Advice on Roger Mayer Octavia?

[Mark Hammer]

Was introduced to a very impressive guitar player in a store yesterday by forum member Boogietube.  Chatting about gear, he expressed great interest in the tone of Andy Timmons and the octave fuzz he was using.  It's a Roger Mayer "spaceship" (no transformer) type.

I built one from Justin Philpott's schematic about a decade back and was untrhilled but I know (and expect) a little more now.  I've checked through the archives here for advice and ideas, but I was wondering if any of you who might have built one for yourself, and had to fine-tune it, might have some suggestions for what brings out its finer qualities, and I suppose especially its "classic" tone.

My own first inclination is to use matched germanium diodes instead of silicon, for slightly less gating of the cotave sound, and maybe a resistor+trimpot arrangement instead of the 2M2 resistor, to provide for better level-matching of the two complementary signals, and better octaving.

I also found, when making a Tychobrahe Octavia, that simply following the schematic and nominal parts values didn't work.  I needed to add a little more resistance in series with the Gain pot to bring the minimum gain down to a point where the octave stood out and wasn't swamped by fuzz.  So I figure I might try some experimentation on the gain-setting resistance for the RM, in case I run into the same issue.

Apart from that, anything some of your pioneers have run across with this circuit that I should know about or consider?

[boogietube]

If you're going to build this for him, I've done a little research.
Fuzz central seems to have a layout:
http://www.fuzzcentral.ssguitar.com/schematics.php
Under Roger Mayer.

[boogietube]

I have all of the parts if you need any.
Sean

[Mark Hammer]

I'm well ahead of you.      If files can be said to fester, those have been festering in my directory of octave units for years, and all the parts are sitting in the bins.  The missing part is, as always, time.  But it seems like a simple enough challenge.  Thanks, though.  And thanks for introducing me.  Helluva nice guy.  I hope I can pull this off for him.  I know he'd certainly make use of it.

[lars-musik]

Reviving this old topic... After two non-functional attempts on the Roger Mayer Octavia (and even doing a simulation of the first part because the 2N3906 voltages looked absolutely wrong) I finally got it. Although always measuring all resistors, a strip of 680 Ohm hid in the 680 "K"Ohm bin. I know you shouldn't enjoy one or two glasses of wine whilst soldering, but...  Nota bene: If your DMM shows .678 in the display from now on I pay attention to the little letter right of that number (in my case it was KΩ and not MΩ).

HOWEVER, now I have got two populated RM Octavia boards and like Mark five years ago, I am underimpressed also. The octave is hardly decernable (as instructed with the neck pickup and volume reduced) and the fuzz is a bit farty and gated.


I replaced the MPSA13 with MPSA14 but my limited datasheet reading skills suggest that wouldn't make any difference. Am I correct?

Have you, Mark, or someone else ever tried the germanium diodes? And do you know if for example trimming the emitter resistors of the darlingtons could make the sound less gated?

This is the schematic I followed:

[Gus]

I posted this a short time ago
http://www.diystompboxes.com/smfforum/index.php?topic=114636.msg1064512#msg1064512



I don't understand why Darlingtons were used in the RM with the two Vbe drop and a 9VDC supply

R14 and R15 are used to leak some current around the 22uf caps to start the Si diodes conducting.  So the voltages at the collector and emitter and the resistance values across the caps could be fine tuned. The collector is a higher voltage than the emitter that is why the resistor values are different. The cathodes go to ground via the 100K.
So adjusting the voltage divider R8 and R9 you can adjust the C and E voltages
Use Ohms law to calculate the current in the diodes

It might be different selected value resistors around the 22ufs might gate less

[lars-musik]

Hi Gus,
this one wasn't on my radar, unfortunately. Looks much easier to tweak than the Roger Mayer thing. But now I already built two of the others so I will try to make them work first, but yours will follow....

I changed the Si-diodes to germaniums (Vf approx. .255V) and the gating is gone. Now I'd need something to make the octave effect more pronounced. I replaced R15 (2M2) with a potentiometer (actually two 1Megs because I hadn't anything larger at hand) but hadn't had the time to really test it yet.   

[Mark Hammer]

After building a bunch of different ones (though admittedly not all), the most consistently robust octave up I've run into is the Foxx Tone Machine.  One of the things that seems to be distinctive about the Foxx is the feedback network that I gather pre-shapes the signal fed to the phase-splitter.

Gus's circuit is certainly simpler than the Mayer circuit, but the U1 op-amp gain stage is essentially flat, with no pre-shaping prior to the phase-splitter.

Is my perception of the FTM off-base somehow?  Does the Mayer Octavia circuit engage in any pre-shaping that is not present in Gus' circuit, or that is comparable to the FTM?

[Renegadrian]

I just buit a RM octavia following this vero here and it works good. yeah the octave is not full throttle, but you may like the overtones it produces. I liked it and hope its new owner will too...
on a side note, there's a hendrix related mxr octave fuzz, it's just the octavia with an IC buffer and not a tranny buffer.

[lars-musik]

I just tried it with my amp and my humbucked semi-acoustic. I don't like it. The pots (for R15) have only slight impact on the octave effect.
I dunno, if Mark says the FoxxToneMachine it is, then the FoxxToneMachine it shall be.

Before I am getting myself into trouble again, what would you think about the "ultimate" version in this document:

http://pedalparts.co.uk/docs/FoxxTave.pdf

??

[Mark Hammer]

I've had better luck with the transformer-based Tychobrahe Octavia than with the fully transistor-based Mayer unit.  I don't know why; it just is.

You will note that C2/C3/R4/R6 in the pedalparts drawing forms a midscoop filter, similar to what you see on a Superfuzz, except here it sits between the emitter of Q2 and base of Q1.  If I've understood RG's explanation of the FF, that scoop would result in a little more "push" for the mids in Q1.  I'm guessing that's what results in a stronger octave.

The topic has come up many times here over the years.  One of the things I recommend using is a 3-position toggle for the octave switch.  You will note that both outputs of Q2 must pass through a diode.  This rectifies the signal and provides doubling when the two complementary signals are combined, using the switch.  But when only one of the outputs is used (non-octave setting) there is some crossover distortion imposed on the remaining signal by that single diode.  As a result, many people tend to find the octave setting more pleasing than the simple non-octave fuzz setting.

A SPDT 3-position toggle allows one to connect the 2nd diode for octave, disconnect it (in the middle position) for non-octave, and bridge the other diode in the other side position for a cleaner (but still overdriven) output.  So, you get to keep everything about the stock unit, but add another potentially more pleasing overdrive sound.

Note as well that the unit does get a raunchy enough sound without that pair of clipping diodes just before the tone control.  So, a diode-lift toggle there provides another useful option.  Just keep I mind that the unit becomes much louder and more dynamic when those clipping diodes aren't in circuit.

[lars-musik]

Thanks for chiming in, Mark. Have I got this right? Do I have to fuzz about biasing the transistors or forward voltages of the germanium diodes or will it probably just work?

[Mark Hammer]

What you show in your modded FTM is exactly right.  You don't have to change anything else.  I think you'll be pleased.

[LightSoundGeometry]

I built myself an escabedo push pull and have been happy with it ever since. makes me sound like srv and hendrix 

[lars-musik]

Great, thanks for the answers. Next on the list will be the FTM. Here's the layout I am planning to use (just added another clipping option).
Layout:

Build Document:
https://www.dropbox.com/s/q9agji82j9mp30d/Build_Doc_Foxxx.pdf?dl=0