Transistors hFE ~400?

[zombiwoof]

The Clyde McCoy wah mods page on Fuzz Central suggests BC109B's in I believe the 350-400 hfe range to get the original Clyde sound with the original value components.  My BBE Wah has BC547's in it and sounds very close to my old Clyde McCoy Picture Wah I had when I was a kid and stupidly sold for very little money (when wah became a cliche sound at a certain point).  I have a number of BC109B's I got that are in the correct range, I bought a bunch from an electronics retailer when they were really cheap and most measured within the range I wanted.  I plan on putting them in a couple of other wahs I have, one is a US made (Dunlop) Vox V847 I have slowly been modding to Clyde specs using the Fuzz Central article as a guide.
Al

[kaycee]

Why not just increase the gain of that section of the circuit? I seem to remember something about changing out the emitter resistor to up the growl?

[thermionix]

Why not just increase the gain of that section of the circuit? I seem to remember something about changing out the emitter resistor to up the growl?

That makes sense, but for my own goofy reasons I want to try to achieve this sound with the stock component values.  Does a smaller Q1e resistor have exactly the same sonic effect as a higher Q1 hFE?  Probably so, but I'm basically just experimenting and learning.  My wah is built on an eyelet board, so I can swap components 'til the cows come home and not damage any traces, or even move a wire.  I've tried most of the common mods out there, mostly just to see what they sound like, and I keep going back to stock values.

One thing that surprised me, and I don't understand, is that different pots seem to have different tones.  Most recently I switched from a Chase Tone pot to a Dunlop Hot Potz II, both 100k, both set as far to the treble end as possible without risking physical damage.  With the Chase Tone, I had added a 2n2 cap in parallel with the stock 10n sweep cap to get the overall frequency response where I liked it.  When I put the Dunlop in, I had to take the 2n2 cap out.  No idea why.  But I like that it sounds "right" now with the stock sweep cap value.  Keep in mind, I'm talking about the tone across the entire sweep, not just the extreme toe-down end of it, it's not just the different tapers.

I'm planning on getting one of the original Hot Pots from Small Bear soon to try that out.  My buddy has a nice vintage crybaby with that pot and I like it.  His wah is basically the main inspiration behind my experiments.  It just sounds classic as all get-out.  He has the stack-o-dimes inductor, and I have a Whipple halo, so they'll never be exactly the same, but still I'm chasing it down.  No, I haven't pulled his transistors to check hFE, woudn't risk that.  But my feeling is that they're right around 400, at least Q1.

I put in a Tayda order last night, including 30 BC550Bs, so I'll check those out when they arrive in a week or so.

[PRR]

> Chase Tone pot to a Dunlop Hot Potz II, both 100k, both set as far to the treble end as possible without risking physical damage.  With the Chase Tone, I had added a 2n2 cap in parallel with the stock 10n sweep cap to get ....  When I put the Dunlop in, I had to take the 2n2 cap out.  No idea why.

Did you measure the pots?

10nFd to 12.2nFd is about 20%. Pot tolerance is around +/-20%. A low-R pot could need more C to hit the same corner. 90K and 110K are both "100K" but could give the difference you found.

[thermionix]

Ah, good point.

I'm actually on my third pot, the first was a CTS-made one that came with the Whipple inductor, looks just like the "black bear" that Small Bear sells.  I added the 2n2 parallel cap when I was using that pot, kept it for the Chase Tone, then took it out for the Dunlop.

Values (all out of circuit):

CTS:  90.7k
Chase Tone:  98.4k
Dunlop:  104.2k

So the resistances get higher, I don't know the calculations for how that effects the frequency response in the wah circuit.  Does it seem to jive, or is it opposite of what you'd expect?

I'm very curious now, if I get one of the old type Hot Pots from Small Bear and it reads a little low, I could add some series resistance on the "heel down" end.  If indeed that's what's making the difference.

Edit:  Dang, re-read your post.

A low-R pot could need more C to hit the same corner.

Seems that was it.

[thermionix]

Got my stuff in.  HotPotz I from Small Bear came yesterday.  111k, nice!  Installed it without any other mods.  I sure do like the taper.  Seems a little more spread out, not as extreme of a "hump" as some others.  Also very familiar, it's what I grew up with I reckon.  Tone was fairly snarly, still a bit too bright and honky.

Today I got the BC550Bs from Tayda, pack of 30.  Selected one at approx. 400 hFE for Q1, and one at about 380 for Q2.  (I never know precise hFEs because the only thing I have to check with is my RG/ Geofex "FF Select" tester, and I never have a battery that's exactly 9.0v, and it does make a big difference in readings.  Someday I'll invest in a decent transistor analyzer.)

Anyway, the new transistors warmed the tone up in a very pleasant way.  Thick and syrupy, but not distorted per se.  The "honkiness" it had before is smoothed over.  Of course I checked it out with a clean amp first, then I ran it into my BMP and I could barely put the guitar down.  Very happy with the results, to say the least.  I might actually be done (!) messing with this thing, some 20 months after I initially built it, lol.

[One thing I might do, is put a 3PDT toggle inside so I can bypass the FoxRox output buffer.  3-pole so I can cut power to it.  That would save a bit of battery when I don't need the buffer, about the only reason to do it.  I don't want a little switch lever sticking out of my wah and getting broken off, so I would mount the toggle internally with a bracket, or maybe it can poke through under the treadle and not interfere with operation.  If I had thought about it at the time, I would have added three DIP switches when I built the FoxRox.]