Another Tone Bender Mk1 question! High pitched fizzing...

[BluffChill]

Hi guys! I'm working on a Tone Bender Mk1, using the classic DAM schematic that's floating around the web. I've managed to source a batch of 2G381s, although they're called 2G381 T - not sure what difference that makes (I know 'D' marks 'driver' for OC81s, maybe the same principle?

Anyway it sounds good with the stock values from the schematic. I've read that the 470K should be 180K but that seems to refer more to the gating, but that's not my principle here. I get a very distracting high pitched noise, only when playing a note, behind the 'main' tone (which sounds great). It disappears if I roll of my tone knob. It sounds a lot like what happens when you miss the small capacitor off Q2 in a silicon fuzz face - it's like the high end is being amplified into noise, and it sounds fizzy and horrible. Does anyone know what I should be adjusting to control this?

I'm using a 25K trimmer set about halfway for the Q3 collector but everything else is fixed as per the schematic. I'm using:

Q1. OC75 - 66hFE, 100uA
Q2. 2G381T - 120hFE, 300uA
Q3. 2G381T - 87hFE, 160uA

[antonis]

It sounds a lot like what happens when you miss the small capacitor off Q2 in a silicon fuzz face

Try a 47-100pF between Q2 Collector & Base..

[BluffChill]

Haha, I was trying to avoid that! And trying to keep it as accurate as possible. Could leakage be an issue here?

[antonis]

Leakage could be healing here.. 

[moterrac]

I'm not sure if you fixed your issue but a 20k trim pot replacing q3 base resistor to ground will usually be enough to sort out the problem; a Zonk uses a 15k in place of the 8.2k and I've seen other builders use a 10k also. I'm not sure if you're U.K based but I just lost out on a lot of 2G381s ebay and am desperate to get a few for an authentic build! I've been obsessing over MKIs and the odd 'no-bias' germanium circuits for quite a while now. If you're interested in the silicon side of things the Sunn Buzz SB-1G can be a good starting point for messing around, along with Mac's Silicon Tone Bender schematic on the forum.

Hope you manage to sort it out!

Tom

[BluffChill]

Thanks! I think I might have got the 2G381s you were after I will try that. I've rectified it for now with a 1nF from Q3 base to collector. Sounds beautiful!