Thank you AG and celadine - I just solved the problem, then came inside to read your values and comments.
I suspected that the 47n in the 3rd phase shift stage of the little voodoo vibe was the incorrect value, and so I had a think about why these particular values were chosen. Grolschie was in my workshop this afternoon and I had a brainwave as to why these values and their particular order etc., after we both listened to my little voodoo vibe in it's 1790 case, and decided something was not quite right - sounded like a bad acid trip
Anyway - having read the technology of the univibe by RG Keen, in which he gives the original values, although not understanding why they were chosen, it dawned on me that, it does not matter which order the capacitors are inserted, and that this was possibly to confuse people trying to clone the univibe - who knows
Okay - there are 4 phase shift stages with a different value capacitor being used in each stage. Why
- to give a more complex harmonic phasing sound like a rotary speaker does
who knows. I believe these capacitors were chosen to represent 4 distinct frequency bands of operation, and it seems reasonable that they should be evenly spaced in the frequency domain.
So - out with my logarithmic graph paper and ruler.
First - I used 470pf for the highest phase shift frequency, because several people including RG Keen stated this as one of the correct univibe values.
Second - I chose to stay with the 100n capacitor used in the little voodoo vibe for my lowest phase shift frequency - although RG Keen stated that 220n was the correct value, I noticed that one of the schematics I had previously downloaded had a suggestio to use 100n for less bass response. This was possibly Roger Mayer's chosen value in his modifications to Jimi's univibe, and why it appears in the little voodoo vibe
Third - I measured with my ruler on the logarithmic frequency axis of my graph paper, and divided the linear distance between 470pf and 100nf into three even spaces. This gave approximate values for the remaining two capacitors as 2n7 and 15n. So I removed the 47n and 4n7 and replaced them with these calculated values.
How does it sound now - fantastically smooth, bubbly and a lot brighter in the top end due to the 470pf capacitor. Before the mods the vibrato made me feel like a seasick drunk and there was no way i could get an evn sounding chorus - it lurched - no matter where I set the bias, symmetry or intensity controls. Now all these controls have very usable ranges and as Roger Mayer states in his literature you should start with all these controls set in the centre of their travel and tweak to personal taste.
Now the chorus does indeed sound much more like a rotary speaker cabinet - before it sounded like a badly balanced washing machine on it's spin cycle
I don't think it makes any difference which order these capacitors are inserted, but it does make a very noticeable difference using these values compared to green fuzz's or kaputella's values which are clearly incorrect after my listening tests
If you want a lower bass response then use these values - 470pf, 3n9, 27n, and 220n. Or experiment with a wider or narrower range, just keep the linear distance between each value on a logarithmic frequency axis the same, and read off the values.
Another way to calculate values is to first decide the highest frequency capacitor and multiply it by the same amount.
For example: choose 470pf, then multiply by 8. This gives 3760pf (3n9).
Then multiply 3760pf by 8, this gives us 30080pf (30n). then finally multiply 30080pf by 8, this gives us 240640pf (240n)
So you could use 470pf, 3n9, 30n, 240n for a different but still very smooth sounding chorus effect.
Have fun
bajaman
