Modding an Ibanez PT-909 Phase Tone

[dacaumodo]

Hi all
I'm thinking of modding my nice PT-909 so the "depth" control - and perhaps also the "feedback" - let me reach more extreme modulation. Can anyone tell me how to do that, or suggest relevant links?
Thanks
Guillaume

[Mark Hammer]

According to this schematic (http://www.freeinfosociety.com/electronics/schematics/audio/ibanezpt909.pdf), there should be two trimpots on the board, labelled (or maybe not) VR101 and VR102.

VR102 sets the maximum amount of feedback attainable.  Normally, because more feedback increases the risk of oscillation breaking out, such pedals are often set up to include a trimpot.  At setup, the Feedback/Regen/Repeat/Recirculation control is set to maximum, and the trimpot is adjusted so as to be just at the point where the oscillation stops.  A person CAN change the trimpot setting, but of course you run the risk the trimpot was put in there to avoid.  Still, that doesn't mean there isn't the potential for just a little more resonance/feedback before all hell breaks loose.

One trick that has proven successful in other contexts is to set imits on howmuch feedback can occur in one or both of two ways. A) sticka a back to back pair of germanium diodes in series with, say, a 1k resistor, and put that in parallel with R112 and C106.  This is often what is done with flangers to make sure the input+feedback signal never exceeds a certain level.  It will produce some clipping with high regen/eedback levels and strong strumming, but maybe that's what you want. 

The other route to take is to shave off some of the bass from the feedback path by reducing the value of either C119, C120, or both.  Most of the feedback signal and risk for oscillation occurs in the low end, so trimming some bass will permit higher feedback settings (with the trimpot) before oscillation breaks out.  Dropping either, or both of those to a value of .022uf - .033uf (or maybe one of each) will reduce (but not eliminate) bass content ion the feedback path and let you crank it higher.  Just note that the frequency where the cut occurs will depend on the joint product of R113, VR102, and C119.  Stock, with VR102 set to maximum resistance, the sum of VR102+R113, time C119, yields a rolloff below around roughly 80hz.  Making VR102's setting smaller (to get more feedback) results in a cross-product that raises that rolloff frequency.  So, if you're going to reduce VR102's setting, then you don't want to make C119 too much smaller. Maybe .033uf or even .039uf is optimal.

Note that while this will permit high amounts of feedback before oscillation occurs, it also means the intensity of the feedback will vary across the sweep range.  Trimming back on bass means that there will be less resonance at the lowest part of the sweep compared to the highest.  If tha's of interest, let her rip.  If not, then maybe the diode soft-limiter is the optimal route to pursue.

Certainly let us know how it turns out and what you notice different about the tone.  If you can make a before-vs-after sound clip, I think that would be appreciated by all.

[dacaumodo]

Thanks Mark for an amazingly detailed and knowledgeable answer. From what you say the first solution (the diodes) sounds the best to me , first because it sounds easier to undo if I mess things up (which I am very likely to do) and second because I would'nt have to alter the amount of bass in the feedback.

I see VR 101 is labelled "Bias": what would be affected , in terms of sound, if i fiddled with this? I could of course go ahead and try but I'm afraid I might mess everything up (I'm still not too clear about what bias is...yep, I'm a beginner) Could it increase the width?

Thanks a lot Mark

Guillaume

[Mark Hammer]

Oh yeah, I forgot to mention that.  Like any FET-based phaser(e.g., P90), the FETs have to be biased on their gates to be "in the zone" where the Drain-Source resistance can be easily varied over a decent range.  You CAN sometimes shift the range of sweep over a bit without taking away from the sweep width using this trimpot.  You will find that the entire range of the trimpot is not useful, just parts of it.  Higher amounts of regen with higher sweeps can be quite striking.  I modded my Ross Phaser to sweep much higher than usual with more regen, and when it keeps going after most phasers would have turned around and started back in the other direction it is pretty striking.

[dacaumodo]

Oh oh, this sounds JUST like something I want to try! Well within the limited scope of my abilities, too!
thanks
guillaume

[Toney]

Just to mention...

The pt 909 is a great pedal! It's actually become reasonably valuable as a collector's item believe it or not.
Well...maybe it's just the machinations of Ebay, but you may want to consider this before going at it.

[dacaumodo]

Yes I've seen it fetches high prices. Thing is, I like the sound it makes more than the money I could get for it... And also it was given to me a while ago by a guy who got it from a friend who died in a motorcycle accident. He had me swear to keep it. I know it sounds silly and melodramatic, but I'd feel uneasy selling it, if only for this reason... but also because it's a great sounding pedal.
Plus I don't think the mod proposed my Mark is very dangerous. If I ever become cynical and decide to sell it after all, I can always reverse mod it....

Thanks anyway.
Oh, and by the way:
http://cgi.ebay.fr/Ibanez-phase-tone-PT-909-vintage-rare-Phaser_W0QQitemZ150023714727QQihZ005QQcategoryZ46671QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

cheers
Guillaume