Great Cheddar project at geofex: some questions

[Mark Hammer]

Finally powered up a fully-wired Great Cheddar board last night ( http://www.geofex.com/Article_Folders/grtchedrschm6.PDF ).  Sounded okay, I suppose, but did not evoke any sort of "where have you been all my life?" response.  I'll be the first to admit that I've never heard a "real" one, so I had no expectations other than Lovetone products generally being appreciated by their owners/users.

A couple of questions arose, however.

1) Because I have so many of them sitting around, I used a pair of NE5534 op-amps in place of the suggested 741s.  Am I missing or subverting anything "magical" by doing so?

2) One of the switch positions (rightmost, if that helps) gets me no output unless I turn the gain down a bit.  The setting of the P1 trimpot determines at what point that drive/gain adjustment becomes effective.  The resulting sound when everythng is tweaked, is also nothing to write home about.  What exactly is it supposed to be?

3) I'm getting a sort of octave-skipping from it, that is very reminiscent of a misbehaving Jordan Bosstone.  Any particular reason for this?

4) I'm assuming that any rotary switch whose solder lug locations correspond to the spacing on the board will make the appropriate connections when switching between modes.  Is this naive, or faith reasonably placed?

[R.G.]

And you think I remember this after that long?? 

I could go dig out the Mother Pedal I reversed from. After paying nearly $400 for that one, and intending to sell it after reversing, I never got it sold. It's in one of those boxes in the pile in the northeastern corner of the workshop. I think.

It was never a life-altering experience for me, either, but some people seemed to like it. If no one comes up with good answers I'll try to dig it out. The opamp use wasn't anything special that I remember, so I don't think the NE5534s would be a problem.
PS:
I checked the schemo. No special requirements on the opamps, they're just AC gain stages. A possible issue is that the NE553x opamps have unusually low input impedances, down in the 100K range, so they could maybe be doing some loading.

Your number 4 assumption may be a problem. I don't know this, but it's something I'd check. Hit the Mouser site, look up the Lorlin switch by part number, get the connection diagram and then use your ohmmeter to test your particular switch.

[hank reynolds 3rd]

1.I built a version using a TL072 and it sounded alright,so I'd say it doesn't effect the sound that much
2.this is the lowest position on the rotary right ?? I found the trimmer was very useful for tweaking this setting,as I got no sound initially with the fuzz pot maxed,finding the sweet spot on the trimmer should give a sort of broken,blatty type of sound,similar to the 'forbidden' setting on the bixonic expandora,
3.I haven't noticed this in the one I made
4.I'm not sure on this one

[Mark Hammer]

And you think I remember this after that long?? 

I could go dig out the Mother Pedal I reversed from. After paying nearly $400 for that one, and intending to sell it after reversing, I never got it sold. It's in one of those boxes in the pile in the northeastern corner of the workshop. I think.

Is it going to require a forklift to get to?

It was never a life-altering experience for me, either, but some people seemed to like it. If no one comes up with good answers I'll try to dig it out. The opamp use wasn't anything special that I remember, so I don't think the NE5534s would be a problem.

I didn't dig around for any in-depth descriptions, but from looking at it, the gist of it is that it is a Fuzz Face, with a buffer on the input and output, a defeatable BMP-style tone-control (in one of the switch positions), and a bit of additional clipping on the output from the Fuzz face portion (via D1 and Q3).

In view of this, perhaps there are some special requirements for transistors Q1 and Q2, much as there might be for any attempt at a Fuzz Face-style topology?  I used a pair of BC547s, without really thinking much about it.  Since I unsoldered them from an ill-fated earlier attempt using the V1.5 board, I checked them prior to re-installation on the new board just to make sure they were okay.  The measured hfe was around 270 for each.  Perhaps that is too high for that sort of topology.  Maybe this calls for the 85/120hfe thing?

PS: I checked the schemo. No special requirements on the opamps, they're just AC gain stages. A possible issue is that the NE553x opamps have unusually low input impedances, down in the 100K range, so they could maybe be doing some loading.

I gather this would pose the largest potential problem for the first op-amp?  I'll try and swap that out for a 741 or TL071 over the next few days.

Your number 4 assumption may be a problem. I don't know this, but it's something I'd check. Hit the Mouser site, look up the Lorlin switch by part number, get the connection diagram and then use your ohmmeter to test your particular switch.

Can do, although a quick look over the layout now, suggests that the pins DO go to where I think they ought to go, based on a quick continuity test last night.

One thing I am still in the dark about.  There is a path from the collector of Q2 to the emitter of Q1, via C4/R7.  maybe it's just an oversight on my part, but I don't recall ever seeing such a path in a FF configuration.  What does it do?

[R.G.]

Is it going to require a forklift to get to?

'sOK. I have the front end loader on the tractor.

I didn't dig around for any in-depth descriptions, but from looking at it, the gist of it is that it is a Fuzz Face, with a buffer on the input and output, a defeatable BMP-style tone-control (in one of the switch positions), and a bit of additional clipping on the output from the Fuzz face portion (via D1 and Q3).

In view of this, perhaps there are some special requirements for transistors Q1 and Q2, much as there might be for any attempt at a Fuzz Face-style topology?  I used a pair of BC547s, without really thinking much about it.  Since I unsoldered them from an ill-fated earlier attempt using the V1.5 board, I checked them prior to re-installation on the new board just to make sure they were okay.  The measured hfe was around 270 for each.  Perhaps that is too high for that sort of topology.  Maybe this calls for the 85/120hfe thing?

BC549's were what was in there. I don't think it makes a huge difference, given the amount of feedback used in the pair.

I gather this would pose the largest potential problem for the first op-amp?  I'll try and swap that out for a 741 or TL071 over the next few days.

I would think so.

One thing I am still in the dark about.  There is a path from the collector of Q2 to the emitter of Q1, via C4/R7.  maybe it's just an oversight on my part, but I don't recall ever seeing such a path in a FF configuration.  What does it do?

There is a whole family of "two-transistor feedback pair" circuits similar to this. They're what EEs used before opamps were easy to get and cheap. The classical two transistor pair has two inputs - the base of Q1 and the emitter of Q1 - and two outputs, likewise the collector and emitter of Q2. This gives you many ways to provide feedaback. One is from Q2E to Q1B, that being overall negative, and usually set for DC bias stability. With a series resistor on Q1B, it gives you an Rf/Ri gain for the pair from Ri to Q2E.

Signal from Q1B to Q2C is positive; butFrom Q1E to Q2C is negative, so C4/R7 is providing negative AC feedback on that last path to lower the overall gain of the pair.

The two transistor pair has many quirks and full use of this configuration is something I have never dug all the way through. The last masters of this configuration retired years ago, as they were current in 1960-72 and would have been gently nudged into a growth career path like greeting at Walmart by their employers by now.