What do these diodes do?

[markbr]

Hello,

This is the Danelectro Honeytone guitar amp.(attached). The preamp is a TL072. I checked the diodes - initially thought they were soft clippers but they dont seem to be doing anything, there wasn't a difference when the were removed. Do you know what they are for then? Also,

1) what do: R2 and R9 do?

2) how is the tone control part working (ie R6, R7, C7, C8, C9, C11)? How would changing their values affect tone?

Im pretty new to all this but im learning so i'll really appreciate it if someone could help explain it! Thank you.

[jrfrond]

Those are definitely clipping diodes, but will not clip until the output of the first stage reaches the diode conduction point of .6V-.7V.

R2 and R9 provide bias to the inputs of the opamp, which is necessary when running a bipolar opamp from a single-ended supply.

The tone control pot essentially pans between a high-pass filter (C8) and band-pass filter (C7, 9,11 and R7). R7 and C7 form a high-pass shelving filter (limiting lows) before C9 and C11 which form a band-pass filter.

[markbr]

Those are definitely clipping diodes, but will not clip until the output of the first stage reaches the diode conduction point of .6V-.7V.

That's what i thought too. But upon removing them, there was no difference to the sound. The overdrive pot A100K acts as a second volume pot, turning them up amplifies the signal into the voltage supply rails...so it's this pot which is clipping the signal not the diodes. This is what i've found out thus far, but please correct me if im wrong.

[jrfrond]

If you are seeing clipping at the output of the first opamp, then the diodes are CLAMPING diodes, to limit the signal level. However, assuming that you see more than .6V-.7V of signal after the 1K resistor, you should see the diodes clip. Being that you now have a much higher Vcc on the op amps, the op amps will hit the rails later and more clean signal will be applied to the diodes, causing THEM to clip.

You can also move the back-to-back diodes into the feedback loop of the first op amp, right across the cap and pot.

Incidentally, you might already know this, but you might want to retain a slight asymmetry in the clipping. Most tubes amps do not clip symmetrically, and will probably yield the most "tube-like" tone. This is a topic that is always open to debate, and it's really a matter of personal preference.

[markbr]

If you are seeing clipping at the output of the first opamp, then the diodes are CLAMPING diodes, to limit the signal level.

I think clamping is their most likely function. How would two diodes in parallel clamp the signal though?

[jrfrond]

I think clamping is their most likely function. How would two diodes in parallel clamp the signal though?

They are back-to-back. One diode clips the negative side of the waveform, the other, the positive.

[teemuk]

1) what do: R2 and R9 do?

They reference the inputs of the opamp stages to a proper potential, which is about half of the power supply’s rail-to-rail voltage.

The schematic seems to have incorrect component values, BTW. Also, it's incomplete and missing vital parts.

2) how is the tone control part working (ie R6, R7, C7, C8, C9, C11)? How would changing their values affect tone?

It’s a fader between a low-pass filter and an (approximately) all-pass filter. It’s a basic RC circuit like all filters - not even a complex one. The effect of component value changes is electronics theory 101 and I don’t really want to start explaining that. Google RC filter.

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As said the diodes clip the signal. The input stage has so much gain on tap it should be able to drive them to clip with any guitar that has pickups with average output. If they don’t clip then they have failed. The diodes aren’t necessarily the sole contributor to distorted tones so even if you hear distortion after removing them doesn’t mean they weren’t working. As far as I know that output chip is driven pretty hard so it likely contributes a lot to the distortion as well.