Why isn't my clipping stage working?

[antonis]

I'd proceed with a DPDT switch for either "soft" or "hard" clipping..

[Steben]

Shouldn't there be a DC blocking cap in series with the feedback diodes from collector to base?

[antonis]

Shouldn't there be a DC blocking cap in series with the feedback diodes from collector to base?

There should be more than that cap (it's Buzz-Fuss-alike..) but if OP likes it, let it be..

[drainyoo]

Shouldn't there be a DC blocking cap in series with the feedback diodes from collector to base?

There should be more than that cap (it's Buzz-Fuss-alike..) but if OP likes it, let it be..

Never heard of the Buzz-Fuss. Will check that out.

Why should there be a cap and more in the feedback loop? I'm not trying to go for a standard overdrive/distortion. Trying to come up with a different sound that is a little messed up.

[antonis]

Why should there be a cap and more in the feedback loop?

To "isolate" DC feedback (bias) from AC one..
In your ciruit, diodes overcome (shunt) 100k feedback resistor..
(you can see that in op-amp configurations but doesn't affect bias there..)
This results into 2 X Diode forward voltage drop(*) + V_BE on Emitter
(100k resistor is practically redundant and stage gain is unpredictably variable(*)..)

(*) much lower that nominal due to very low forward current..

Also, for stage gain set by 100k/2k2 ratio, "open loop" gain (feedback resistor disconnected) should be the maximun availiable..
(Emitter resistor AC bypassed via a 22 - 47 μF capacitor..)

I do know I confuse you but the above need analysis beyond the scope of present thread..

P.S.
As I said before, if you like it, ignore the above recommendations and go for it..!!

[drainyoo]

Why should there be a cap and more in the feedback loop?

To "isolate" DC feedback (bias) from AC one..
In your ciruit, diodes overcome (shunt) 100k feedback resistor..
(you can see that in op-amp configurations but doesn't affect bias there..)
This results into 2 X Diode forward voltage drop(*) + V_BE on Emitter
(100k resistor is practically redundant and stage gain is unpredictably variable(*)..)

(*) much lower that nominal due to very low forward current..

Also, for stage gain set by 100k/2k2 ratio, "open loop" gain (feedback resistor disconnected) should be the maximun availiable..
(Emitter resistor AC bypassed via a 22 - 47 μF capacitor..)

I do know I confuse you but the above need analysis beyond the scope of present thread..

P.S.
As I said before, if you like it, ignore the above recommendations and go for it..!!

Yeah, took your advice and I removed that second gain stage, and just put a switch after the 100k resistor to remove it from the circuit. Once removed it turns into high distortion and it sounds great to me. Thanks for the suggestion! Still don't get why that second gain stage didn't work, though.

There is one other issue. When the Gain pot on the transistor emitter is turned all the way down, the signal level gets really low. I assume this is kinda normal since it is a gain pot, but is there a solution to reducing how much the level drops? I changed the pot to a B50k, and that helped a little.

[antonis]

When the Gain pot on the transistor emitter is turned all the way down, the signal level gets really low.

Of course, 'cause gain is set by R_Collector / R_Emitter

is there a solution to reducing how much the level drops?

For a 20k Collector resistor value, Emitter gain pot value shouldn't be higher than 5k say (better 1k or so..)
(for a normal guitar signal amplitude..)
And don't forget to make 47nF cap much bigger, unless you wish for high gain at high frequencies..
(kind of Treble booster..)

[drainyoo]

Thank you for all your help. I will try these out.

[Vivek]

Should there be a DC blocking capacitor near the diodes near the base and collector ?

[antonis]

Should there be a DC blocking capacitor near the diodes near the base and collector ?

Shouldn't there be a DC blocking cap in series with the feedback diodes from collector to base?

  

[antonis]

I will try these out.

Based on your own design, I'd try the following configuration..

[drainyoo]

Should there be a DC blocking capacitor near the diodes near the base and collector ?

Shouldn't there be a DC blocking cap in series with the feedback diodes from collector to base?

  

🤣🤣

But why? Explain like I'm a 5 year old. I guess what will help me understand is, what bad comes out of not having a cap there? Is it just the proper way of doing things?

I'll tell ya, it sounds great to me without a cap there.

[antonis]

But why?

I presume you didn't understand post #24 above..

Go to "Biasing with Collector Feedback resistor" and place a diode across RB

[drainyoo]

But why?

I presume you didn't understand post #24 above..

Go to "Biasing with Collector Feedback resistor" and place a diode across RB

I didn't. It was beyond my knowledge, but I appreciate you explaining. This link looks helpful, I'll read through it.

For reference, I'm biasing the transistor to 2.5v because I like the busted up sound it makes. It's what I'm going for. It reminds me of what a Super Reverb sounds when you overdrive it with a lot of bass. Love that sound.

[antonis]

I'm biasing the transistor to 2.5v

With what Collector & Emitter resistors values..??

[drainyoo]

I'm biasing the transistor to 2.5v

With what Collector & Emitter resistors values..??

A 5k gain pot, and 10k & 6.8k at the collector. The resistor with the diodes is still 100k.

[antonis]

A 5k gain pot, and 10k & 6.8k at the collector. The resistor with the diodes is still 100k.

If diodes are C-B directly coupled (no series capacitor), you didn't bias it..
It is self-biased, in the mean of V_CE is 2 X Diode forward voltage drop + 1 X V_BE..

In case of diode series DC blocking cap existence, 100k feedback resistor value should be raised..
(for more voltage drop across itself, hence lower Emitter voltage and lower Collector current, resulting in lower voltage drop across Collector resistor - Collector more close to (+9V + V_Emitter) / 2 ..)

P.S.
2.5V Collector bias calls for 1.75V negative headroom, which should be OK for Si diodes but should result into asymmetrical clipping for diodes of higher forward voltage drop..
(e.g for LEDs ..)

[BennyBoy]

I'd proceed with a DPDT switch for either "soft" or "hard" clipping..

+1 this.

Easy and interesting solution!