Vox AC-30 Mod: Tremolo Speed Pot

[martthie_08]

Hello, I'd like to mod a 60ies non-TB AC-30 to have a continuous adjustable tremolo speed instead of the standard 3-position rotary switch. Looking at the schematic, this shouldn't be too complicated. The rotary switch connects three resistors (330K, 560K and 1M) to ground that set the tremolo/vibrato speed. In stock condition, the lowest tremolo speed setting is still too fast for my taste. So my plan would be to wire something like a 100K resistor in series with a 1M linear pot to replace the switch. Are there any issues to be aware of or any suggestions?

Thanks, Marten

[Rob Mercure]

Marten,

The 1 M resistor sets your slowest speed (and have you measured it's actual resistance?) so replacing the switched resistances with 1.1 M of resistance, while allowing you variable speed, probably won't slow the speed significantly. Assuming that all the phase shift capacitors have maintained their values - a common cause for old tremelos not to oscillate at all - and the associated resistors you might try putting your potentiometer in series with the 330K resistor which should, all things being equal, slow your lowest speed to about 75%. But why not add resistors in series with the existing 1 M to determine just how slow you want the oscillator fire - and determine whether it will "fire" that slowly - and then determine your parts value. Hell, you might like a 2M pot in there. And, again, replacing the caps - which look like C28-30 (kinda blurry) might drop the speed with existing parts. I don't specifically recall the parts used in a Vox tremelo but USA manufacturers often used the cheapest ceramic caps in the tremelo oscillator and this is why replacing them usually gets older tremelos to oscillate again.

Rob

[martthie_08]

Rob, thanks for your post, I didn't know the 1M set the lowest speed, I thought it was the other way around, thanks for clearing this up! Good idea to try out the approximate resistance for the slowest speed with a fixed resistor and to then choose the value of the pot. Since the fastest setting is fine I think I'll then stick with the 330K in series with a pot of >>1M, whatever it turns out to be.
Regarding the caps, I don't have a capacitance meter yet, but since the caps are not electrolytics or tantals, I'll probably leave them alone, will see...

[soundmasterg]

Ceramic caps at their best are only 20% tolerence and at their worst are 50% or more, and they aren't all that stable. If they used ceramic caps in the trem section, I'd replace them with film caps of some sort. I'm planning to do this mod to my Korg reissue AC30 but haven't gotten around to it yet.