gain of MPSA13 (Dunlop GCB95) vs. BC109 trannies

[powerplayj]

I have replaced my stock Dunlop GCB95 Wah transistors  with a pair of BC109's and noticed a significant volume drop.  Is there a big difference in the gain between these tranny types?

[phillip]

Yes, the MPSA18s that are used in the Crybaby have gains typically around 700 or 800, while the BC109C has a gain of around 400.

The BC109 has several different gain ranges, though.  There's the BC109, BC109A, BC109B, and BC109C.  I believe that the "A" is the lowest gain, while the "C" has the highes gain.  I personally like the BC109B in wahs, but sometimes I can get a pair of Cs that have pretty close to the same gains that I like.

If you convereted to true bypass switching, you could replace the 68K resistor that's in series with the input with a 47K.  This will help get some of the lost volume back.  You could also replace the resistor on the emitter of Q1, usually a 390 or 470, with something smaller...I used a 330.  That helps increase gain and bass response.

Phillip

[al3151]

Yup!Check it out here http://www.datasheetcatalog.com/
The GCB95 one mps A13 and two mps A18s.

[powerplayj]

That makes sense then because I got the BC109 "workarounds" from Small Bear not knowing there was more than one type.  I'll try and audition some B's and C's instead.

I have already changed the resistor values to increase the volume/gain in other areas like mentioned above so it looks like I'll put the MSPA18's back in until I can find some BC109b's and BC109c's.

Thanks guys!

Jason

[Transmogrifox]

I don't see how one can improve above the MPSA18 transistors.  Those are very low noise high performance transistors.  I can see the "mojo" appeal in the BC109's, but honestly, I think Dunlop made an improvement by putting MPSA18's in their crybabys.

[powerplayj]

I don't see how one can improve above the MPSA18 transistors.  Those are very low noise high performance transistors.  I can see the "mojo" appeal in the BC109's, but honestly, I think Dunlop made an improvement by putting MPSA18's in their crybabys.

It's a sickness........... I haven't taken on a pedal from scratch yet so, I try to tweak the pedals I already have.  It's sometimes more of a fishing experiment than a known improvement.  I'm in need of a compressor so maybe I'll tackle the Orange Squeezer and leave my other pedals alone for once.

[petemoore]

I replace that 68k Resistor with a 100k pot wired as a V/R, with a [I forget eggzackly] 33k-47k resistor in series to set a min. for the variable resistance, I should probably set it for unity, then pull the pot and resistor out, and measure the resistance they were providing there in the circuit, and replace with [give or take a little] the same R value resistance of fixed value, using one or more fixed resistors to get close'.
 I used BC109c's, gain is workable, sound is different than with the originals...oh I started with VOX847 boards, their inductors outward appearance is different.

[phillip]

I don't see how one can improve above the MPSA18 transistors.  Those are very low noise high performance transistors.  I can see the "mojo" appeal in the BC109's, but honestly, I think Dunlop made an improvement by putting MPSA18's in their crybabys.

The gain of the transistors in a wah pedal has a lot to do with the overall "voice" of the wah.  The lower gain transistors tend to mellow out the tone...so that it's not quite as ear piercing and shrill sounding.  The lower gain BC-series adds a very noticeable amount of bass response.

There are other 2N-series transistors that would work nicely, but the BC109s are noted as being very good sounding audio amplifiers, usually lower noise than most.

Phillip

[powerplayj]

So far I have TB'd my Dunlop, added a Fulltone inductor and made several resistor changes.  

Vocal mod - 33k resistor -> 68k (tried 100k but, lost too much of the sweep range)

Volume mod - 68k -> 47k

Gain & bass response - 390ohms -> 330ohms (Actually, I might increase this to 500 like the stock Vox if the old MPSA18 transistors allow the volume not to drop when the pedal is engaged.  I like cleaner/light fuzz wah tones)

midrange - 1.5k -> 2.2k  Didn't notice much improvement from this one

[brett]

The gain of the transistors in a wah pedal has a lot to do with the overall "voice" of the wah. The lower gain transistors tend to mellow out the tone...so that it's not quite as ear piercing and shrill sounding. The lower gain BC-series adds a very noticeable amount of bass response.

Other way to get a mellow sound is convert to the JH-1 circuit.  Only 2 extra capacitors (and there's probably already a space on your circuit board for them).  This change doesn't affect the strength of the "wah" effect, which swapping to lower gain transistors might do.

cheers

[RedHouse]

Phillip's got it right, the originals were much lower hfe.

My old early 70's Thomas/Vox Crybaby has much lower gail transistors than my friends Dunlop GCB-95, and they create a different sound texture, neither one seems better, but when you A/B them they do sound different. The newer high hFE ones have a more "peaky" (sharp) resonace while the older, lower hFE ones have a smoother sound in the same circuit.
(even w/o the input buffer)

I prefer the lower hFE (100-350) transistors (2N5088 or 2N3904) to recreate the older sound.