Univibed Phase 45 - Help?

[mydementia]

I built Torchy's version of MXR Phase45 w/Univibe mods per this vero/schem:


The first night, it fired right up and vibed just fine - not much phasing...but I didn't have the right FET's so I figured I'd try again when they arrived (I used 2N5457's in stead of the prescribed 2N5458's).  When I tried using J201's last night (after reading about subs on the standard Phase45 project) I hosed something up good!  In my investigations, I found that I have zero voltage on my Vref strip - not good.  I'm guessing that I killed R3 (10k) somehow.  Sound reasonable?

The only component I'm not sure of is the Zener diode... I used is a 1W 5.1V type (1N4733A) - I noticed that the unmodded Phase45 uses a 4.7V Zener (1N5230) - does it matter?

Thanks in advance for your help.
Mike

[R.G.]

When I tried using J201's last night (after reading about subs on the standard Phase45 project) I hosed something up good!  In my investigations, I found that I have zero voltage on my Vref strip - not good.  I'm guessing that I killed R3 (10k) somehow.  Sound reasonable?

You can have zero voltage there two ways - either the R3 feed resistor is open, or something else is shorting the Vref to ground. If you unsoldered/soldered the JFETs when you changed them, I would bet on a solder thread shorting to ground. It still could be that R3's solder connection is open, or that the body is cracked, but based on the kind of errors that I make....  ... solder shorts are more likely.

The only component I'm not sure of is the Zener diode... I used is a 1W 5.1V type (1N4733A) - I noticed that the unmodded Phase45 uses a 4.7V Zener (1N5230) - does it matter?

It matters, but only a little, and that's probably not your problem. The zener can actually be anything greater than the Vgs(off) of the JFETs and less than the 9V supply, and the bias control will make it work. JFETs take  varying amounts of back-bias on the gate to turn them from fully on (Vgs=0 lets Id be Idss) to fully off (no conduction). Somewhere in the middle of that is the resistive region we need for phasers.

The only reason not to use a big zener is that the same reference is used for the opamp's bias point as for the JFETs, and if the opamps get too high or too low, there's not enough headroom left for them to operate on a 9V battery and you get distortion from the opamps not being able to swing as wide one way as the other. Ideally, the reference would always be half the power supply, but then you would not have a constant place to bias the JFETs and the JFETs would not always be in their ideal resistance region. So its a compromise. The zener needs to be approximately half the power supply - which varies from 9.5 or more with fresh batteries down to about 7V with low batteries. MXR chose 3.2V for their zener, a bit below the 3.5V of half a 7V supply. This made them choose a low Vgsoff JFET in the 2N5292. If you count on higher supplies, you can use 4.7V or 5.1V zeners. You'll just run out of opamp range before distortion sooner on batteries. This may or may not matter if you use a power supply.

The JFETs are going to have to be matched somehow, whether by matching fixture, manual selection by ear, or luck. Otherwise, you won't get much phasing.

[mydementia]

Thanks for the quick response RG.
I always socket FETS and OpAmps - so I don't think a solder tail is my problem...but I've made dumber mistakes!

I run my pedalboard on a OneSpot daisy chain that puts about 9.3V to each output...so my Zener choice should be just fine...right?  Would it be advantageous to drop to a 4.7V zener just to give the op-amp a more even swing?  (If so, it's off to the surplus store for me!)  What about 1W vs. 0.5W?

Once I get my 2N5458's from steve@smallbear, I was going to use the 'match by ear' approach.  In my searching, someone gave a link to one of your threads that showed the 'more scientific' approach - but it was dead.  Do you have a working link to a FET matching fixture for this project?

Thanks again.
Mike

[jrem]

I can tell you that even with matched fets you don't get a deep swoosh, you get the nominal Robin Trower / Jimi tube vibrato feel.  I worked that project to death, and built one for my brother (www.zipperheadrocks.com) and he doesn't think it's enough swoosh, so it's on to the phase 90.

I matched fifty or so fets per RG's doc, it's at http://jrem.webhop.org/stuff/phase/   email me offline if you need a couple.

[R.G.]

I always socket FETS and OpAmps - so I don't think a solder tail is my problem...but I've made dumber mistakes!

I bet you haven't made any dumber mistakes than I have   

I run my pedalboard on a OneSpot daisy chain that puts about 9.3V to each output...so my Zener choice should be just fine...right?

Yes - with 9.3V available all the time, it will be fine.

What about 1W vs. 0.5W?

That's pretty much immaterial here. The zener just doesn't have that much power through it with a 10K series resistor. I estimate it at about 2.1mW, so there's a lot of room there for either half or one watt.

Once I get my 2N5458's from steve@smallbear, I was going to use the 'match by ear' approach.  In my searching, someone gave a link to one of your threads that showed the 'more scientific' approach - but it was dead.  Do you have a working link to a FET matching fixture for this project?

It's on the entry page at GEO - 4/16/02 is the date.

[mydementia]

Interesting...
After checking the suspect 10k resistor (which was just fine) I decided to follow my own advice and scrape between the vero tracks with a scrwedriver and what do you know - it works again! 

I got my 2N5458's from smallbear today - none of them would work in the circuit.  I found a couple J201's that worked at the end of the bias pot travel.  What I'm getting now is a gentle phase effect and a huge vibe.  The switch doesn't seem to do much - makes it a little brighter/darker?  Anyone have a few sound samples of this build so I can see (hear?) if I'm in the ballpark?

Thanks.
Mike

[tiges_ tendres]

I had trouble matching jfets in this one, and also noticed that I almost got a swoosh at the end of the trim pots travel.  I put in a larger trimpot and this helped somewhat.  I didnt match any jfets, I just used sockets, so when I found a pair that worked well I noted that the trim pot was probably set way above the 100 k listed on the schematic for the trim.

what this means?  I have no clue, I am still painting by numbers!