Help with Vox Wah v847 mod: transistor problems

[simo2guit]

Hi,

  Can anybody help with this? I have been modding my vox v847 back closer to Clyde McCoy Specs. I did most of the resistor and caps swaps which went fine with a noticable tone improvement. I had read that the transistors in the old wahs were lower gain and a good replacement for them would be some BC109Bs. I got some NTE123As which are supposed to be the same thing... I installed them making the assumption (I know!) that the curved indication on the pc board would match with the curve of how the pins are positioned coming out of these resistors, as the casings are not curved themselves like the stock transistors (mine say MPS A18 - f0053). When I put them in, I lost a lot of volume as well as most of the wah effect. I ended up switching them back and the tone was restored.

  I did some more research and found that the transistors in the newer v847s had an assignment of ecb emitter/collector/base (is this true?) for the pins from left to right while the NTE123as have an assignment of ebc. Apparently, you can just bend the pins as long as they don't touch to fit the correct connections. Excited that this was my major problem, I did just that, and replaced back to the transistors I had used before after bending the pins appropriately. I did take time to clean and brush the pins and pc board before resoldering. This time, when I engaged the wah, I got no sound period. As it is now wired in true-bypass it still worked fine when switched off. Frustrated, I reversed the transistors just in case I had it backwards and tried again. This time I just got hissing when it was engaged. My wah now sits on the bench without transistors. I have no idea what I'm doing wrong at this point, but am stubborn because they really SHOULD work as I have read of others switching to these with better success. Does anyone have any suggestions as to what I can do other than going back to the originals???

Thanks in advance if you actually read all this and took time to respond!

[jonathan perez]

different pinout? not on a vox 847.

[simo2guit]

Thanks.. I threw in some NTE123APs and positioned them normally and it worked!! Definitely mellowed the sound a little.. I almost want more highs.. but it's nice and thick. I may lower the input resistor a tad and see if this helps a bit and/or play around more with r3. Any suggestions?

Still don't know why those BC109Bs didn't work.. Oh well.. Maybe I'll try again at some point...

[jonathan perez]

keep the pinout correct on them bc109Bs. do you understand what low/high gain transistors will do, sonically, to your wah?

dont just mod for the sake of modding.

2n4401s sounded good. i think that youd be satisfied with something with the gain around 400.

here are some key modifications:

33k (Q)-100k.
1k5 (mid)- 2k
470r (bass/gain)-330r
.01uF (sweep cap)-.012, .015, .022...so forth...
.22uF(to wiper, for a "sweeter" taper) to .33uF, .44uF

true bypass...is a must with that wah hahaha

[erickliban]

hi
i think i just did the same thing
i think its because the transistors were too low gain
anyone out there who could tell me if this is correct?

[zombiwoof]

If you were following the instructions on the Fuzz Central site about returning a wah to Clyde specs, you will notice he suggests you need to keep the hfe of the transistors in the 350-400 hfe range to work nicely with the stock components.  So you probably did have trannies that were out of the range.  It's always a good idea to measure the gain of the transistors in a case such as this.  Even BC109's that are sorted into the "B" range can vary from the "ideal" gains that he recommends.

Personally, I try to avoid NTE replacements myself, see Small Bear's comments on the variable quality of their products, which are mostly re-labeled transistors of unknown original designation.  The Thomas Organ repair notes on the Cry Baby mention 2N3900 as a good sub, but I have found those just about impossible to find.  Any good NPN silicon in the 350-400 hfe gain range should work with the vintage values.  If you vary from that range you have to change other components to compensate, which is what Dunlop does because they use high gain MPSA18 transistors, and part of the reason their modern Cry Baby's don't sound the same as the vintage wahs IMO.

Al