Modified old transistor receiver fuzz pedal

[Pmelius]

Hey folks

I'm just poking around here for fun. I haven't really successfully built anything yet but I've been doing a whole lot of research and just messing with junk I find at thrift shops.

So I have this old Walkie talkie that I stripped some Walkie talkie junk out of and threw in a 9v adapter and some jacks. I've only tested it on this little solid state amp I throw around the house and it sounds CRAZY. So I'm trying to get it all legitimate (or as legitimate as I can get it) before plugging it into one of my more important amps.

My big question is, currently the output impedance is 2k ohms  (also this thing has 2 little transformers inside of it) and I THINK the impedance should actually be more like 100k. Is this correct? How can i increase this value? I've been poking around with a 1/2w 5% 100k resistor in different areas with no success. Again I'll mention that I have no idea what I'm doing hahaha.
Is there anything else I should be wary of before throwing it on my pedalboard? I understand  that the input of a tube amp probably wants to see something specific and I want to be very careful about what I plug in.

I can't find a schematic anywhere online for this thing either.

Any help is appreciated! Thanks so much!

[thermionix]

In what way does it sound CRAZY?  I'm no expert on this stuff, but I don't think you can hurt anything with a low output impedance.  I think the general rule is that you want lower output impedances and higher input impedances.

[Pmelius]

In what way does it sound CRAZY?  I'm no expert on this stuff, but I don't think you can hurt anything with a low output impedance.  I think the general rule is that you want lower output impedances and higher input impedances.

It's just a super heavy sounding fuzz! i couldn't find anything online telling me definitive numbers in layman's terms so i just tested the output impedance of a few of my pedals with my VM and got around 100k on each.

I did manage to throw two resistors before the output line to shield and got the value I was looking for but now it sounds different (in a bad way). Need to learn more now hahaha

[PRR]

> the input of a tube amp probably wants to see something specific

No.

Driving a tube from 2K is quite fine.

[Pmelius]

> the input of a tube amp probably wants to see something specific

No.

Driving a tube from 2K is quite fine.

Interesting. Is there a range that's safe?

[GibsonGM]

ANY range is 'safe', bud!  Have at it!!!   "Bad" input or output impedances will result in a bad sound, but won't hurt anything.   

Typically you want a high input impedance to a device...maybe 50k, 100k, or even more as you stated.   

You want a low output impedance feeding the next device, so <10k is 'typical'....mismatched impedances in our devices will cause 'tone sucking', or loss of high freq's...sound like mud.   Impedances can be purposely set to enhance something (or to LOSE something, like brittle high tones...).

Your output impedance and the input impedance following it create a voltage divider, and this also plays with the next device's input capacitance to create a filter. 

Enjoy the fuzz, let us know what you end up with! 

A place to start:  http://www.geofex.com/article_folders/impednc.htm

[Pmelius]

ANY range is 'safe', bud!  Have at it!!!   "Bad" input or output impedances will result in a bad sound, but won't hurt anything.   

Typically you want a high input impedance to a device...maybe 50k, 100k, or even more as you stated.   

You want a low output impedance feeding the next device, so <10k is 'typical'....mismatched impedances in our devices will cause 'tone sucking', or loss of high freq's...sound like mud.   Impedances can be purposely set to enhance something (or to LOSE something, like brittle high tones...).

Your output impedance and the input impedance following it create a voltage divider, and this also plays with the next device's input capacitance to create a filter. 

Enjoy the fuzz, let us know what you end up with! 

A place to start:  http://www.geofex.com/article_folders/impednc.htm

This is awesome, thank you!

[duck_arse]

measure at the input and the output of your fuzz for DC. if present, you probably want to add a blocking cap and pulldown resistor.

[Pmelius]

measure at the input and the output of your fuzz for DC. if present, you probably want to add a blocking cap and pulldown resistor.

Relative to my knowledge, I understand what you're telling me to do but I have much research to do before figuring out how to do it haha

[GibsonGM]

You have a DMM? (digital multi meter)   Then clip the black probe to battery " - ", set meter to "DC volts", and use the red probe on the wire that goes into, and then out of, the circuit.   Those are your input and output connections.

You should see a SMALL dc potential there, probably unsteady, like 50 or 100mV or something.  This is due to leakage, and is ok.  Much more than that, tho, may mean there is really DC flowing on those wires, which is BAD (noisy), so you'd need to add a capacitor in series to block it.   Only AC should be flowing in or out, from your guitar etc.  The DC is used inside the  circuit to run the transistors and such.

So, what did you find?

[Pmelius]

You have a DMM? (digital multi meter)   Then clip the black probe to battery " - ", set meter to "DC volts", and use the red probe on the wire that goes into, and then out of, the circuit.   Those are your input and output connections.

You should see a SMALL dc potential there, probably unsteady, like 50 or 100mV or something.  This is due to leakage, and is ok.  Much more than that, tho, may mean there is really DC flowing on those wires, which is BAD (noisy), so you'd need to add a capacitor in series to block it.   Only AC should be flowing in or out, from your guitar etc.  The DC is used inside the  circuit to run the transistors and such.

So, what did you find?

Finally had time  to do this tonight. Thanks for the thorough walk through! I'm I'm doing it correctly (with the batter connected? I'm actually getting 8v dc on the input and output jacks.

[ashcat_lt]

Finally had time  to do this tonight. Thanks for the thorough walk through! I'm I'm doing it correctly (with the batter connected? I'm actually getting 8v dc on the input and output jacks.

Just measure DC voltage between the tip and sleeve of each audio jack.  If it really is 8V, that might explain why it sounds so crazy.   

You'll want to block that from going to your guitar and your amp.  It probably won't actually hurt anything, but it might cause noises or popping or (probably non-fatal) malfunctions.  Put something like a 1uF cap in series between the tip of each jack and the rest of the circuit.  This will probably be electrolytic and polarized.  The negative side goes toward the jack in both cases.  You should also then put a 1M resistor between jack and sleeve of each jack.

I'm kind of afraid that the large DC voltage was messing with that amp which caused a large part of what you liked about the sound, but I'm hoping it'll work out for you.

Those caps, BTW, are the reason your other pedals measure ~100K output impedance.  The DC voltage that the meter uses to figure the resistance can't get through the cap, so the only path it can find to complete the circuit is through the volume pot, which I'm guess was all the way up.  If you measured on the other side of the cap, it would probably be a lot smaller.

[Pmelius]

Finally had time  to do this tonight. Thanks for the thorough walk through! I'm I'm doing it correctly (with the batter connected? I'm actually getting 8v dc on the input and output jacks.

Just measure DC voltage between the tip and sleeve of each audio jack.  If it really is 8V, that might explain why it sounds so crazy.   

You'll want to block that from going to your guitar and your amp.  It probably won't actually hurt anything, but it might cause noises or popping or (probably non-fatal) malfunctions.  Put something like a 1uF cap in series between the tip of each jack and the rest of the circuit.  This will probably be electrolytic and polarized.  The negative side goes toward the jack in both cases.  You should also then put a 1M resistor between jack and sleeve of each jack.

I'm kind of afraid that the large DC voltage was messing with that amp which caused a large part of what you liked about the sound, but I'm hoping it'll work out for you.

Those caps, BTW, are the reason your other pedals measure ~100K output impedance.  The DC voltage that the meter uses to figure the resistance can't get through the cap, so the only path it can find to complete the circuit is through the volume pot, which I'm guess was all the way up.  If you measured on the other side of the cap, it would probably be a lot smaller.

Oh okay I see what I'm measuring now. Before I was connecting the negative probe to the actual battery jack and the positive to my input and output.

So I'm getting 112mv consistently  across the input jack and nothing across the output jack.  what does this tell me? GibsonGM had mentioned that 50-100mv shouldn't cause the issue. Where can I go from here?

[ashcat_lt]

That indicates to me that the "DC blocking" (more properly "AC coupling") caps are already there, though the one on the input is a little leaky.  So from here, you should be good to just plug in and go.  That little bit of voltage won't hurt anything in the guitar, but it might cause a little bit of a pop if you're switching it (or even possibly a pedal before it) on and off.  It's only a problem if it's a problem.

There was never any chance you'd damage an amp with a 9V circuit, but now you know it won't cause any strange malfunctions and should play nice with whatever you hook it up to.

[Pmelius]

That indicates to me that the "DC blocking" (more properly "AC coupling") caps are already there, though the one on the input is a little leaky.  So from here, you should be good to just plug in and go.  That little bit of voltage won't hurt anything in the guitar, but it might cause a little bit of a pop if you're switching it (or even possibly a pedal before it) on and off.  It's only a problem if it's a problem.

There was never any chance you'd damage an amp with a 9V circuit, but now you know it won't cause any strange malfunctions and should play nice with whatever you hook it up to.

Well that's great to know! Is there anything I can do to troubleshoot the cause of the noise beyond what I've already done?

[ashcat_lt]

You haven't mentioned noise before, so...

You said you added a 9V adaptor?  If that means you're now running it off of a wallwart, then it's likely the original circuit didn't include any kind of power supply filtering.  Running off battery power, it wouldn't need it.  So like a smallish resistor in series with the PS + wire (I'd also add a series Shottky diode for reverse polarity protection) then a big cap between that and ground, and often a smaller cap in parallel to help with the high frequency weirdness of electro caps. 

Then like shield the thing?  Is it still in the plastic walkie body?  Just loose on the floor?  It really wants to be inside a (grounded) metal enclosure, but wrapping it in aluminum foil might help.