Help a noobie out!! fuzz bias led indicator.

[sabah310]

Hey guys,
Im in university studying nothing related to anything electricity (medicine) and ive taken up building some pedals for fun at home in my spare time.
I got a soldering iron last month and practiced and just built my first build ever a Vintage style clyde mccoy wah!

Now Im going to build a germanium fuzz face.
I have a couple of questions:
1. Id like to put an external bias knob with an led that glows when q2 is at the right voltage! what kind of pot would i need? a 10k linear?
2. I tried researching and reading on how to do the led bias indicator but im really lost. I read through the part about window comparator circuits and using opamps to achieve that; however if im using a vintage style pcb like this: https://pigeonfx.com/collections/pcb/products/fuzz-face-iss1-pcb how would i wire that up exactly?

and please be easy on me with the terms and such still learning.
thanks guys!

[garcho]

Welcome to the forum!

have you read these?:

https://www.electrosmash.com/germanium-fuzz
http://www.geofex.com/article_folders/fuzzface/fffram.htm

Id like to put an external bias knob with an led that glows when q2 is at the right voltage

So you can mess with the sounds you get from a "mis-biased" Q2 and then get it back where you want it? If there's a "right" voltage, why make it on an external pot?

I read through the part about window comparator circuits and using opamps to achieve that; however if im using a vintage style pcb like this: https://pigeonfx.com/collections/pcb/products/fuzz-face-iss1-pcb how would i wire that up exactly?

Well, you wouldn't. There's no way to "wire" that up. If that's your fuzzface PCB, you'd need a daughter board for the comparator.

what kind of pot would i need? a 10k linear?

What schematic are you using? 10k is most likely a reasonable value to start with.

[deadastronaut]

why not just use a cheapo small dc led meter in your build, then you can get it bang on every time, and still vary too...

a visual aid...better than an led in my opinion....and instant.....just my 2p.

[mozz]

I've got a few hundred of these, i think it will work, if anybody wants to try I'll be glad to mail you a few.
https://www.mouser.com/ProductDetail/ON-Semiconductor/MC34164D-5R2?qs=dFKnUM%2FquJZ8YxrC07sA6Q%3D%3D
Datasheet is on the page. Supposed to work at 4.3 volts so close enough you could get the led to light then bump it up a bit.

[iainpunk]

put a led in series with the collector of Q2. if there is light that means there is current and the transistor in the conduction region. but playing with and changing the bias of the fuzz face is really fun, its capable of letting it sound really different, from thin and velcro gated to fat and muff-like

i don't think a led can account for taste

[sabah310]

Welcome to the forum!

have you read these?:

https://www.electrosmash.com/germanium-fuzz
http://www.geofex.com/article_folders/fuzzface/fffram.htm

Id like to put an external bias knob with an led that glows when q2 is at the right voltage

So you can mess with the sounds you get from a "mis-biased" Q2 and then get it back where you want it? If there's a "right" voltage, why make it on an external pot?

I read through the part about window comparator circuits and using opamps to achieve that; however if im using a vintage style pcb like this: https://pigeonfx.com/collections/pcb/products/fuzz-face-iss1-pcb how would i wire that up exactly?

Well, you wouldn't. There's no way to "wire" that up. If that's your fuzzface PCB, you'd need a daughter board for the comparator.

what kind of pot would i need? a 10k linear?

What schematic are you using? 10k is most likely a reasonable value to start with.

Holy, Thanks alot for the answer! i never got a notification for this post and just saw this! i appreciate your help!

1. Yea so i can mess with the bias externally and when i want it back to "normal" the led lights up when i turn it. why external? cause i saw it on the kingtone germanium mini fuzz face and thought it was hella smart 
2.I assumed as much, but i do remember reading something on the forum about wiring it up to the bottom of the pcb board without the addition of another. if i do need another i guess id have to keep reading!
3. No schematic honestly just from what ive read on the forum and such

[sabah310]

why not just use a cheapo small dc led meter in your build, then you can get it bang on every time, and still vary too...

a visual aid...better than an led in my opinion....and instant.....just my 2p.

Hey! thanks for the help! never got notifications so just only saw this!
Well goes to show how new i am haha. Never thought of it, do you mind expanding on this idea or lead me to any links?

[sabah310]

I've got a few hundred of these, i think it will work, if anybody wants to try I'll be glad to mail you a few.
https://www.mouser.com/ProductDetail/ON-Semiconductor/MC34164D-5R2?qs=dFKnUM%2FquJZ8YxrC07sA6Q%3D%3D
Datasheet is on the page. Supposed to work at 4.3 volts so close enough you could get the led to light then bump it up a bit.

Hey thanks for the answer! I never got notifications so just seeing this!
Are you saying that you have a few hundered of these bias indicators built up? Could i PM you?

[sabah310]

put a led in series with the collector of Q2. if there is light that means there is current and the transistor in the conduction region. but playing with and changing the bias of the fuzz face is really fun, its capable of letting it sound really different, from thin and velcro gated to fat and muff-like

i don't think a led can account for taste

Hey thanks for the answer! never got notifications so only seeing this now!
Sound interesting! will look into it
I understand that tastes are different haha but i was thinking more that the led would be lit to have a benchmark of the "normal" fuzz/bias then you could always mess with the bias and still return to normal"

[deadastronaut]

why not just use a cheapo small dc led meter in your build, then you can get it bang on every time, and still vary too...

a visual aid...better than an led in my opinion....and instant.....just my 2p.

Hey! thanks for the help! never got notifications so just only saw this!
Well goes to show how new i am haha. Never thought of it, do you mind expanding on this idea or lead me to any links?

A led small dc volt meter, it has 3 wires,  power, ground, and the probe/test/measurment wire,   use the measurment wire on your lug 2 of your bias pot....the display will tell you the voltage...everytime.   have fun.

[nocentelli]

I understand that tastes are different haha but i was thinking more that the led would be lit to have a benchmark of the "normal" fuzz/bias then you could always mess with the bias and still return to normal"

You could just do this by ear, like you would with any other control, or add suitable padding resistors so the supposed magic voltage  (or even better, the most useable sound) is at 12 o'clock on the pot. You'll probably find it is a bit too clean and loud at higher voltages, sounds good (subject to taste) at some point between 6 and 3+ volts, and fizzles downwards into spluttery gated nonsense towards the lowest voltages: This sort of external bias is often labelled as a "starve" control in some pedals based on a modified fuzzface circuit.

[amptramp]

Is bias voltage more important than collector current?  You could use a simple meter in the emitter or collector that would be more indicative of fat or thin sound.  There are some very simple, tiny and cheap meters that are designed as battery charge meters or signal strength meters for tuners etc. that cost about a buck surplus.  The bias needed to maintain a given current will change with temperature for a transistor circuit so current may be more important than voltage because transistors are current amplifiers and h_fe varies with temperature.  There is no extra circuitry needed.

Plus, an analog meter on a stompbox always looks good and if you are playing a gig with the light in your eyes and you look down at your pedalboard, you can spot the meter right away and know which pedal it is.

[antonis]

IMHO, both voltage or current monitoring can't say anything if you can't figure out the meaning of their variations..
(you just might be able to do one pot turn instead of two..)