Fender Blender Octave mods...

[Toney]

 I would like to add an octave cancelling mod to the FB...ala the Foxx Tone machine.
Thats to say cancelling one side of the phase splitting diodes.  But it seems, reading the various methods people have employed, to be a little uncertain.... not exactly the  same as the FTM.

Here, after some discussion, John Lyons had success by shorting out the diode from Q4's base to Q3's collector via a 10uf cap (blue color below on schem)

After some searching I found a post by Ed Rembold whereby, after happy accident, he achieved this effect by switching out the 10uf cap from Q3's collector and retaining the 27k to ground from the "upper" diode to Q4's base. Red color on schem below.

thread here




Any other idea??

[tcobretti]

In case one of the big brains doesn't respond, I am pretty sure that is the correct way to kill the octave.  I am no EE, but I believe Q3 is acting as a phase splitter, and then the two out of phase signals are combined to generate the octave.  So, by using an spst to break the signal at one of Q3s diodes you kill the octave.

A cool mod for this pedal might be to put the Sustain pot just before the clipping diodes,  This way, the blend pot would blend between fuzzy octave and clean octave.  Also, Q3 gets a stronger input so the octave is more pronounced.  You might need to tweak the value of the blend pot for it to work right.

[Mark Hammer]

The two diodes in the phase splitter are critical to ending up with two complementary "clones" of the input signal that, when combined, produce frequency doubling (or something close to it).  If you want to cancel the octave effect, then you need to factor those diodes out of the picture and have ONE output from that phase splitter providing the entire signal.  In another thread somewhere, I suggested the following:

1) Cut the trace between one of those diodes and the base of Q4.
2) Get a SPDT toggle (or stomp, if that's your leaning), and wire the common of that switch to the junction of one of those two diodes and the base of Q4.
3) Run a lead from one of the outside lugs of the switch to the diode whose trace had been cut.
4) Run a lead from the other outside switch lug to the junction of the other still-connected diode and the 10uf cap and 27k resistor.

In one switch position, both diodes are connected, yielding the octave effect.  In the other position, one path is cancelled, while the diode in the remaining path is bypassed, allowing the entire signal to pass, not just one half-cycle.

[John Lyons]

I just bypassed the top diode so I guess I am still rectifying the bottom path?

Woudn't it take a DPDT switch (or DPST if you can find one) to make your mod  work Mark?

Like this.
One of the switch poles breaks the top diode connection and the other pole bypasses the bottom diode.
Both common  poles would be connected together and run to Q4.

Seems like I tried this and it didn't work... Have to try it again.

John

[Toney]

 Well, the way Ed Rebold did it, was cutting the link between the 10uf cap and the diode(at Q3's collector) but maintaining the diode/27k to ground and leaving the second (emmiter) set as they are. Not sure if this would react any differently to just shorting the diode, but it seemed to work for him...

Heres the second link from above...http://www.diystompboxes.com/smfforum/index.php?topic=54455.0

[tcobretti]

Mark and I just had this same discussion in another thread, and he was right there, too.  The remaining diode still affects the sound, so ideally BOTH diodes should be removed from the signal path.  What Mark describes is a switch that routes the signal around the remaining diode, and can be done with a SPDT.

[John Lyons]

I can see how this will work in theory but I can't see how a SPDT switch will work to do it.

I can't see how you can use less than 5 contacts.
Top diode and Q3 base = two lugs, Bottom diode connected and bottom diode shorted 3 lugs (common, shorted, diode connected)
Can anyone post a drawing?

John

[tcobretti]

This way, in one position, one diode is bypassed by the signal flowing toward the least resistance and the other diode goes nowhere, and in the other position both diodes are connected to the collector of Q4.

[Mark Hammer]

Thanks for the drawing and confirmation.  What allows a "mere" SPDT to do it is the fact that both diodes tie to the common point of the Q4 base.  Lucky for us.

[John Lyons]

Oh...ok...I see now. The top diode is just jumped to Q4.
Thanks...I'm slow I guess.
I figured it had something to do with the common Q4 point shared by the diodes. I'll try that out now.

John

[Toney]

Ok thanks guys, excellent input.
I don't have time to build this at the moment, else I'd test myself.
I will add the mods to my layout as soon as I hear of them being tested in the field..
Cheers, Toney