I’m trying to understand the cut off frequency of this tone control… but need help!

[giovanni]

This is the way I've always seen it:

View attachment 88156

I find that confusing because the emitter should be at a higher voltage than the collector in a PNP, so to me E is at the top, C at the bottom.

 

[Flying]

I find that confusing because the emitter should be at a higher voltage than the collector in a PNP, so to me E is at the top, C at the bottom.

This is the first thing I've done with a PNP, so I'm still getting to grips with the differences!

 

[JTEX]

Easy. Put the whole thing in Tina-ti and see if it works. I have $20 that says it doesn't. But what if I'm cheating and I've already done that?

(I haven't)

 

[Flying]

Easy. Put the whole thing in Tina-ti and see if it works. I have $20 that says it doesn't. But what if I'm cheating and I've already done that?

(I haven't)

Hi @JTEX

Are you saying the schematic as drawn doesn't work in Tina-Ti? Only I've just spent the last 20mins playing through it on my bread board. The only thing I omitted, as an oversight, was a 100uF cap between 9V and GND.

I've not yet had a chance to follow up on your link that says how I can get Tina-Ti to work on my Mac, but will do later.

 

[JTEX]

Ok, now I'll really put it into Tina if I find some time. As drawn, it really looks to me like it shouldn't work. Will let you know.

 

[giovanni]

Ok, now I'll really put it into Tina if I find some time. As drawn, it really looks to me like it shouldn't work. Will let you know.

It won’t unless it can simulate the leakage current of those two caps (a resistor in parallel should do that trick). That’s my bet.

 

[Flying]

Hi @JTEX,

Only put it in Tina-Ti if you have the time and inclination, I'm hoping that I'll be able to get it to rum on my Mac. I've attached the Schematic with the missing 100uF capacitor.

@giovanni, Is that the reason for using two 47uf capacitors, rather than one 100uF, because it doubles the leakage?

 

[JTEX]

Well, I'll be damned. It seems to work in Tina! You should have taken the bet, you'd be up $20 The only way I can imagine it working is if the PNP transistor is simply used as a passive clipping device. Maybe someone more knoweledgeable could explain how the darn thing actually works??

 

[Sturdag Lagernathy]

A capacitor couldn't leak enough current to turn on a transistor and still be considered a decent capacitor..

 

[JTEX]

^ there's a weird path to ground from the emitter of T1 which might explain why it works:

 

[giovanni]

A capacitor couldn't leak enough current to turn on a transistor and still be considered a decent capacitor..

Then I’m out of theories! This circuit is very weird. Maybe somebody more knowledgeable can chime in.

 

[giovanni]

Another theory could be that the transistors are both in a different mode than the usual low signal amplifier mode, like saturation or cutoff. I am not at all familiar with that stuff though.

 

[Flying]

I doubt this will shed any light on the matter but here is what it looks like on my scope. The top trace is from the Base of the Ge PNP transistor. the bottom trace is from after the 470nF capacitor just before the tone and volume control. The bottom trace looks very simular to @JTEX's Tina-Ti simulation.