Any problem if I put a mini-voltmeters on Q1&Q2 collectors?

[Tone-Analyst]

I made a typical Fuzz Face with a pot for the bias and switches that alternate between different transistors. You know on Ebay they sell tiny voltmeters with display. They have a 100k input impedence. Just wondering if I wire two permanently, are they going to affect the sound? Thx

[willienillie]

are they going to affect the sound?

I have no idea, but definitely yes.

[MaxPower]

Note to surgeon: Lobotomy may have been too extensive.

[Tone-Analyst]

The first guy be pulling my leg? 

Ok but what is the issue, it may induce hum or what?

I switch between Qs of different gain and the voltages change.
I wanna learn how the different voltages affect the sound, you know when it sounds compressed and spitty, or why is 1.2v the textbook value for Q1.. how does it sound at 1.6v... just to have a visual.

[Phoenix]

A few potential issues I can think of off the top of my head.
Most inexpensive voltmeters share their negative input with their power rail, so in a positive ground fuzz face you could only measure from -9V to collector. I'm not sure if this is the measurement you want or not.
The input impedance of 100k is a problem, across a typical Q1 collector resistor of 33k this lowers to ~25k, and across a typical Q2 collector resistor of 8k2 this lowers to ~7k6. This will upset the bias without adjusting the collector resistor values.
Many inexpensive voltmeters are poorly decoupled as they're usually intended for monitoring battery voltage and do not require decoupling in that role, so clock noise can creep into the audio signal. This may be difficult to eliminate, especially if the power and voltage input are common.

But the only way to be sure if you can make this workable is to try it. If you find you have any problems then I'm sure you'll get many suggestions on how to tackle them. Good luck!

[willienillie]

The first guy be pulling my leg? 

Well not really.  I have no clue about the circuitry of the voltmeters you're considering, nor specifically how you would connect them in your Fuzz Face.  But with a circuit as simple and finicky as a Fuzz Face, anything you add will surely effect the sound.  In some cases you may be able to compensate, such as DC bias shift Phoenix describes.  Are you looking at digital meters, or analog with a swinging needle?  I would expect the latter wouldn't have clock noise, but I really don't know anything about it.

[PRR]

Little DVMs are naturally 200mV (199.9V).

You want to read to 9V. Which really means to 19.99V so the numbers come out right.

So a 100:1 divider. 4.7Meg:470k   4.7Meg:47k   is close enough and should have "no" effect in most small audio. (But musicians can hear changes better than calculations suggest.)

The polarity issue is a real problem.

[amptramp]

Little DVMs are naturally 200mV (199.9V).

You want to read to 9V. Which really means to 19.99V so the numbers come out right.

So a 100:1 divider. 4.7Meg:470k is close enough and should have "no" effect in most small audio. (But musicians can hear changes better than calculations suggest.)

The polarity issue is a real problem.

Are you sure about that divider ratio?  It looks more like 11:1 with those resistances.

[Tone-Analyst]

I ordered the 3-wire voltmeters. They have a separate supply and reading wires. I will be able to pull the supply from the 9v power and read each collectors individually. The effect of the 100k input imp will be balanced by the bias pots, 10k on Q2 and 100k on Q1. The fuzz in NPN so we're all positive.

I don't understand the "divider" suggestion however..

[willienillie]

I think Paul and Ron both meant to say 10:1....?

[PRR]

> on Ebay they sell tiny voltmeters

They sell many types of little meters. If you get the kind that "read battery voltage" without extra parts, sold for cars/motorcycle charge displays, you may be fine as is. (However they also sell 2-wire meters for that purpose, knowing that if a car battery is down to 5V it is dead to most people, and these will load-down FF guts.)

> Are you sure about that divider ratio?

Lost count of my thumbs. Corrected above. This would make 200mV meters read 20V (or 4V) directly. Thanks!

[antonis]

I don't understand the "divider" suggestion however..

It's called the full-scale rating expansion imposture..

https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-design/
https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-impact-measured-circuit/
https://sound-au.com/articles/meters.htm

[amptramp]

Suppose you have a divider with 4.7 megohms as the upper resistor and 470K as the lower resistor and you put 11 volts on it.  You get an exact 1 volt at the output.  The total resistance is 5.17 megohms.  The divider ratio is 470/5170 or 1:11.

[amz-fx]

3.24 megohms as the upper resistor and 360K as the lower resistor.

Both of these values are available from Mouser as 1/4w through-hole metal film 1% types.

10v in and 1v out

regards, Jack

[EBK]

I'm asking this in an open-minded, non-judgmental way:  Out of curiosity, why do you want to add two permanent voltmeters to your circuit?

[Marcos - Munky]

If you got the same kind of mini voltmeter I have, it reads about 3V-300V range. I may be wrong, but I think the suggested voltage dividers won't work, since you already have a voltage under the lower limit it can measure. You want to have something 1V at the input and 10V (or 100V) at the output, if it's the same meter I got.

[Tone-Analyst]

I installed one on Q2, it induces a bad hum but it was educational. You can see how the voltage drops when you play. I'll leave the voltmeter there but I'll add a switch to power it down. I think I may have solved the carbon battery myth however 


https://youtu.be/aLJCs0EBWkg

[bluebunny]

I think I may have solved the carbon battery myth however 

Which myth is that?

BTW, you can see in your video that the LED display flickers.  I'd say this is the source of your hum.

[Tone-Analyst]

Ok I don't want to look like I'm trying to bump this thread up and hog attention haha.. but just to conclude, the reason I want the voltages diplayed is because when I switch between transistors of different gain, the collector voltages swing way out of specs. For example when I switch my Q1 to germanium, its collector voltage drops from 1.4v to 0.7v, then if I change the Q2 to the lower gain germanium as well, Q1 voltage drops to 0.3 volts. I could probably include resistors in my switching to keep all the voltages right. I thought it might be cool as well to see where I'm at when tweaking volt starved sputtering sounds. I prefer to have one fuzz that's more versatile than multiple fuzz boxes too.

[Electric Warrior]

Ok I don't want to look like I'm trying to bump this thread up and hog attention haha.. but just to conclude, the reason I want the voltages diplayed is because when I switch between transistors of different gain, the collector voltages swing way out of specs. For example when I switch my Q1 to germanium, its collector voltage drops from 1.4v to 0.7v, then if I change the Q2 to the lower gain germanium as well, Q1 voltage drops to 0.3 volts. I could probably include resistors in my switching to keep all the voltages right. I thought it might be cool as well to see where I'm at when tweaking volt starved sputtering sounds. I prefer to have one fuzz that's more versatile than multiple fuzz boxes too.

That behaviour is expected and generally known. Why are you assuming the voltages are going "out of spec"?
With consistent Vbe, Q2's collector voltage mostly depends on Q1's gain and leakage while Q2's gain and leakage will hardly make any difference. For germaniums, I'd consider leakage even more important than gain.

1.4V on Q1C is on the high side for silicon fuzz face, but still within the range used in vintage pedals. You are probably using a low gain silicon transistor here. That will bring Q2C down to around 3V. Higher gain silicon transistors were more commonly used. Q1C is usually between 1.2 and 1.3 V.

0.7 V is very high for a germanium Fuzz Face, but not unheard of. Q2C would probably be close to 4V; 0.3 V is low, but not uncommon at all in vintage units. Q2C will obviously be much higher. They can still sound great in that range, but they may fart out more quickly with rising temperature.

When they're just about to fart out, I found that a fresh battery can improve things a lot. They voltages may look "worse", but it may gate nicely and controllably with a 9.9V battery. Interestingly, the output volume was higher at 9.0 V, though..