Home Forums GuitarPCB Build Support Dr Phybes Board with The Vibe parts list just sounds like a mild reverb

Viewing 4 posts - 1 through 4 (of 4 total)
  • Author
    Posts
  • #6846
    Anonymous
    Inactive

    As the title says, Playing a P-Bass through just this circuit to my Fender Amp. bypass is fine, most settings on trim pot just sound clean, about 1:00 I get a very slight reverb when speed knob is almost maxed. otherwise very underwhelming.

    45/90 mod added, same sound in either switch position. loud pop when flipping this switch. Resistors are mounted to switch but double-checked that they are wired according to schematic.

    Parts substitutions:

    Q1-4 2N5457 mounted reversed for backwards pin out. (mounting per diagram yields nothing but clean signal no matter what the settings)

    Q5  2N3906

    All other parts exactly per “the Vibe” parts list.

    I chose the Dr Phybes board because I needed the mounting hole, I chose the Vibe parts because I had them all on hand.

    The Jfets:

    Idss ma: Q1=3.36 Q2=3.45 Q3=3.83 Q4=3.58

    Vp: Q1=1.77 Q2=1.78 Q3=1.79 Q4=1.80

    Transconductance ms: Q1=2.21 Q2=2.27 Q3=2.23 Q4=2.18

    The Voltages:

    IC1 pin1 = 85mV pin2 = 4.2V pin3 = 4V pin4 = 0V pin5 = 83mV pin6 = 4.2V pin7 = 9.12V pin8 = 5mV

    IC2 pin1 = 4.1V pin2 = 4.1V pin3 = 4.1V pin4 = 9.1V pin5 = 4.1V pin6 = 4.1V pin7 = 4.1V pin8 = 4.2mV pin9 = 4.1V pin10 = 4.1V pin11 = 0V pin12 = 4.1V pin13 = 4.1V pin14 = 4.2V

    IC3 pin1 = 21mV pin2 = 5V pin3 = 5V pin4 = 0V pin5 = 11mV pin6 = 2V pin7 = 9V pin8 = 21mV

    Q1: D = 4.19V S = 4.19V G = 2.22V

    Q2: D = 4.19V S = 4.20V G = 2.19V

    Q3: D = 4.19V S = 4.19V G = 2.19V

    Q4: D = 4.20V S = 4.19V G = 2.22V

    Q5: E = 4.2V B = 3.65V C = 2.27V

    Pics to come. In the pics you will see a jumper on IC1 between pins 2-6, this is because the pad lifted on the solder side of pin 2 and my sockets don’t let me see the component side to see if the solder wicked through or not so I added it as a precaution.

    I really hope you guys don’t say I have to de-solder any existing parts, I already destroyed one Dr Phybes board and had to order a second because multiple pads lifted when I tried to re-heat them. Haven’t had this issue on other boards, just these for some reason.

     

    #6847
    Barry
    Keymaster

    Thank you for supplying the detailed info.

    Short suggestion is to use a matched set of 2N5952 to work in conjunction with the stock phaser components (Vibe or Phybes). This board has been around for 10 years and will absolutely work fine if built per the BOM.

    For a lengthy explanation please read Mark Hammers 2nd post in this thread which exactly covers everything.

    I will Copy and Paste the info here for reference:

    The FET has 3 pins: the drain, the source, and the gate. When a voltage is applied at the gate, the path from drain to source (which is the pins connected between ground and the capacitor in each phase shift stage) changes its resistance. If the FET is able to change that resistance over a very wide range of possible values, then the sweep of the phaser, from lowest to highest point, will be very wide. If a voltage at the gate goes from, let us say, +3.5v to +5.6v (this is just an example, and not the actual voltages) produced a change from 3k to 1.4M in one type of FET, but only produce a change from 44k to 300k in another type of FET, the first one would give a wider sweep than the second one.

    In the Phase 90 (which is what I guess you are building, since you mention 2N5952s), the combination of the FET and .05uf caps allows the notches produced to go as low as around 145hz. Look at the schematic, and you will see that there is a 22k fixed resistor in parallel with the FET in each phase shift stage. Remember that when two resistances are in parallel, their combination can never be higher than the smallest one. So, even though the FET may be able to have a drain-to-source resistance of several megohms, when combined with a 22k fixed resistor, the combined parallel resistance will NEVER be any higher than 22k. If the FET only had a narrow, and low, range of possible resistance values for different gate voltages (e.g., 100k to 300k), then the parallel resistance would not change much (from around 18k to around 20k). For example, 100k to 300k, in parallel with 22k and using a .05uf cap, would mean that the notches produced would never go any lower than 155hz (not a huge change at the low end) but never really go above 353hz (for the upper notch at highest sweep point), only about 1.3 octaves. Not much movement in the location of the notches at all.

    Because it is the combined parallel resistance (the FET plus the fixed resistor) that sets the resistance range covered, in this case, that is good news for you. If you find 4 matched 2N5457 (or any other) FETs, and they don’t sweep over a very wide range of resistance values, you can simply change the 22k resistors for a different value. In our example of 100k-300k, if we changed the 22k resistors for 47k values, the resistance range covered would be 32k – 40k. Use 100k instead, and the resistance change is now 50k-75k. So the bigger the value of the fixed resistor, the more usable range can be gotten from even a FET with very limited resistance range.

    One thing you can see is that every time I increased the parallel resistor to change the range, the actual resistances went up. With 7k, the lowest notch would be at 42hz. This is much too low. Changing the cap to .015uf, though, moves that lowest notch up to 142hz, which is very close to stock.

    #6848
    Barry
    Keymaster

    Also in this thread by GEOFEX and R.G. Keen you will see further explanation regarding different JFET characteristics as well as the ideal Vgs range for phasing being -2.40 at minimum and -2.60.

    I hope that helps.

    If you are looking for 2N5952 Small Bear may have the them..

    or better get a matched set from PedalPartsAndKits.

    #6853
    Cybercow
    Moderator

    I’ve built a couple of these and they do require a quad-matched set of 2N5952 N-Channel FETs. I’ve tried matching my own quad set of 2N5457 FETs and it would not work right. Perhaps my “matching” technique was flawed, but I chose instead to acquire a proper set of professionally quad-matched 2N5952 FETs. It worked fine after carefully dialing in the trimpot. But I cheated on the trimpot and spent a few extra pennies for the 10-turn version trimpot and found that dialing in was much easier.

Viewing 4 posts - 1 through 4 (of 4 total)
  • You must be logged in to reply to this topic.