MKII Tonebender bias pot question.

[Rambozo96]

So I been messing with the MKII circuit for a while and wondered if it's best to put a bias pot for Q2's collector or Q3's collector. I currently use a 20k trimmer in place of the 8.2k resistor off of the collector on Q3 and messing with the bias voltage on it doesn't seem to help with the overly gated sound I been getting. Thanks in advance!

[PRR]

Post your voltages.

What are you using for Q1? If it is not an old-production (leaky) part, it WILL "gate".

If we missed you: Welcome!

[BetterOffShred]

I read up extensively today, and PRR nailed what I seemed to find as well.  Q1 needs to be like 50-70 hfe and rather leaky.  Then your technique seems to be conventional wisdom.  20k trimmer for the 8.2k on Q3, and then mess with the 470R for volume. 1k seems to be a widespread acceptance.   

I have a couple Si Mk II, about to build one.  Let us know what you find

[Electric Warrior]

Post your voltages.

What are you using for Q1? If it is not an old-production (leaky) part, it WILL "gate".

If we missed you: Welcome!

The stock circuit is set up for three transistors of identical types. They should all be in the same general gain and leakage range. Too much leakage on Q2 will also make it gate (try touching it for a couple of seconds). It's very interactive. A trimmer on Q2 can certainly help dialing out the gating.

Bias only needs to be set for Q1 and Q2, as Q3 depends mostly depends on Q2's specs and setup. Sola Sound used 100ks on Q1's base and Q2's collector for low-ish leakage OC81Ds and (more commonly) 10k on Q1B and 47k on Q2C for the leakier OC75s. The voltages were in the same ballpark for either version.

If the voltages are in the right ballpark already, I wouldn't bother with trim pots at all and simply select a set of transistors that play well together.

I read up extensively today, and PRR nailed what I seemed to find as well.  Q1 needs to be like 50-70 hfe and rather leaky.  Then your technique seems to be conventional wisdom.  20k trimmer for the 8.2k on Q3, and then mess with the 470R for volume. 1k seems to be a widespread acceptance.   

If you leave the 8k2 alone, messing with the 470R should not be necessary. This circuit has plenty of output.

[fretbuzz2003]

Bias only needs to be set for Q1 and Q2, as Q3 depends mostly depends on Q2's specs and setup. Sola Sound used 100ks on Q1's base and Q2's collector for low-ish leakage OC81Ds and (more commonly) 10k on Q1B and 47k on Q2C for the leakier OC75s. The voltages were in the same ballpark for either version.

Sorry to bring up an old post but I have a question. What is the cutoff between low leakage and high leakage when choosing a 100k or 10k on Q1's base?

I built a MKII using AC125's. The leakage in Q1 was 0.210ma and I used a 10k on Q1. It biases correctly but it doesn't clean up like I would like. Would I be better off using a 100k?