

[R.G.]

If you get some glow, and the LFO makes it get brighter and dimmer, close it up and listen to it, then diddle the bias a bit to see if it sounds better or worse at some settings. Try it for sound before you try to get accurately half the brightness. The amount of basic glow with the bias trimmer is highly subjective.

N.B. The resistor in series with the trimmer is 68 ohms, not 68K. I think that was a simple typing error in your post, because the bulb wouldn't light at all with a 68K there.

[powerplayj]

yeah, it was a typo....


Actually, the 200 ohm trimmer is at 0 ohms and I barely get the bulb lit.  Should I sub a 47 ohm resistor (or lower) in for the 68 ohm just to have more room to play with LFO setting or will too low a value fry the transistor?

[R.G.]

Could be a low gain transistor. If you have one, sub another device in there.

I hate to tell you to put lower value resistors in there because that resistor is the DC stabilization for the lamp driver. But, shoot, if I'm telling you to change the transistor, why should I wink at lowering the resistor?

Sure, pop in a 33 or 22 ohm resistor.

Or change the transistor.

JC will pop in here in a second and tell you to darlingtonize the lamp driver. That's OK, it works. Purists always yell at me if I say to change the Univibe circuit in any way, but I'm all for whatever works.

Another consideration is - what bulb are you using?

[powerplayj]

50mA 12V bulb....I have a 25mA bulb but the leads on the 50mA are much more user friendly and it sits upright much better in the light shield.

[Eb7+9]

we've been through this so many times already ...

bulb you want is something more like 25mA/6v rating - found at Rshack ... I recommend getting the bulb right before altering circuitry

FIY, my mods to the Vibe are mainly extension mods - to extend the performance of the LFO-bulbDriver at both ends of the speed range ... and, also to make that old dynosaur easier to adjust (still takes me a good 2 days of playing to tweak to perfection) ... you don't need all that if you just want basic Vibe operation ...


purists yelling at you all the time RG ?! - gawd, life must be hell for you ...