VOX FX

jaminjimlp

jaminjimlp

Member
Ok a buddy of mine sent me this video... The distortion sounds fabulous!!! Well this led me down the rabbit whole...
https://fb.watch/w3Sw80ybvq
then I did some digging and found the FX on the VOX site along with the schematic of not just this one but others as well, then to my dismay I find that the "Yellow Dot Transistor" is no longer made and the some more digging and can't really find out the specs on the yellow dot so here I am here to see what you guys think of a good replacement for it. All I got is its a NPN so a while back I bought a but ton of NPN and PNP old Germanium transistors and I am gonna try some of them (the NPN's) and maybe try some 3904's to start... anyways hopping someone knows more about the yellow dot's and some good replacements...

here is where i found the schem and more FX https://www.voxshowroom.com/equipment/effects.html
 

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there are mentions on the web about these units using originally ASY51 or ASY50, then switched later on to BC108. This is a fuzz face topology so I would look at the original FF variants, but yeah these yellow dotted ones are most likely BC108A. Any low gain NPN would be a good replacement.
Try out with whatever you have available and tweak the biasing to your taste!

I want this guitar though, I need it 🤤
 
I don't believe the Starstream series guitars ever had the germanium Distortion Booster circuit in them – to my knowledge that circuit was only in the earliest of the long red plug-in units shown in the fsb link (and besides they're PNP positive ground which wouldn't work with all the other NPN negative ground circuits in the guitar). I have built a couple of variations on that circuit, though, and it's one of my all-time favourites. Any low-gain, medium-leakage germaniums are worth trying if you want to have a go at that one. (I did not have good luck with either higher-gain and/or low-leakage Ge trannies, though.)

The "yellow dot" transistors in the silicon FF-like circuit that was in those guitars were probably 2N2924/5/6 TO-98 types when they weren't BC108As, and the yellow dot indicates a gain range of around 150 to 300 hFE, so as Thomas says above, any low gain NPN should work just fine, including 3904s.

Another good link for more info on these guitars' circuits is https://www.acidfuzz.com/pages/vox-guitar-fx...
 
pacealot said:
Any low-gain, medium-leakage germaniums are worth trying if you want to have a go at that one. (I did not have good luck with either higher-gain and/or low-leakage Ge trannies, though.)
Click to expand...
That was a very informative post, thank you! Do you have ballpark hFE and leakage figures for what you consider low gain/medium leakage?
 
Well, the best results I had were with a pair of NKT214Fs I blundered into (I also got decent results with a couple of ACY types, though I forget the exact number and don't have it to hand readily), and as I recall the NKTs were both around 26-28 hFE (yes, pretty low!) with leakage around 50µA or so. I also played with the 33k Q2 collector resistor to get it to bias more reliably (basically the same technique as with a Fuzz Face, even though the circuits are not very similar otherwise besides both being 2-transistor fuzzes). With much less leakage, the stages won't bias correctly (not unlike an FZ-1 or a Mk I). It has a very strange forward feedback aspect which I still don't quite understand (I'm definitely not an EE in any way!), with the 330k resistor connecting both collectors. It's a good circuit to try if you have germaniums that are considered "too low" hFE for the usual FF/Mk II sort of applications.

It's really a lovely lower-gain, very '60s fuzz – I could see how it would've appealed to young Brian May. Has some sonic similarities to the Carlsbro 2-knobber that Chuck did his thing with a few years back. They also share having fairly unusual circuit topographies by modern post-germanium standards!
 
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