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.







43 comments:

  1. Thanks! This looks super exciting!

    ReplyDelete
  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.

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

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

      Delete
  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.

    ReplyDelete
  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.

    ReplyDelete
  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.

    ReplyDelete
  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?

    ReplyDelete
    Replies
    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.

      Delete
    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?

      Delete
    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.

      Delete
    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!

      Delete
    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!

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

      Delete
  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.

    ReplyDelete
    Replies
    1. Brilliant, thanks for verifying!

      Delete
  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?

    ReplyDelete
    Replies
    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.

      Delete
    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.

      Delete
    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!

      Delete
    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.

      Delete
    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.

      Delete
    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!

      Delete
    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

      Delete
  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.

    ReplyDelete
    Replies
    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.

      Delete
    2. This comment has been removed by the author.

      Delete
  11. playing around with this one again. . . .decided to box it up in a box something else is vacating. The size of the box made me decide to try a pot instead of a switch for the tone control, just to save space. So FYI, a B100k pot works pretty well to pan/blend those two input caps.
    Also, I hooked up a JFet buffer in front of this to see if it would further reduce clock noise. It did not, but I swear this thing sounds more present and solid with the added buffer. Now to find room in the box for the buffer. . .

    ReplyDelete
  12. Hey this is my first time building pedals and I'm going to go buy parts tomorrow should I build this tremolo or another one and is the expression jack also included in the layout
    I'm also going to see if I can get parts for a sho boost a Emerson Paramount and the silicon tone bender you have here

    ReplyDelete
  13. Sorry again for the probably newish question but if I can't source the transistors where van I go to see for replacement transistors or can I just use one with a similar gain

    ReplyDelete
  14. Hi, how to wiring/connect the CV jack ?

    ReplyDelete
  15. Hi!
    47uf & 4.7uf was very much put 4.7uf & 200nf. but the problem is very loud clicks. where I could make a mistake?

    ReplyDelete
  16. Hi,
    I recently got the Skippy board and also observed that the board I have is v1.1 yet the documentation is for v1.0. my homepage
    Is there paperwork for v 1.1 someplace that I missed out on?
    Many thanks!

    ReplyDelete
  17. Just completed this. Had loud ticking same as the EQD Hummingbird I finished 2 days ago. After some research on this problem I found the following mod reduced ticking by 90-95 percent: replace 33 ohm resistor for 100 ohm (start increasing the value until satisfied with reduction, I stopped at 100R, beyond that I stopped noticing a difference) and lastly isolate ground leg from the 100uf electro cap and the grounded end of 33 ohm (or whatever the value you my hva replaced it with) and connect them directly to the DC socket negative pin. I have applied this to both the Skippy (2n6027) and Hummingbird (2N2646) which both suffered from loud ticking and now drastically reduced. NOW it is faint and not all that noticeable but not 100% either. My goal is reduce it further. Aside from that it is a great pedal although I may adjust the tone until I find a value which resembles the guitar natural tone.
    Some pics:
    https://www.dropbox.com/s/kbu9g7tcikp3izo/Moosapotamus%20Skippy%204.JPG?dl=0
    https://www.dropbox.com/s/fwreejt7e2zc524/Moosapotamus%20Skippy%205.JPG?dl=0
    https://www.dropbox.com/s/n9hn292mzni5md9/Moosapotamus%20Skippy%20Gut%201.JPG?dl=0

    ReplyDelete
  18. Also I placed a 20k resistor in series between Rate 1&2 and the 4u7 cap. Moosapotamus has a 20k trimmer to dial in the effect. I tried the trimmer and it worked when maxed out. I tried 18k resistor but I lost the effect near the top of the rate pot and replaced with the 20k to work perfectly

    ReplyDelete
  19. Good stuff Chance. Which 100uf electro do you mean there are two going to GND. Also did you lift the legs off the two components on the board and connect them directly to GND that way or isolate them with track marks? Just trying to visualise it. Thank you.

    ReplyDelete
    Replies
    1. 100uf electro top right on this layout (C6 I believe on Moosapotamus schematic) above the PUT. To make it tidy and easy this is what I did: I had the POS leg remain in place and re-directed it's GND leg. Place a cut somewhere on the third row up from the bottom just to the right of the Q1 emitter which will give us an unused track. Bring the bottom leg of the 33R (or whatever value you may adjust it to) to this track we have isolated. Place the NEG leg of the 100uf on this track also. If you look at my pic I had to bend the leg to the right to achieve this, also had to give it some height as it sits over resistors, no prob as long as you have space in the box. Lastly run GND wire from this track to the DC NEG pin. That's what I did anyway plus change the 33R resistor and it help alot. I'm not an expert here, I can follow a map, but I took notes from R.G. keen and JaviCAP to come up with this solution (thanks guys). Hopefully I can develop an even greater understanding about how these effects work and eliminate the damn ticking from everything 100 percent for good, bloody headache they can be. Pic below:
      https://www.dropbox.com/s/3cm7kdaoav9kgwm/Skippy%20with%20GND%20MOD.JPG?dl=0

      Delete
    2. Thats great thank you for taking the time to explain this. I'll definitely implement this when I return to my repeat percussion build.

      Delete
  20. This comment has been removed by the author.

    ReplyDelete
  21. TICKING... If anyone has a minute can they please have a go at the following mod to deal with this ticking... try replacing (or just perform a quick bypass) the 33 ohm resistor with say a 2k trimmer. I tried a 500 ohm and have now settled for 2k. You should hopefully notice as you adjust the ticking/pulsing can be reduced or near eliminated (with full tone setting if you are using near the 100nf in the schematic, this mod is not so effective on the low value cap like the 4.7nf, too much signal loss needing to much level which then makes the tick more pronounced once again). Between zero and halfway you'll notice the best reduction but beyond that too much resistance starts to minimise the 'chop' and starts to act like a depth control and I guess sounds like a standard tremolo. Test and adjust, you should find a happy medium. It does affect speed you will notice but it's of no real significance, tone is unaffected. Make sure you isolate the GND from the 100uf (c6) and trimmer (what was the 33 ohm resistor) and send it straight to DC NEG pin.

    ReplyDelete
    Replies
    1. I just did some further testing with this mod and I highly recommend either placing a 510 ohm trimmer (2k works better in the hummingbird) in series with the 33 ohm resistor or just replacing the 33 ohm resistor with the trimmer. Placing the trimmer in series just allows you to retain the stock value if you back the trimmer right off. I now have a squeaky clean Skippy, no tick/pulse but max effect.

      Delete
    2. If you apply too much resistance with the trimmer you will notice the effect start to degrade.

      Delete