fender vib optocoupler neon - replace with LED?

[Alex R]

This guy:

David Lamkins - Guitarist - Even better Vibro-King "Ticking Tremolo" fix

...says you can cure a Fender bf ticking vibrato by making up an optocoupler bug with an LED instead of a neon. He reckons the 100K tail resistor will deal with the 350vDC-on-an-LED issue. But even if that did get the voltage drop and current limiting exactly right, wouldn't the voltage swing coming off the vib tube plate be too wide for this to work? A customer found the site after I failed to fix his vibrato tick.

DR schem attached.

[R.G.]

Both LEDs and neon bulbs are current-driven devices. Both have a minimum voltage below which they won't conduct or produce light at all, and above which they produce light proportional to current. The LED minimum voltage is less than 5V, the neon is more than 70 or so. Both have the behavior that above their start-emitting-light voltage, the voltage is very nearly constant. I have seen neons used like "zener diodes" in tube circuits.

So where does this leave us?

Well, it's possible that there is a case where ticking resulted from a neon being turned on and off by a tremolo. Note that the voltage across the neon+resistor is slightly discontinuous when the voltage ramps up to 70V, then the neon stays constant while the current flows limited by the resistor.

But I think what's happening is more like ticking caused and cured by the other ways that fender LDR tremolo is caused and cured. There's a current spike caused when the neon turns on and off at the bottom of the voltage swing from the trem oscillator. If that spike gets into the audio path by capacitive coupling or by ground lift in an imperfect grounding setup, you get a tick when the current changes rapidly.

In any of these cases, an LED would be on all the time because its turn on voltage would be so low that it would never turn off. Bingo, no tick. But you could do much the same thing by arranging the circuit so it never turned the neon off, too. No discontinuity, no tick.

That schemo shows 345V feeding a 100K, the neon, and another 100K in the cathode of the driver tube. The maximum current that a neon or an LED could see is (345 - tube saturation voltage - light emitter voltage )/200K. So for the neon, that's (*345-90-70)/200K = 925 uA. For the LED it's more like (345-90-5)/200K = 1.25 ma, only about 20% more, and well within an LED's ability to handle. Could work.

But I doubt that it's THE way to stop tremo ticking. It could well be A way.

[Enzo]

Seems like excess effort. Some older fenders lack that 10meg resistor. it is there to reduce ticking.

I almost always can cure ticking by simply redressing the wires of the trem tube away from the wires to tubes in the signal path.

[Raybob]

Best way to cure the Fender tremolo tick is to use shielded wire to v5, pin2. I ground it to the tube socket bolt and leave the other end ungrounded, attached to the capacitor. Really makes the tremolo silent, as far as ticks go.

[Alex R]

Thanks guys. I suspect the red LED would reduce the vib depth too much, or indeed entirely, but all the same when/if the amp comes back I might give it a try just to see.

On old idea I've been meaning to try is a conversion to Vibrochamp-type vibrato, where a similar oscillator wobbles the bias on the final preamp stage cathode. I wonder if that would work on the Deluxe mixer stage cathode. On the VC it works beautifully with the bypass cap reduced to 2uF; on the Deluxe the mixer stage shares a cathode resistor so that would have to change...