Wednesday, 2 April 2014

Moosapotamus Skippy Tremolo / Ring Mod

Based on the Vox Repeat Percussion but with greater control options, and another advantage is this version uses the 2N6027 Programmable Unijunction Transistor which is still available cheaply and in quantity, so you don't have to search around for 2N2646's and pay top dollar for them. 

I haven't included the connections required for external expression pedal rate control, opting to keep it self contained, but if you do want to include one, make the connections to the control socket to where the rate pot currently connects to the board, as shown in the scheme on the Moosapotamus site.

Info from Moosapotamus about their pedal:

The SKIPPY tremolo is based on the vintage Vox Repeat Percussion. The original VRP is well known for it’s very hard, percussive tremolo sound, or chop. Certain combinations of speed settings and playing style can almost trick your ears into thinking that you are playing through a delay instead of a tremolo. For these reasons, the sound of this tremolo effect is often described as synth-like.

Originally, the VRP had only one control, Rate, to adjust the speed of the effect. In addition, the version that was built into the Vox Beatle Super Reverb amplifier also had an input jack for a foot pedal that enabled you to control the speed of the effect with your foot.

With additional controls and features, SKIPPY covers a much wider sonic range while still retaining all the character and bravado of the original VRP. Check out the video…

CONTROLS
Depth - Adjust effect depth from subtle throb to hard chop.
Rate - Adjust effect speed.
Level - Adjust output volume to compensate for perceived volume loss with certain extreme settings.
Tone - Switch original Vox low-cut filter on or off. When on, output has a lot of treble and very little bass.
Mode - Switch between hi and low speed ranges. Low speed is normal tremolo range. Hi speed is pseudo ring-modulator range.

FEATURES
Plug CV pedal into control jack to vary speed with foot while playing.
Capable of ultra-wide range of speeds, up to pseudo ring-modulator speeds.







27 comments:

  1. Thanks! This looks super exciting!

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  2. It is.. and a hell of a lot easier to source than the trannies in the Vox repeat percussion and it sounds pretty impressive! I like it enough that I built one even though I have a VRP.

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  3. I am ordering the PUT now. Definitely going to build this.

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    1. I just got in my 2N6027's! going to build this right now.

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  4. I found socketing the caps for your rate control to be a very good idea. If I recall correctly (and it's been a little while and from a different layout) the speeds were all too fast for my tastes and I was able to adjust the entire range to my liking by auditioning a few different caps. I'll open mine up and check which ones I socketed and what values I used next chance I get.

    Of course someone smarter than me can probably take one look at the layout and tell you what caps are responsible, but I still get a bit cross-eyed following the signal path on vero layouts.

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  5. Pretty sure it was the 4u7 and the 47u caps... and I want to say the 68n cap... but that might have been me switching stuff out of ignorance.

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  6. Yep, the 4u7 and 47u set the fast and slow mode speed ranges: Smaller cap = faster rate. I would advise connecting the mode switch lug 2 direct to the input or output jack ground, and not to the board's ground strip (bottom row) which is shared with the audio portion of the circuit: This is an attempt to minimise clicking that is common in this type of design. It may still click a bit anyway.

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  7. Got a crybaby shell+pot crying to have one of these bad boys fitted in it!

    Not sure what the pot value im the wah is but since the rate pot looks like a simple variable resistor summing the value of two caps, am I right to think that it wouldn't matter if it wasn't a 100k?

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    1. The fastest rate would remain the same but it would change the slowest rate, so it may be worth adding an extra column to the layout so you have space to add a resistor in series with the pot should you want to fine tune the control.

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    2. Thanks a lot Mark! Would that be a case of the higher the added resistor the slower it'd get then? Or am I looking at things the other way round?

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    3. Yes so you'll probably want to socket the resistor to find a happy medium between the slower and faster rates or adding a trimmer would probably be the best idea to fine tune it. If you look at the schematic at the Moosapotamus website linked above, the original circuit had a trimmer in there which I omitted because I wanted to control the rate with just the pot which would then allow you to achieve the fastest speeds possible.

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    4. Thanks again Mark. This is definitely in the pipeline and hopefully will be done in not too long. Will report back and share my findings!

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    5. In my build I found that when turning the rate pot towards "faster," the tremolo quit working before the pot was swept to wide open . So I think the idea behind that trimmer is so you can tune the minimum resistance set by the wide-open position of the rate pot, and avoid having a dead zone in the pot sweep. In my build for example, I found that an additional 12K resistor in series with the 100k pot gave me use of the full pot sweep. Using a 2.2mf cap for the fast mode, the fastest speed is really, really fast. . .

      Thanks for all the cool layouts Mark - lots of fun!

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    6. Thanks for the info Andy, I think I'll add a layout with the trimmer then so people have the choice. Cheers

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  8. I have built this from this layout and it works fine so can be verified.
    I forgot to try nocentelli's suggestion with the earth wire and it clicks a bit straight in but with a Nobels ODR-1 overdrive into it (not JohnK's layout but an original because I wanted the buffer - from Thomanns, 3 working days Germany to UK!) the clicking is barely noticeable. I had already build the board so did not give myself the socketting option. The high range runs from about 100bpm from the bottom to very fast before the end of the range but the Slow only works in the middle of the range so Nocentelli is right again. I may try other values later but the 4u7 (fast) is near enough for me. I don't know how much use the low range is (it's REALLY slow) but if someone needs it my best guess would be to try a 22u instead of the 47u.

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    1. Brilliant, thanks for verifying!

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  9. I can't source any mpsa18's, so i thought I'd try 5088's (not working out, and I don't have any 5089's, which I think would be a better alternative). What would be a good substitute, or must I have the mpsa18's?

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    1. Not working as in no sound, or doesn't sound good? Any of those transistors should get sound out of it and so if you're not getting anything something else must be wrong. Measure your voltages and post them here to see if there is anything obvious wrong with it.

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    2. Oh no there is deffinitely sound. My apologies for not being specific. Here is what is happening: i get a tremolo effect, which sounds fine and ordinary, except that the tremolo flattens out completely when the rate reaches about 2 or 3 oclock. I don't hear any change in the depth, but I'll troubleshoot that after I get the rate set right, if it's still a problem. I assumed that at least the rate defect was a transistor issue.

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    3. okay, here are the voltages:
      Q1
      E - 0
      B - 0.37
      C - 0
      (obviously a problem, and it explains the depth not working. It seems the base isn't getting enough voltage to turn the transistor on?)

      Q2
      K - 0.96
      G - 1.05
      A - 1.88

      Q3
      E - 0.43
      B - 1.02
      C - 0

      Thanks for all your help!

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    4. Okay, before I've led a wild goose chase, I found a solder bridge between depth 3 and tone 2. I'm sorry, I wouldn't have posted if I had seen that when I checked before. I am going to see if that fixes it.

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    5. No Q1 collector is ok, it's grounded and so should be 0V. Q3 collector though definitely isn't. That should be connected to the supply via the 33K resistor and so definitely shouldn't be 0V. Have you missed that link, and is all the soldering good around there? Anyway that's the most obvious standout problem which I think needs sorting first so check all those connections around Q3 collector.

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    6. Hi IvIark,

      Great job! However, I think it would be very helpful if you could include the vero layout for complete version of this effect that include trimpot and expression input. Many thanks!

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    7. Trimmer is only needed it the 2N2646 UJT is used. The trimmer isn't connected in the original if the 2N6027 PUT is used.

      As for the expression input -
      "I haven't included the connections required for external expression pedal rate control, opting to keep it self contained, but if you do want to include one, make the connections to the control socket to where the rate pot currently connects to the board, as shown in the scheme on the Moosapotamus site."
      http://moosapotamus.net/ideas/skippy-tremolo/
      You can see how it's wired with a stereo jack: http://moosapotamus.net/images/Skippy-v1-scheme.jpg
      +m

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  10. I have this built and working, it is a fun tremolo! However, the ticking from the lfo is very pronounced and seems to get slightly more pronounced when turning the depth pot down. I know the trick about separating lfo and signal ground and connecting them off board but I don't see a simple way to do that with this layout. Maybe I have something else wrong with the circuit though it seems to work fine other than the ticking. Any suggestions to tame the ticking are appreciated.

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    1. Looking at the schematic I had a question, is the 47u for the range switch supposed to be a 470n? I ask because they used the non polarized cap symbol and labeled it u47 instead of 47u and also this range seems unusably slow. At any rate, I'm going to experiment with those caps.

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