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Foxx Tone Machine with a problem.

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8/1/2000 10:21 AM

Foxx Tone Machine with a problem.

I have just completed a FTM, and while I can see it has tonal possiblities, I can't help but think it's not operating correctly.

What makes me think this is;

(1) To get a reasonable amount of fuzz I have to have the Gain/sustain control fully clockwise, distortion wise this would be similar to a Rat pedal with the gain set at about 10 o'clock (only sufficent distortion) the pedal doesn't have a lot of sustain, the note dies off fairly quickly.

(2) The Volume of the pedal is quiet as well, if I have the gain set fully clockwise, I would have to set the volume pot at three o'clock to achieve the same volume as when the effect is bypassed.

I have checked the pcb (etc) for errors, and I haven't found any.

The transistors used in the pedal are BC549's, these transistors according to a data manual should have a Hfe of 110, which you'd think is enough.

The voltages around the circuit are as follows.

Q1 Collector 3.22V

Base 0.614V

Emitter 0.06V

Q2 Collector 6.47V

Base 3.22V

Emitter 2.6V

Q3 Collector 7.49V

Base 0.684V

Emitter 0.146V

Q4 Collector 8.24V

Base 0.634V

Emitter 0.101V

Battery Voltage 8.9V

Measurements taken with a Fluke 77.

Thanks for any info and help.

Yours Sincerely

Mark Abbott.

8/1/2000 12:58 PM

There are several schematics floating around, one of which has component values for the tone control off by a factor of about 10 or so (RG drew it, lord bless his pointy little benevolent head

). Like you, I built one and while it functioned according to plan, it wasn't very loud at all. Then I stumbled onto the component discrepancy in the tone control. When I realized the cap values were wrong, I changed them to the appropriate values, and the problem went away. It's still not an overwhelmingly loud unit, but you don't have to leave it up full to avoid a volume drop when engaged.

This may or may not be your problem.

Another thing you might try is replacing the Ge diode pair with an Si pair. Not exactly "vintage repro" but it will raise the clipping threshold roughly twofold. It is important to realize that diode clipping thresholds also vary from diode to diode because of normal tolerances. You can get Ge diode pairs that clip at 200mv, and other pairs that clip at 370mv, as well as Si pairs that clip anywhere from 450mv to 750mv. Since the clipping threshold sets your maximum output level, you'll want to make sure that this threshold is at an optimum level (i.e., max volume for consistent clipping).

8/1/2000 7:21 PM

I used BC549s and Ge diodes too, here are my voltages:

Q1 - c 2.14, b 0.74, e 0.16
Q2 - c 8.30, b 2.14, e 1.54
Q3 - c 6.96, b 0.88, e 0.29
Q4 - c 7.87, b 0.88, e 0.30

Battery was 9.81V

Although pot taper will affect this, with the gain on full, the volume has to be at about 1 o'clock to equal the bypassed volume.
Anything past 3 o'clock is noticeably louder and it can get about as loud as a Rat.
I used the schematic on Justin Philpott's site, but I'll check I didn't sub any parts for what was available.

Hope that helps,

Stuart

8/4/2000 6:32 AM

Much thanks and an idea.

Dear Mark and Stuart

Thanks for your help, since posting my question, I have noticed that the 4.7k in the tone control is in fact 47k in my circuit, I don't know how this will affect things, but I'll let you know.

I am trying to remove as many variables from the equation as possible, today I have bought new transistors which I've checked the Hfe (650-700). I did notice the value of Hfe varied from 300-700.

I also decided to dabble in "mojo", I have a green cap for the 1000pf negative feedback cap, and I decided to put a ceramic cap in its place. (I have found from guitar amps that the poly type caps seem to pass more signal than the ceramic caps). I'm hoping to pick up some gain here.

I was also thinking the first and last stage don't have emitter bypass caps, which also could be a source extra gain.

I'll keep tinkering and I'll let you know how it all goes.

Yours Sincerely

Mark Abbott.

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