Help with another SHO build

[Lost_soul]

I built another super hard on for one of my friends and idk why but this circuit specifically has some black magic or something as it acts weird when everything looks fine.

First while adjusting the pot the signal goes away until i leave the pot alone (ie: if i keep adjusting the pot back and forward and play the guitar at the same time it would work as a tremolo)

Second thing is the voltages. They don't look right (or do they?)
Specially the voltage of the source. It should be zero when it's 0.75v

Here are the voltages:

Fully anti-clockwise
6.81
2.52
1.64

Fully clockwise

5.07
2.16
0.75

Battery: 8.62

I used a 2N7000 instead of the BS170 And i know the pinout difference. And used a linear pot instead of the reverse log.

So what could be it?





Also this would be a great opportunity to start learning something. How can i calculate the voltages that are supposed to be at the red dots so when i measure a voltage for a circuit i'd know the part where something may be wrong.

Thank you.

[PRR]

the voltage of the source.

Ah, the drawing is a BJT. These do not have a "source". So what else is wrong?

[Lost_soul]

the voltage of the source.

Ah, the drawing is a BJT. These do not have a "source". So what else is wrong?

The drawing symbol for the transistor is wrong yes but the circuit uses a MOSFET indeed. BS170

[PRR]

> The drawing symbol for the transistor

Lets fix that. And because I can't reach your place to point, put labels on points.


By inspection, D has to be twice G (because 10Meg+10Meg).

If P1 is at zero, D has to be twice the MOSFET's threshold voltage. I have assumed "2V" which is in the ballpark for BS170. Then D must be 4V.

When P1 has resistance, it gets trickier. However S and thus G will stand at higher voltage due to drop in P1, so you expect D to be higher than 4V.

[PRR]

I asked my idiot calculator. It does not have any small MOSFETs, only a IRF150, which is an old 24 Amp part. It may not wake-up for half-mA jobs, so I raised the voltage and lowered the resistances.


At these currents the threshold voltage is more like 3V. With Source practically grounded, Source will wander to twice that, 6.5V. OTOH with a large Source resistor the current is less and the Drain sits higher.

[antonis]

How can i calculate the voltages that are supposed to be at the red dots so when i measure a voltage for a circuit i'd know the part where something may be wrong.

post #36

P.S.
If you got yourself familiar with BJT CE amp bias, you'll not find difficult to MosFet voltages calculation..
Just consider zero Gate current and replace V_BE with V_GS..

As for "tremolo" effect caused by pot swing, it's absolutely normal..
(by continuously altering gain you alter output amplitude..)