General Guitar Gadgets - Phase 90 Clone issue

[bettsaj]

So as I said in the video... changing C10 has smoothed the phase out, and achieved it's intention.

Something you said at the end made me think you expected to fix it.  No worries, as long as you know.

How would I go about changing the sweep length? I intend to change the pot back to a 500k rev log as soon as I can as I need the 100k for something else.

Basically you have reduced the resistance by 1/5 and that means you need to increase the 15uF cap by a factor of 5, so 5 x 15uF = 75uF.   So to do that you will need 47uF in parallel with 33uF (or 25uF).   The other thing is if we make the cap bigger we need to make the 4.7k resistor (R20) five times smaller, so about 1k; that ensures the maximum speed is correct.     It's a bit of a pain but OK for a temporary fix.

You could also do a "nearly there" fix by changing the 15uF cap to 47uF and R20 to 1.5k.   That will give you upto about 4.5 seconds, which is a lot better than 1.5 seconds.

When you say the 15uf cap, are you talking about C7, the tant cap?? DOH!!! Of course you are, just looked at the schematic and realised that's the only 15uf cap on the circuit.

I've just ordered some 47uf tant caps... will be about 3 to 4 days I guess so I'll not post anything until then. In the mean time  have ordered 4 JFets from GGG for the Fuzzdog build and i'm about 3/4 of the way through that build. I just need to buy a couple of extra components that I don't have in my stock... As soon as I get those Jfets I'll let you know how the Fuzzdog build goes. I suspect the GGG JFets will be better than those that I bought from the UK with a higher VP value, so i'm fully expecting that build to be a straight "plug and play" affair (I hope).

[Rob Strand]

so i'm fully expecting that build to be a straight "plug and play" affair (I hope).

More likely.   There's still room for a lot of variation in the results with a 1V and a 2V JFET.   Normally people adjust the pot for best sound and that kind of hides the differences.

The funny thing is once you tweak the LFO to suit the JFET, like you did, the sound difference between low VP JFETs and high VP JFET is vastly reduced.   Basically what I'm saying is don't expect large improvements in the sound by changing the JFETs.    IMHO you would probably gain more by adding mods to make the trimpot easier to adjust.   (Some of the 80's and 90's Phase 90s actually did just that!).

[bettsaj]

Rob,

The latest... And likely the last instalment. I changed the 15uf tant cap as suggested to a 47uf cap, and also R20. I didn't have a 1k5 resistor so i used a 1k8 resistor... and this is the end result.... i'm quite pleased, and can't thank you enough Rob

[bettsaj]

so i'm fully expecting that build to be a straight "plug and play" affair (I hope).

More likely.   There's still room for a lot of variation in the results with a 1V and a 2V JFET.   Normally people adjust the pot for best sound and that kind of hides the differences.

The funny thing is once you tweak the LFO to suit the JFET, like you did, the sound difference between low VP JFETs and high VP JFET is vastly reduced.   Basically what I'm saying is don't expect large improvements in the sound by changing the JFETs.    IMHO you would probably gain more by adding mods to make the trimpot easier to adjust.   (Some of the 80's and 90's Phase 90s actually did just that!).

What sort of mods are you talking about?

[Rob Strand]

The latest... And likely the last instalment. I changed the 15uf tant cap as suggested to a 47uf cap, and also R20. I didn't have a 1k5 resistor so i used a 1k8 resistor.

Yes it's much more usable like that.  No prob with 1k8.

What sort of mods are you talking about?

If I explained all the options it's probably going to get confusing.    So I'll explain what I think is best in your case.

So going back to the start.  The sweep sounded bad so I suspected your JFETs had low VP.  Then I suggested a mod to see if we were on the right track.  And all-in-all things worked out pretty well.  Which is what you have now.  So you could just leave it there.

Now phasers are hard to adjust accurately at the best of times but in your case it's made harder.  When I look at the audio from each of the demos I can see the phaser adjustment has moved around quite a bit between the video.  That's because it's hard to adjust.  You can get away with some misadjustment but if you compared a misadjusted unit side by side with a factory adjusted unit you will hear a difference.

I'll try to explain the degree of the problem.   Suppose you have a trimpot that adjusts a filter frequency.   Suppose the trimpot "knob" goes from 0 to 10.   On your unit one octave change in the filter frequency corresponds to only 0.057 change on the 0 to 10 scale.  That's nuts, so it's hard to adjust.

So an improved mod would be to make that adjustment less sensitive.   An improved mod achieves what we have so far but allows the pot to be adjusted more easily.   So for the improved mod I'm suggesting has one octave change in the filter frequency for 0.44 change on the 0 to 10 pot scale.  So that's 8 times easier to adjust.

So the downside of this mod is you have to add a resistor.   The previous mod didn't change add anything it only changed values.   Note also at the moment your trimpot is probably at setting position 9 but with this mod you it will be to around 1.5;  no big deal I'm just mentioning it.

I just want to be clear that 150k on the Improved Mod is *the same* as 100k on the the First Mod.  The reason is the 1M is present.

[bettsaj]

Hi Rob,

That's brilliant.... Really helpful. So, to do the improved mod I need to replace the 1M resistor back to where it was at R17, and graft in the 150k as indicated.

When you say "for same sweep as original perhaps use slightly higher value than those indicated", what resistor values, both 150k and 1M, or just 150k?

This is all going to be really helpful on my second phase 90 build....

[Rob Strand]

That's brilliant.... Really helpful. So, to do the improved mod I need to replace the 1M resistor back to where it was at R17, and graft in the 150k as indicated.

Yes.  So you have to find a home for the added 150k.

When you say "for same sweep as original perhaps use slightly higher value than those indicated", what resistor values, both 150k and 1M, or just 150k?

For the "Improved mod", up the 150k a tad and leave the 1M as is.

The originals have a bit wider sweep than yours does at the moment. 
If you like how sounds now just leave it.

This is all going to be really helpful on my second phase 90 build....

Whether to tweak or not depends on the VP values of your JFETs.    Should be no worries anyway just tweak that added resistor until it sounds right.  You will need to tweak the pot each time you try a resistor.

[bettsaj]

So.... my new supply of 500k Rev log pots arrive, I change the pot on the phase 90 and i'm now left with a sweep that lasts about 10 seconds on its slowest setting 

[Rob Strand]

So.... my new supply of 500k Rev log pots arrive, I change the pot on the phase 90 and i'm now left with a sweep that lasts about 10 seconds on its slowest setting

Normally you end-up with about 7.5secs.  I've seen a few real units go out to about 10 secs though, which I put down to extreme tolerances on the 500k pot and 15uF cap.

With 100k you were getting about 1.5 secs.  So from that you would predict a 500k pot would be 5 times longer 5 x 1.5secs = 7.5s.   If you used the same cap I wouldn't expect it to be too far off that.  Can you measure the pot resistance?

Did you change the cap back to 15uF and the resistor back to 4k7?  (I expect you did, otherwise it would be more like 20 seconds.)

At 12 O'Clock it should be about 1 sec but that can easily vary 20% because pot tapers aren't that accurate.

[bettsaj]

I haven't changed anything.... Only the pot. so I still have the same cap, and R17 is still 100k.

Maybe my timing isn't that accurate, but the longest sweep is loooooong

[Rob Strand]

I haven't changed anything.... Only the pot. so I still have the same cap, and R17 is still 100k.

Maybe my timing isn't that accurate, but the longest sweep is loooooong

Ah, OK that' explains it.   

The slowest setting is so long I can't make out the whole sweep either. 
I think it's probably around 20 to 30 seconds!

In the video the 12 O'clock setting with 47uF + 500k pot is about 3.5 seconds.    With the 47uF cap in there instead of 15uF we would expect the sweep to be (47/15)  = 3.1 times longer.    Also  I expect around 1sec with 15uF at the mid setting so that means we should expect 3.1sec with the 47uF.   So it's all working as expected.

If you change the 47uF back to 15uF and the 1k8 back to 4k7  the sweep will be how it is supposed to be.

[bettsaj]

All changed, and  now working like a charm.... Many thanks Rob

[Rob Strand]

All changed, and  now working like a charm.... Many thanks Rob

Good stuff.  No worries.

[bettsaj]

All changed, and  now working like a charm.... Many thanks Rob

Good stuff.  No worries.

Rob, now I have this phase shifter in my guitar rig it's now quite noticeable there's a slight volume drop and loss of bottom end when I engage the effect. If I can I'll try and record it going through my rack, but even on the small practice amp i use for testing pedals there's a slight drop in volume.

The effect chain in my switcher is:

• Tuner/Mute
• Dyna comp
• Electric Mistress
• Uni-Vibe
• Phase 90
• Big Muff
• Power boost
• Empty loop

I have tried placing the phaser directly after the guitar then going into my pedalboard, but even then there's still a slight drop of volume and more concerning a loss of bottom end...... Any suggestions?

[Rob Strand]

I have tried placing the phaser directly after the guitar then going into my pedalboard, but even then there's still a slight drop of volume and more concerning a loss of bottom end...... Any suggestions?

For the LF issue try increasing the 10n cap input C1 to 47n, also increase the C9 to say 100nF (you could go higher).

For the level try increasing R26 by one step 150k to 180k.  *OR*  decrease both R27 & R24 by one step 150k to 120k.   One might be better than the other.  I'll need to check.

[bettsaj]

I'm thinking the loss of volume might just be the lack of bottom end.... i'll adjust the LF caps and see what it's like after that

[Rob Strand]

I'm thinking the loss of volume might just be the lack of bottom end.... i'll adjust the LF caps and see what it's like after that

The same thing crossed my mind about 2 mins ago.   The input cap is the first place to start.

FWIW,  if you need more volume I'd go with the feedback resistor R26.  It has slightly less boost than the input resistors R27 & R24  but it should be enough.   If you really want to fine tune it you can use smaller steps like 160k.

[bettsaj]

So i changed the LF caps... And it's made a significant difference.... I was right, it is just the lack of bottom end that gives the impression of volume drop.... I may stuff a slightly higher cap in C9 and see what that's like

[bettsaj]

Changed C9 to 150nf and it's perfect.