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Wurlitzer 200a/206a - Vibrato Speed Mod DIY Query

Started by amorrison651, March 30, 2023, 04:20:11 AM

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Hey All

So I came across the below thread about a DIY vibrato speed control for a 200

I have a 206a, which has had vibrato circuitry added, but no rate control - something which I would like to have for live performance use, not looking anything extreme, just to be able to slow it down a bit for a couple of the slower numbers my band does

Has anyone done a similar mod with a 200a/206a, and if so any suggestion which R?? controls rate and resistor and pot values to use?

I realise there are new well designed amps out there with this functionality already, but I've already rebuilt much of the original preamp and amp and got this wurli sounding great and very clean so don't feel the need to spend a great deal on a totally new amp at this stage - I also like taking the DIY approach to things
Wurlitzer 206a - 1983


Hi, i'm the OP from that post you reference! So long ago, I didn't even have a camera or phone to document the mod. I've haven't even logged in here in forever, somehow I randomly browsed to this post looking for something else. But, that original mod is still working great and definitely is useful at every gig.

I've not done this mod to a 200a amp but inspecting the schematic, my guess is you want to focus on R12, shown as a 27k resistor in my copies of the manual. From a circuit topology standpoint, this is the equivalent component to the one I mod in the 200 amp. It's a "twin-T" oscillator, and this is the resistor that has a junction with two caps on one side and ground on the other.

As to your second question, first check that the resistor value is actually 27k - it's possible it could have been altered from the schematic to tune the circuit. Once you know the resistor value, remove it, and in its place try a 25k linear pot in series with a resistor that's a bit lower than the value of the original component. Say an 18k or 22k to start if it was a 27k.

What you want is to be able to test the range of outputs for both musicality and also stability of the oscillator. I found that extreme values will cause it to cease functioning. You might need to try different pot values if the first one doesn't provide a broad enough range on top of the series resistor.

I've also done this type of mod to 140b amps numerous times. IMO if you're DIY-inclined it's worth trying, at least on the bench before you commit to hard-wiring and adding an external pot. Happy soldering...