AC125 or 2n1309 vs. the OC 75 in the Tonebender Mk 2

[mordechai]

I saw a youtube demo the other day of a tonebender circuit that had AC 125s rather than the OC75s.  It sounded great, though not exactly as dark or woolly as the usual OC-types.  Yet I've heard many people out here suggest that AC125s are good substitutes, so I wonder if it was just a matter of biasing.  What are your thoughts on this substitution?  Same question re: the 2n1309, which someone suggested to me would be a little closer to the OC75 in tonal response...

[Scruffie]

That is nearly impossible to answer, two sets of OC75s could sound completely different and one set could sound completely different on its own if you changed the three transistors around infact you could take two from the other set and swap them round and put them with one from the first set and it'd sound different and your head would explode from all the variables.

Just go with what you have, if you think it sounds good, it's good! Biasing will play a part yes, adding a Trimmer to Q2 & Q3s collector is useful as you can bias it up with the Q2 one then take it from Harsh to smooth with Q3s Trimmer.

The 100k/10k on the input also plays a part in the tone, higher values sound harsher to me (in a good way) than lower and the roll off cap can also be increased to roll off more highs or removed to let them through (like in the Marshall Supafuzz).

But you should get your set of three transistors and see what sounds best on the breadboard with different biasings, it's the only way you're gunna find out... I haven't even mentioned the different gains and leakages of transistors that come into play... some transistors may have a different sound but it's not cut and dry and you could use whatever you wanted that had hFes and leakage simmilar to that of the original (around the 100s for hFe and 200-350uA Leakage) and you'll end up with something that sounds like a Tone Bender with experimenting.

It's a complex one!