Disabling the octave in the Fender Blender

[John Lyons]

If I want to disable or switch in/out the octave effect in the fender blender should I lift one of the 27K resistors to ground?
Or is it a matter of breaking the connection between the negative side of one of the 10uf caps and connecting directly to the base of Q4?
Or am I way off?

Thanks

John

[R.G.]

Break the connection between one of the diode anodes and Q4.

[John Lyons]

Thank you kind sir!

John

[John Lyons]

Ok, I broke the connection between the top diode at the cathode (Anoode or cathode should not make and difference I don't think) and put a SPST switch between the two now open connections to switch between octave and non octave.

In the octave mode the sustain is fine and ringing as usuall, In the open mode with the top diode not coonected to Q4 the sound is misbiased sounding and decays less with a slight gating... more of a threshold where the note decay fizzles out .

I change my arangemnt so the switch just bypasses around the top diode and the sustain and decay are fine. There is a little noise now but I think that's because octave circuit tend to cut out the signal while idle due to the rectification process.

The circuit is splitting the phase correct?

Any thoughts on this or better ways to do this? It seems to work fine but it goes aginst R.G's advise slighly so I wonder why the "gating/misbiasing is going on.

John

[doug deeper]

try putting a jumper over the diode left in the signal.

[doug deeper]

i need to read first@!

[Meanderthal]

By bypassing the diode rather than breaking the connection you will still be getting both 'halves' (mostly) of the waveform split by the symetrical bias of  Q3. So yeah, it's misbiased...

[Jaicen_solo]

I'd use a DPDT switch and bypass the top diode, then cut the connection between the emmiter and the 10uF cap. That makes more sense to me at least.

[John Lyons]

Jacien

Is this what you are talking about?

[R.G.]

Ok, I broke the connection between the top diode at the cathode (Anoode or cathode should not make and difference I don't think) and put a SPST switch between the two now open connections to switch between octave and non octave.

In the octave mode the sustain is fine and ringing as usuall, In the open mode with the top diode not coonected to Q4 the sound is misbiased sounding and decays less with a slight gating... more of a threshold where the note decay fizzles out .

I change my arangemnt so the switch just bypasses around the top diode and the sustain and decay are fine. There is a little noise now but I think that's because octave circuit tend to cut out the signal while idle due to the rectification process.

The circuit is splitting the phase correct?

Any thoughts on this or better ways to do this? It seems to work fine but it goes aginst R.G's advise slighly so I wonder why the "gating/misbiasing is going on.

Bummer. Jacien's right, open the other side and short a diode. It's convenient to do this:

1. Break the connection between the lower diode and Q4 base.
2. Install the pole of a 1PDT switch on the base of Q4
3. Connect one throw to the lower diode anode where you broke the connection in 1
4. Connect the other throw to the cathode of the top diode.

Now one position of the switch is stock, the other is one diode opened, the other shorted.

It'll probably pop like crazy when you flip the switch because it'll have big bias shift. I'll see if I can gen up something to not do that. A DPDT switch could do it easily enough.

I had thought that the constant forward bias on the remaining diode when breaking one would let signal pass through. Apparently it's not that way and the diode plus signal rectifies on to the coupling cap.

[John Lyons]

Like this?

[Mark Hammer]

That looks like it to me.  Keep in mind that the octave is created by creating complementary halves of the signal from the phase splitter that spits out opposite phase versions of the input signal.  The diodes chop half off, and when you mix the two complementary halves you end up with interpolated peaks that produce an apparent doubling.  Lose one of those complementary halves and all you have is one half cycle of a normal signal.  No wonder it sounds like crap.

A pity this one misbehaves.  The Foxx handles the octave/nonoctave transition really well via a simple SPST.

How come?

[Jaicen_solo]

Hehe, I got the better of R.G for once? Wonders never cease it would appear!

[John Lyons]

Well...  breaking the bottom diode connection to Q4 basse and bridging to top diode does not work. The gain gets cut and it the throws off the bias (didn't measure it, just sounds like it)

Shorting the top diode only seems to sound the best (leaving eveything else the same as per the schem) .

John

[R.G.]

That's what I get for not putting the thing in the simulator and just doing it in my head.   

I put it into the simulator and was surprised to find that it does not act the way one would think it does with those resistor values. In fact, the octave is distinctly impaired.

The problem is that the 27K/27K/100K biasing on Q4 and the two diodes load down the phase splitter too much. What you get is not the expected full wave rectified waveforms, but distorted and asymmetrical waves. There is an octave there, but it's much more subdued than it has to be.

You want to play? Change the two 27K's to ground to 100K, and change the 100K to 470K. Mr. Simulator says it's interesting. 

[R.G.]

And the SPDT octave switch works much better too.

[John Lyons]

I've been studying the FB schematic compared to the Foxx Tone Machine. I don't have a FTM here but I got to try out an original unit a while back. From what I remember it was thick and smooth with a nice fuzz element.
The Fender Blender is very thick and sort of "rubbery" sounding, a bit sluggish, tons of bass!

I'd like to bridge the gap between the FTM and the Fender blender. If I could get a little more bite to the high end and a bit more articulation I'd be happy. The input cap isn't really too large at .1uf but maybe cutting it in half and lowering the coupling caps here and there will do it.
But then the FTM uses the same values ... actually all the coupling caps are 10uf in tha one. I don't usually use the tone stack so it's pretty much bypassed  as stock (when in tone bypass). Other than the input and couple caps I can't see where to clean up the mud.

Question. The 3.9K/.001 combo across the Base and collector at the end of the circuit. Is this a high end taming measure?

RG, Thanks for taking your time to sim the octave switching. I have it set up as a switchable shorting of the top diode right now and it seems to do the trick and null the octave. Very slight pop and shift when switching but really nothing. I was planing on trying the 100k/100K/100K for the The octave section and into Q4. I'll try the 470K off the base to the supply rail as well.

John