I tried to do something cool -- Foiled!

[italianguy63]

I picked up a couple of these small 3 wire digital voltmeters....

The plan was to wire it into a 4 knob PNP FF and use the voltage measuring wire on the Q2 tranny bias voltage.  Hence, when you messed with the Sag or Bias, it would give you a digital readount (so you could dial in a cool tone again later).

But, the cheapy things will only read positive voltages, and is funky/picky about being hooked up.

The BEST I can get it a psuedo reading of how much you dial DOWN the bias voltage. (a lower bias voltage becomes a larger number).  I also can't seem to get a range of better than about 0.0 to 4.0

Not what I wanted...   

[Seljer]

More mojo you should have used analog voltmeters, they're not so picky

[bool]

Perhaps not the answer you'd want to hear, but imho the best way to tame the fuzzes wrt "presets" would be indeed to measure all the sweet spot settings but then use a custom built controler to set the operating point accordingly (either analog or digital).

For starters, if analog, you would need a voltage regulator with a remote sensing probe to read in the Q2 Vc and set the V+ so it matches the desired voltage. This is heavily simplified and in the real world this would mean that your fuzz would have separate supplies for Q1 and Q2 but this way things can be automated to a certain degree. (since you actually need to tweak the Q2's Ic which in turn "produces" the desired "voltage" bias ...)

[italianguy63]

Bool--

Is that a Stephen King "Bool!"

??

[tca]

But, the cheapy things will only read positive voltages, and is funky/picky about being hooked up.

But any negative voltage is a positive voltage. That thing has only one (+) probe, doesn't it?

[italianguy63]

It seemed to be neg. voltage protected somehow.  They potential of the + has to be higher than the - connections.  So + goes to ground, and - to - power in this case.  Then the voltage sensing wire will only read postive voltages...  I tried "fooling" it by making the bias voltage the common.  It just wouldn't work. 

I screwed around with it for about an hour trying different wiring configurations...

[italianguy63]

Maybe I will need to build a NPN Si fuzz...

[slacker]

Does the figure the meter gives you actually matter for what you're trying to achieve? So long as it shows some value that you can return to to get a particular sound it's doing it's job.

[italianguy63]

Yes.  I know it is gadgety, but, for a couple bucks, I thought it would be cool to incorp. into the face of a pedal... 

[italianguy63]

I would prefer it measured the voltage.. It just "means" more to me.. but, it's all relative anyway.  I may do one anyway... Stupid pedal tricks.

[slacker]

What you could do is run the bias voltage through an inverting opamp buffer biased around -4.5 Volts, and connect you meter to the output. This would flip the voltage reading around -4.5 volts so the voltage on the display would be what you want ie:- a lower bias would read as a lower voltage, it would still be displayed as a positive value though. For example a bias of  -4 Volts relative to ground would become -5 Volts on the output of the buffer, -5 Volts measured relative to the -9 volt negative rail would read as 4 Volts on the display.

[samhay]

I did something like this a few years ago on a negative ground FF with piggy-back Si transistors with variable again. I used an on-off-on toggle to read either the supply voltage after a 'sag' pot, or the bias voltage. It seems to throw quite a lot of noise onto the signal ground, so it is good to be able to turn it off. It also has a significant current draw, so loads down the supply a lot more than the FF. Wound I do it again - probably not. You can dial in the bias by ear quite easily.

Sorry about the photo - it's had a hard life.

[italianguy63]

That was the idea... Forgot about the LED noise.  That would have to be isolated too... Looks like this is a "do it on the bench" project before I go through the major pain of cutting a square hole in a 125B!

This one may die in development.

[Jdansti]

Some of those meters have trouble if the power supply for the meter is also the power supply for what you're measuring. I had this problem and had to power the meter with a separate battery to get it to work correctly. This may not be causing your problem but it's an easy thing to check.

[DDD]

Ge-based stompbox.
No noise, battery friendly (50 uA max. consumption), easy to read, vintage image.
http://forum.gtlab.net/cgi-bin/yabb2/YaBB.pl?num=1399689316/0#15

[italianguy63]

I just liked that the digital readout was blue... matched my  power LED.  For like $4 I thought it would be cool.

I'm OCD OSO sometimes.  Obsessive Compulsive Disorder Other Shiny Object.  Easily distracted.   

MC

[amz-fx]

I picked up a couple of these small 3 wire digital voltmeters....

The plan was to wire it into a 4 knob PNP FF and use the voltage measuring wire on the Q2 tranny bias voltage.  Hence, when you messed with the Sag or Bias, it would give you a digital readount (so you could dial in a cool tone again later).

The problem with the 3-wire digital voltmeters (at least the ones that I've used) is that they are only good down to about 4v or so since they are being powered by the same voltage that they are measuring (in part).  I used one in a lab power supply that I built and it had a measuring range of 4.5v to 30v (or so). Since I was measuring 5v to 24v, this worked perfectly - I powered it off the 9v tap as I recall.

There are digital voltmeters that go down to 0v but they require a separate power supply for the meter, isolated from what is being measured. - they have 4 or 5 wires.  Power the meter with a separate 9v battery and it will measure anything you want, up to the limit of the meter.

regards, Jack

[Jdansti]

Some of those meters have trouble if the power supply for the meter is also the power supply for what you're measuring. I had this problem and had to power the meter with a separate battery to get it to work correctly. This may not be causing your problem but it's an easy thing to check.

The problem with the 3-wire digital voltmeters (at least the ones that I've used) is that they are only good down to about 4v or so since they are being powered by the same voltage that they are measuring (in part).  I used one in a lab power supply that I built and it had a measuring range of 4.5v to 30v (or so). Since I was measuring 5v to 24v, this worked perfectly - I powered it off the 9v tap as I recall.

There are digital voltmeters that go down to 0v but they require a separate power supply for the meter, isolated from what is being measured. - they have 4 or 5 wires.  Power the meter with a separate 9v battery and it will measure anything you want, up to the limit of the meter.

Two thumbs up for separate power.

BTW-digital meters may not react fast enough for them to be useful for quickly changing voltages. Even if they do react quickly, the readings might go by so fast that your brain can't capture them. This is where analog meters have an advantage. Your eyes will capture the maximum deflection of the needle and your brain will retain the level after the needle has moved again.

[duck_arse]

if you use a dual-gang pot on the variables you want metered, you can connect the other track between "a" voltage and "another" voltage. connect the meter to the wiper, and you're measuring the, I dunno, something.

[Jdansti]

^ That's a quote waiting to happen.