http://lamkins-guitar.com/music/article/vk-trem-fix
David Lamkins picked up his first guitar a long time ago. As best he can recall the year was 1967: the year of the Summer of Love. Four decades later David has conjured up an amalgam of folk, rock and jazz solo guitar music for the occasional intimate Portland audience.
location: Portland, OR USA

Facets: amplifiers, mod, fix, Vibro-King, @musings info
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Vibro-King "Ticking Tremolo" Fix

The Fender Vibro-King has a well-known problem with ticking tremolo. It's present in all the Vibro-Kings I've played (about five of them), and I've heard about many others that have the same problem. It's kind of surprising to encounter such a persistent problem in a $3,000 amp, but it's there. Fortunately, the amp has other positive attributes to recommend it...

The trem ticking is really not so much a design flaw as it is a problem inherent in the design of the circuit. Older Fender amps have this problem; the well-known fix is to install a 0.022 uF cap across the neon lamp. Unfortunately, that doesn't apply to the Vibro-King. Not only is the circuit slightly different, but that cap is already present.

A posting on the FDP describes a fix attributed to Bruce Zinky, the designer of the Vibro-King and other Fender Custom Shop amps. Zinky's fix is to cut open the trem bug (an assembly consisting of a neon lamp and one or two LDRs wrapped in a length of shrink-wrap tubing) and put some distance between the lamp and the LDR(s). Folks who have amps with the single-LDR trem bugs have reported reasonable reductions in ticking thanks to this fix. I had far less success applying the fix to my dual-LDR trem bug, so I went looking for further improvements.

Before I outline my fix, I'd like to explain why the ticking is a problem inherent in the design. It's all the fault of the neon lamp. A neon lamp has an interesting behavior as the voltage increases across its leads. From zero up to about 90 volts, the lamp is dark and non-conducting. At a certain threshold in the neighborhood of 90 volts, the neon gas in the lamp ionizes and conducts electricity and the lamp emits light. But here's the interesting part: the voltage doesn't stay near 90 volts; it drops suddenly to around 60 volts. That's why you always see a resistor in series with a neon lamp: to prevent it from drawing excessive current once the lamp starts conducting.

Got that? OK. It's the sudden change in voltage across (or current through) the lamp that's responsible for the ticking. There are two possible explanations for the ticking; I haven't taken the time to do the experiments to find out whether one or both is applicable. The first hypothesis is that there's capacitive coupling from the neon lamp to the nearby LDR. You have a fast 30 volt change on one of the electrodes of the lamp; that gets coupled into the LDR which then passes a brief spike to the audio line. The second hypothesis is that the sudden current change (about 4 to 5 ma) when the LDR turns on induces noise in the power supply or ground line, and that shows up in the audio as a tick.

So, what to do? Zinky clearly thought that the capacitive coupling was significant. His fix addresses that by increasing the distance between the lamp and the LDR, thereby reducing capacitive coupling. The old-line Fender fix (the 0.022 uF cap across the lamp) seems to tackle the same issue: the cap reduces the rate of change of the voltage across the lamp.

I suspect that the Vibro-King actually suffers from a combination of the two effects. In the VK, the 0.022 uF cap is across the driver tube, not across the lamp. Moving the cap to shunt the lamp rather than the tube actually made the problem worse, turning the lamp into a relaxation oscillator during the transistion. (Don't worry about what that means, if you don't understand it. What happened is that the tick turned into a musical pitch that was much worse than the tick.) Removing the cap entirely caused a slight reduction in ticking; this suggests that there's some noise being coupled through the supply or ground. Separating the lamp from the LDRs, per Zinky, also caused a slight reduction in ticking. Together, these changes made a noticeable improvement in the ticking, but did not eliminate it entirely. The separation of the LDRs from the lamp also had the side effect of reducing the intensity of the trem, although it remained quite useable.

Not satisifed with "almost" fixed, I decided to eliminate the cause of the problem. If the sudden change in the voltage and current through the neon lamp was somehow responsible, I'd eliminate that change. The key to this mod is the observation that a very small current through a neon lamp will maintain it in its conducting state while not emitting very much light at all. If the lamp is always conducting, then you don't have that 30 volt spike every time it turns on or off. As the current increases, the lamp gets brighter. As the current decreases, the lamp gets dimmer, but never turns off completely.

This mod, as described below, completely eliminated all traces of ticking in my amp. However, there's a tradeoff: you'll lose some trem depth. On my amp, there's no noticeable trem effect until the depth control is set at least half-way. However, the remaining trem is quite useable.

Here's what I did. This easily-reversible mod can be done in about ten minutes, not counting the time it takes to disassemble and reassemble the amp.

But first, the disclaimers:

IMPORTANT: Consider yourself warned about the potential life-threatening danger of working inside a vacuum-tube amp. If you're not familiar with the rules for working safely on this kind of equipment, then get this work done by someone who has. Improper safety technique can cause your death by electrocution.

If you try this mod on your Vibro-King, I'd like to hear how it works (or not) for you.

Instructions
  1. If you've already done the Zinky mod to the trem bug, put it back as close to original as you can.
  2. Remove C26, the 0.022 uF cap next to the trem bug. Set it aside in case you want to undo the mod.
  3. Disconnect one end of R50, the 10M ohm resistor across the leads of the neon lamp, from where it connects to the lamp and R49. Take a short piece of hookup wire to extend the now-free lead, and connect it ground at the point where C26 was grounded. (Not at the lamp; the other end of C26.) Alternatively, you can remove the 10M ohm resistor entirely and try using a larger value resistor (22M would be fine) in place of the 10M resistor and extension wire. I haven't done this myself, but it might give you just a tiny bit of additional depth.
  4. Connect a 68K ohm resistor (1/2 watt rating is fine) from ground to the junction of R49 and the lamp.
  5. Connect a speaker, turn all the reverb knobs to zero, apply power, set the tone controls mid way, crank the volume and the trem depth, and listen for ticking as you sweep the speed control through its entire range. If the ticking is gone and if changing the depth control doesn't produce a significant change in the volume of the amp's background hiss, you're done!
  6. If the ticking is still present (you'll most likely notice it at the fastest trem rates), try a higher value resistor in place of the 68K. If there's too much volume change as the depth control is swept over its range, try a lower value resistor in place of the 68K. You shouldn't need to go lower than 47K or higher than 100K, in any event.

Here's a photo taken after the mod. Click the photo for a larger view:

VK trem fix detail

Notice that my trem bug is wrapped in electrical tape because I had previously applied Zinky's mod. You should leave your trem bug as-is. If you've already tried the Zinky mod, push the neon lamp and LDR(s) back into their original positions and retape the assembly.

June 30 2005 04:10:09 GMT