Germanium transistors

[Gumby212]

I am using a matched set of mullard oc75/76 transistors (matched from smallbear) in a tone bender mk2 and notice that, altho the fuzz tone is great, the sound is gated and fizzles out with not much sustain. No matter how i bias Q3, the issue persists. In fact, the bias for each Q reads pretty close to "what they're supposed to". When i swap trannies for some russian gt402a's or mp20s with matching gains and bias voltages, the sound is no longer gated and starved and i have sustain for days. Is the gating a characteristic of the OC/mullard/glass casing style trannies, or am i missing something? In fact, i have 5 oc75's and 1 oc76, and have been swapping them from multiple tone bender circuits, and whenever i use the OC's in any circuit, i get gated sounds. They are dead quit when not playing. Same with the ac128's i have that are the "hottie" style (metal rectangles with a hole thru them). They sound great, but are gated and sputtery like the OCs. When using any russians, or my ac125's, this does not happen and the sound is more pleasing to me. Any advice would be greatly appreciated. Thanks!

[Electric Warrior]

It's a bias problem. A little bit of gating can sound great in this circuit, though.

Some voltages would be really helpful...

[Gumby212]

Its just weird that no matter the voltage, hfe or leakage, every oc75 i have sounds this way. My russians sound much noisier (as in not gated) which allows for more sustain. Q3 at 4.5v is too low imo. 8.5v sounds way more correct to me, which is where i bias Q3 every time. Its odd that in my testing, the gating seems to be a characteristic of the oc75's i have, as well as the ac128 "hotties" i have

[Electric Warrior]

With OC75s you should be close to 8.5 with stock values, anyway.
OC75s leak a lot and the circuit was set up for this with a 10k on Q1's base and a 47k on Q2's collector. They used 100ks in both positions for lower leakage OC81Ds. You may want to try this setup with your russian transistors.

It depends a lot on the selection of transistors you have at hand, but the gating should be rather easy to tame. Picking a slightly lower leakage transistor for Q2 is usually all it takes. Alternatively you could tweak Q2's collector resistor.

I'd need to see collector voltages of all three transistors to tell if everything biases right.

Here are the voltage readings of my vintage unit for reference:

Battery: 9.67V
Q1 C -9.02V B -0.03V E 0V
Q2 C -0.17V B -0.08V E 0V
Q3 C -8.44V B -0.17V E -0.11V

[ilcaccillo]

Here are the voltage readings of my vintage unit for reference:

Battery: 9.67V
Q1 C -9.02V B -0.03V E 0V
Q2 C -0.17V B -0.08V E 0V
Q3 C -8.44V B -0.17V E -0.11V

Hi EW, those are the voltages you have on a original tone bender mk2?

thanks

[Electric Warrior]

It's an early Supa Fuzz made by Sola Sound. Those got the exact same circuit boards as the Sola Sound and Vox MKIIs when they were being made concurrently.





The Mustard caps are replacements, but 0.015µF is one of the values that was used at the time. I suspect it came with 0.01µFs originally and even got the correct vintage parts, but it sounds killer as is, so I haven't dared to put in the correct parts yet.

The piggy backed electrolytic (50µF in parallel with the 25µF power filter cap) is probably original. They added caps of various types to units that would squeal, but it was usually a 25µF one. Earlier units had a 100Ω limiting resistor that could optionally be wired into the circuit to fix the squeal. Later ones simply had a 50µF or 64µF.