English Channel - Q4 gate won't get to 1.6v

[cheezit]

I've built the ROG English Channel.   It sounds really good up to about 2-3 o'clock on the drive knob, and after that it sounds ok, but has a grainy quality that sounds like a misbiased JFET.  It just doesn't get that saturated Brian May sound at the top of the drive knob.  I looked over the ROG schematic again and noticed something I missed before---the gate on Q4 is supposed to be at 1.6v, or close to it.  Mine is sitting at 0.5v.  Here's what I've tried:
- Rebiased it several times, so I'm sure the bias is set right.
- Swapped around every other J201 into that position, and they all do the same thing.
- Based on the ROG comments, I replaced the source-side 4.7K with a 10K, which made the gate voltage go up to 0.6v.  I then threw a 10k pot in series, to try the range from 10k to 20k ---it then adjusted up to 0.7v, but it audibly reduced the gain---then I noticed that the Q4 drain voltage was rising up well past 5v and I could no longer bias it to the correct level (about half the 9v supply).  ROG sez a 12K is the maximum value they had to put in there.

So -- a couple of questions:
1.  can anyone who knows the circuit well tell me what the effect of not having 1.6v at that point in the circuit would be?  Am i chasing something that doesn't matter?
2. Any idea where to look for issues?  Messing around with the 4.7k source side resistor doesn't fix it, and the drain side is a simple trimmer to 9v+, which leaves the resistors in the little network that tie the gate to the source, but those look okay.  I see those three resistors (1.2K, 4.7K, and 1M from gate) as a little DC voltage divider circuit, but beyond that I don't know where it might be going wrong.

Thanks in advance for any advice...

[kbibs]

The ROG instructions say the J201 Source should be at 1.6v not the Gate. I believe it's the center pin on the J201.

Give it a try!

[cheezit]

Jeez, I used the wrong word in the first paragraph...it's the center pin on the J201 that I'm measuring, and which has 0.5v instead of 1.6v.

[kbibs]

I'm very interested in this topic also. I've recently built the OLC kit of the English Channel and I too measure ~0.54v at the source of Q4. The bias is correct on all the J201's, i.e. ~4.5v on the drain pins except for Q3 which has ~9.0v. I also noticed that the gate of Q4 measures ~0v which also disagrees with the ROG table of pin voltages. According to the table the gate should have ~0.85v?? ... I don't understand these circuits all that well but is it just a coincidence that the ROG voltage difference from Source to Gate, Vgs?, is ~0.55v?? ... the same as what I'm measuring except ROG's is biased up 0.85v??

I agree this pedal sounds very good but I would like to understand these circuits a little better so any help would be greatly appreciated.

Thanks! 

[petemoore]

  The article reads:
  To obtain 1.6v at the source of Q4, you may need to try different J201 or adjust the 4k7 resistor. During testing and development, values as high as 12k were necessary for some J201. Try to get close to 1.6v, but slightly lower values will have no audible difference.
  What I was going to say...

[cheezit]

petemoore - that comment on the ROG site is exactly what I am going off of.  0.5v vs 1.6v doesn't sound like "slightly lower" to me.  And as I stated in my original post, I've tried both swapping in other J201s and adjusting the source resistor.

kbibs- I built it from the OLC kit too!  Hmmm.  Does yours sound like the lead in the ROG clip when you turn the drive all the way up?

[cheezit]

I think i've found a PCB error. I'm going to try a fix and report on the results later...

[cheezit]

Ok, the current OLC board for the English Channel has a trace error!!! 

The IM resistor (R14) that has one end at the Q4 gate is supposed to connect to the far end of the 1.2k resistor from the source of Q4. Instead it connects to ground.

Fixing this resulted in a *lot* more gain and saturation from the pedal.  The sound is still a bit grainy and misbiased-sounding, and unfortunately that sound is now present at lower levels of gain than before I fixed the problem.  The bass also saturates very easily---a real Vox does this too, but what I'm getting is over the top.  So there may be another problem with my build (or another PCB error, I'm going to trace the whole thing tonight).

I'm going to let Mark at OLC know about this, we'll see what he says.

[kbibs]

cheezit, Thanks! ... I was going to check that very same thing this morning, that is the 1M resistor traces. I'll try to trace the board also to see if there are any more errors. If you still have too much gain you may want to try MPF102's. I've heard they have lower gain.

Keep us posted on what you find or hear from Mark.

Great work!

[cheezit]

I heard back from Mark, he is going thru to find affected customers.  BTW, my PCB has the pads for the control pots all lined up on one edge, whereas the previous version had the pot pads in various spots on the board (and didn't have this problem).  So if you have the same type as me that's further evidence you might have the same issue.  After my fix, the Q4 source is up to 1.41v, which is "close enough" for now.

I'd go ahead and contact Mark if you think you are affected.  He seems to be rather inbox-challenged these days so make your subject line attention-getting 

[scott1568]

Thanks Cheezit for looking into this. I built this pedal a few months back. I works OK at low drive settings as you said, but was not sounding Voxish at higher drive settings which was what I was looking for to begin with. I decided I would have to live with it.

The 1.2K R you are referring to, would this be R16? CAn you be more specific as to how to hook it up properly?

For clarification, my PCB is labeled "GNG 2007" and all the pots are on one side of the board.

I would agree that Mark is really busy. I am glad I joined this board especially now. I liked this pedal. If I can get to straighten it out, I hope to love it.

thanks

[cheezit]

Yes, the 1.2K is R16.  What I did as a temporary fix was to pop out the ground end of the R14 resistor and just tack it on to the end of R16 right next to it using a bit of solder.  A better solution might be to cut the trace on the back that leads from the end of R14 to ground, then put in a solder bridge back there.  I would suggest you check the PCB against the schematic to make sure you have the same problem---you have what sounds like a slightly different version of the PCB.

Now that I have made this correction, the circuit has a lot more gain, but it has also taken on a ragged sound at pretty much all drive levels.  I've tried the MPF102s, didn't help.  So I'm curious if you or kbibs do this fix---does the result sound good?  I've gone over the traces and compared them to the ROG schematic and everything appears ok, so I may have a build problem...

[kbibs]

Thanks cheezit for keeping us posted. I haven't tried the fix yet. I plan on cutting the trace on the back side and jumpering over to the junction of the 1.2k and the 4.7k. I did go over the pcb layout Mark sent with the instructions (the one in the instructions is the old one) and the R14 to ground was the only big error that I found. I believe the bass pot (R12) pin indicators for pins 1 and 3 are backwards from the schematic. Pin 3 should be grounded, per schematic, but the layout drawing has pin 1 grounded. I fixed that prior to this.  I'll try to fix R14 tonight and report back. I'm sorry but I hope you have another error in your build cause I'm not liking the sounds of the so called fix .

Let us know if you find anything

scott1568, Let us know what you find.

Thanks
 

[cheezit]

I've got some before/after clips.  The before (before fixing the trace error, that is) is on my pedal page, scroll down to the bottom:

http://home.comcast.net/~jcwilleford/pedals/diypedals.html

The after is something I threw together tonight:

http://home.comcast.net/~jcwilleford/pedals/englishchannel2.mp3

I used the higher-gain sounds.  The pedal still sounds pretty good, but you can hear there's a bit of harshness and splatter...

[kbibs]

I fixed the trace error last night and did not notice a big difference. I didn't get a big bump in gain from previously and no increase in harshness, if anything it smoothed out some of the crunchiness. I'm getting 1.54 at the source of Q4 now. The hardest part of this pedal is figuring out the interactive bass and treble controls of this Vox tonestack.

cheezit, your clips sound great! ... I'm not sure what tone settings you were using but your pedal sounds more crunchy than mine. I might be able to get that tone at high gain settings and treble up but I like the lower gain settings. Also, mine can get pretty clean at low gain. I can't believe the difference in our pedals is just component variations but maybe. One of us might have an error in our build. I'll check mine out and report back if I find anything. Oh BTW I like the sticker you're going to use on it, nice!.

Kbibs

[petemoore]

The hardest part of this pedal is figuring out the interactive bass and treble controls of this Vox tonestack.
  I've one of those in the Vox amp, after seeing Duncans Tonestack Calculator 'work the lines' [interactive TC graphics] I understood how to use the TC better and tried a few things out with expectations that better matched response.

[kbibs]

Thanks petemoore! ... that should help confirm what I'm hearing