Tonebender Mk1.5 voltages

[Greenballs]

I've built a Tonebender mk1.5 to as close to the original spec as possible with the following changes:

4.7uf input cap in place of 5uf.
22uf Q2 emitter cap in place of 25uf.
2N1308 transistors in place of OC75s.

So, nothing too different.

It passes signal and sounds like a 60s fuzz should but from the way some notes drop off relatively quickly I can tell that there's a biasing issue. The voltages I've measured are:

Q1 (hfe 115)

C - 0.34
B - 0.09
E - 0.01

Q2 (hfe 173)

C - 7.35
B - 0.32
E - 0.24

It seems to me that the obvious issue is the 0.34v at Q1 collector, which looks a little low... the thing that's puzzling me is why its so low? I've attached the schematic and my layout for reference so you can see what I'm looking at but I'm a bit stumped to be honest.

If anyone has any suggestions or knows what the optimal voltages are for this circuit I'd be most grateful as I'm pretty sure I don't have them here!

[antonis]

May I ask if +9V supply in your layout is a typo..?? 

edit: Didn't realize 2N1308 as n-p-n BJTs..

I'd start tweaking Q1 Collector resistor value (33k to 39k)..

[FSFX]

May I ask if +9V supply in your layout is a typo..?? 

edit: Didn't realize 2N1308 as n-p-n BJTs..

The 2N1308 is an NPN germanium transistor.

[antonis]

The 2N1308 is an NPN germanium transistor.

That's exactly what I said..
(I was just following OP's schematic..)

[FSFX]

I've built a Tonebender mk1.5 to as close to the original spec as possible with the following changes:

4.7uf input cap in place of 5uf.
22uf Q2 emitter cap in place of 25uf.
2N1308 transistors in place of OC75s.

The 4.7uF electrolytic input capacitor is the wrong way around on your layout picture. It needs +ve to base of Q1 and -ve to the input.

[duck_arse]

well you know what I always say, and it seems I have to say it again. show us photos of your build, please.

[Greenballs]

I've built a Tonebender mk1.5 to as close to the original spec as possible with the following changes:

4.7uf input cap in place of 5uf.
22uf Q2 emitter cap in place of 25uf.
2N1308 transistors in place of OC75s.

The 4.7uF electrolytic input capacitor is the wrong way around on your layout picture. It needs +ve to base of Q1 and -ve to the input.

Ah, yes. Thanks for pointing that out!

[Greenballs]

well you know what I always say, and it seems I have to say it again. show us photos of your build, please.

I'll take a pic when I get home later.

[Electric Warrior]

It seems to me that the obvious issue is the 0.34v at Q1 collector, which looks a little low... the thing that's puzzling me is why its so low? I've attached the schematic and my layout for reference so you can see what I'm looking at but I'm a bit stumped to be honest.

If anyone has any suggestions or knows what the optimal voltages are for this circuit I'd be most grateful as I'm pretty sure I don't have them here!

Your voltages are fine. 0.34V is quite high for a Mk1.5 - you're well into Fuzz Face territory here. MK1.5s are often under 0.2V - but yeah, they can be quite farty sounding, especially when it gets warmer and Q1C goes further down (and Q2C further up). I guess many pedals just crapped out in the summer of 1966 - leading Sola Sound to upgrade their pedals to the MKII circuit, which is much better behaved.

Bringing Q1C down sure will smooth things out..
The fuzz pot is also worth tweaking. IMO its tolerance has a bigger impact than transistor matching (or mismatching)

[Greenballs]

It seems to me that the obvious issue is the 0.34v at Q1 collector, which looks a little low... the thing that's puzzling me is why its so low? I've attached the schematic and my layout for reference so you can see what I'm looking at but I'm a bit stumped to be honest.

If anyone has any suggestions or knows what the optimal voltages are for this circuit I'd be most grateful as I'm pretty sure I don't have them here!

Your voltages are fine. 0.34V is quite high for a Mk1.5 - you're well into Fuzz Face territory here. MK1.5s are often under 0.2V - but yeah, they can be quite farty sounding, especially when it gets warmer and Q1C goes further down (and Q2C further up). I guess many pedals just crapped out in the summer of 1966 - leading Sola Sound to upgrade their pedals to the MKII circuit, which is much better behaved.

Bringing Q1C down sure will smooth things out..
The fuzz pot is also worth tweaking. IMO its tolerance has a bigger impact than transistor matching (or mismatching)

Thanks for the info Electric Warrior, good to know that it was in the right ballpark after all.

Its funny you should mention the rising and falling voltages because something I've just observed is the exact opposite of what you described, taking voltage measurements immediately after connecting the pedal to the power supply, with Q1C rising from 0.10v and settling at 0.39v and Q2C falling from 8.95v and settling at 6.67v. Not sure what this is indicative of but I have a 63v 47uf power conditioning cap on there which my gut is telling me could be responsible???

Regarding the fuzz pot, do you mean switching the value to something higher, like 5k perhaps?

FSFX - I switched out the input cap (it's now a 2.2uf cap because I don't have any decent quality 4.7uf caps currently, but I'm not sure it's made a huge difference anyway) and put it in the correct way around and it seems to be hanging onto the notes with a bit more sustain.

[FSFX]

FSFX - I switched out the input cap (it's now a 2.2uf cap because I don't have any decent quality 4.7uf caps currently, but I'm not sure it's made a huge difference anyway) and put it in the correct way around and it seems to be hanging onto the notes with a bit more sustain.

It was just an observation and the reverse DC voltage across the capacitor would never have exceeded the base-emitter voltage drop of Q1. I have seen some manufactures quote values like 1.5v reverse voltage as being safe anyway.