Wah transistor; MPSA18 or 2N5088....???

[zombiwoof]

Hi zombiwoof,

Additional info : I opened a few days ago a chinese brand new VOX V847 wah, and also found 2xMPSA18 inside. The inductor was a black one, labelled with the VOX in white. Sorry, i didn't have the means to take pics.

A+!

Yes, I've seen the insides of the Chinese Vox wah, and they basically use the same circuit as the Dunlop-made Vox 847 (with the added power jack and the input buffer that most of the Dunlop wahs use now), only it's mostly SMD components instead of the through-hole components on the older pedal.  They used the same transistors, as you said.  The fact that all of these wahs use the high gain transistors is one of the reasons they don't sound like the vintage wahs.  I'm sure they use those MPSA18's because they are cheap and plentiful, being that they are mass-produced pedals, and the sound is not a consideration in their choice.

Al

[Bullet79]

at least my volume drop problem with the BC109B tranny is solved.. 47K input resistor + lowering the Re1 value.. (i use trimmer).. seriously transistor swapping really make a big changes..

thinking of adding a (switched) boost/gain circuit...

[PRR]

[zombiwoof]

Notice how he used a 2N3900A for one of the transistors?.  I read a repair bulletin years ago for I believe the Thomas era wahs and it listed 2N3900A as suitable replacement transistors, and tried for a long time to find those trannies.  They seem to be very rare these days, as I could not locate them anywhere.  It's also interesting that this original schematic called for two different transistors, I would assume he specified that so that there would be two different hfe values, one of those two types probably is typically lower gain than the other type.  That would be in line with Fuzz Centrals's advice to put the lower gain transistor in Q2.   In practice, though, I don't think any of the commercial pedals ever came with two different types of transistors in them, the company must have not wanted to bother with that.  In fact, I bet Vox/Thomas never bothered to even measure the gain of the trannies they put in the pedals.

Al

[PRR]

> two different hfe values

I doubt it.

The year was 1966. Arlo had been arrested for littering, Officer Obie used a vacuum-tube cop-radio to direct the investigation. Transistors were VERY rare and expensive.

Q1 has to have high gain. Q2 hardly matters. The 2N3900A is the distant parent of our friend 2N3904: high gain, but only good for 18V. That's probably all the Plunkett and Kushner had or could afford in the way of high-gain parts. IIRC, the 2N2924 is practically an un-sorted factory-sweepings part number.

I'm thinking that 2N3900 cost a half-day's pay, but was needed to make the ringy work; OTOH Q2 could be anything so they used a low-spec part to keep the cost reasonable.

[guitarist]

I'm about to swap out the A18 transistors for BC109 type (actually 2N700, flipped around though as pinout is reversed). Wondering is the bias need optimizing some?

[zombiwoof]

I would like to add that I recently bought a BBE wah from Ebay, after a long search to find a reasonably priced wah that sounds like an old Vox Clyde McCoy wah I bought when I was a kid, and stupidly sold for $50 at a time when wah sounds were considered to be passe.  I found that the BBE fills the bill nicely. so I checked what transistors they used.  They turned out to be Bc547's, so those might be considered an option if they test in the suggested hfe ranges.
Al