t rex mudhoney tone works backwards?

[erynforrester]

hi folks

Just built a mudhoney and the tone pot appears to be working backwards,
aside from swapping leads over, is there anything else obvious that might be causing this?
I've definitely got them the right way according to the layout.

also, possibly related - the orientation of q1 doesn't seem to matter. - that is it seems to sound the same either way. Could this indicate a mistake in my build?

appreciate any suggestions or thoughts.
cheers

[IvIark]

I'm not sure whether it is a "Cut" type tone control which do behave in the way you describe, but assuming the original isn't then yes you can just swap the 1 and 3 connections.  It definitely shouldn't work if you turn Q1, are you sure the cut to the right of the 22n input cap is complete and with no burrs in the hole making an unwanted bridge?  If that cut was incomplete then I could understand the signal getting through to the opamp

[erynforrester]

hmm...

I'm not sure about the tone knob, but i guess its fine for now since it works.

The more pressing issue it seems is q1. I checked again with another bc546b, and either orientation the pedal works. It doesn't work however with q1 pulled completely.
It is worth mentioning that the pedal appears to work as it should otherwise, sounds good, boost switch is huge.
however, I'd like to sort out Q1 so I know its right.

I've checked all cuts and checked between rows with DMM to make sure there isn't continuity where there shouldn't be.
I've gone through the layout, first checking resistors are in correct place, the electrolytics, then all other caps and diodes. Everything seems to be where it should.

I'll post some pics and get voltages tomorrow after work.
I really appreciate your speedy responses.
If you have any ideas about q1 I love to know what is going on with it.

thanks again,
also, can you recommend any books on pedals. I understand tube amps all right, but really don't know much about how pedals work.

cheers
Eryn

[Frank_NH]

There is a schematic for the Mudhoney online at FSB.  The purpose of Q1 appears to be impedance matching through a emitter follower BJT stage (that is, Q1 doesn't amplify the signal).  I just found this online article, which may explain things...

/bjt-in-reverse-active-mode-of-operation

Q: "What will happen if for a BJT transistor it's Emitter terminal is treated as collector and collector as Emitter in a common Emitter amplifier circuit?"

A: It will work but will have a lower β (beta)

[IvIark]

Thanks for that Frank, I didn't realise that was the case, although I think there must be instances where it doesn't quite work like that.  I can't tell you how many time people have had problems with various layouts on the site until they rotated the transistor 180 degrees.   And with the transistors used being predominantly CBE pinouts, that would suggest there would be no problem in doing that.

[Frank_NH]

Yeah, that surprised me too (though I'm a mechanical engineer by training and not a EE - so don't quote me when it comes to electronics...).  The article suggested that it may not work for all transistors, but in this case, as long as the signal somehow finds its way from the base to the emitter, I guess it can be amplified by the subsequent op amp stage.

However, after studying the online schematic (assuming the schematic is correct), the Mudhoney is really nothing more than a modded Proco Rat.  The BJT buffer can probably be removed entirely and it probably wouldn't affect the sound at all.  The buffers (including the input buffer which remains on the vero layout) are simply a part of the bypass circuit of the original effect (which is quite elaborate, according to the schematic).

[erynforrester]

wow thanks guys!
looks like I was lucky on this one, i'm glad i didn't start ripping up the board yet.

And yes Mark, I hear your point about transistors and chips. After my disastrous first two fuzz builds I invested in ic sockets and stand alone pins for transistors and managed to diagnose a faulty ac127 that caused me to rebuild an entire pedal.

Keep up the good work gentlemen, learning heaps