Inductors and Gyrators (Acoustic 360 Project)

[Rob Strand]

300Hz 100mV p-p input (My input is ±100mV. Is it OK, or I need to inject ±50mV?), Q5 base shall be at 400mV and Q4 base at 1.2V. Q6 base shall be at 800mV.

If I read this CRO well, I have 1.2V at Q4, 400mV at Q5 (but bottom is flipped) and 600mV at Q6. So Q5 is definitely a suspect, just have no idea what's going on...

The way I read it is you would need +/- 50mV to match the 100mV p-p (peak to peak) in the service manual.

It's best to try to match the service manual.   Nonetheless it looks close (are Q4 and Q5 voltages flipped?).  The tone control settings  (or in inaccuracies in the pot tapers) can also cause slight difference unless the service manual says to set them to full or minimum.

I have also measured DC with shorted input, and this is what I get, but have in mind that my B+ is 23.7V, not 25:

The DC voltages are fairly close.  Nothing stands out as bad (your Q1 base voltage is low due to meter loading).  I checked a few voltages with hand calculations and the service manual voltages look like the transistors have a gain of about 400.

So maybe the problem is the gyrator circuit.  That connects to Q5 so it could be affecting the waveforms.   If the gyrator opamps are clipping it could do something like that.     If you have a real inductor you could try that just see if it gets rid of the flipped waveform peak.  You could also look at the DC voltages and waveforms on the gyrator opamps.


Are all the electrolytic caps loaded with the correct polarity?

[rankot]

I don't use electrolytics, I always use 1uF poly for audio path.

But I have almost the same distortion on another unit which has normal inductor. I will see it with CRO.

[Rob Strand]

But I have almost the same distortion on another unit which has normal inductor. I will see it with CRO.

A couple of things to try:
- change the Vari-mid frequency switch to see if it change the waveform shape.   The idea here is perhaps the mid control resonance is making the waveform look weird when the transistor stage clips.
-  It's probably worthwhile lifting the 2x6.8uF caps to isolate the transistor and mid control and see if the inverted tip is present with only the Q5 transistor stage.

[rankot]

Even when mid control completely disabled, there's the same distortion as before (and maybe even more). So it seems that one's not causing it.

Here is ltspice model of Acoustic 360 preamp, no idea what's going on. 

Code Select Expand


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[Rob Strand]

Even when mid control completely disabled, there's the same distortion as before (and maybe even more). So it seems that one's not causing it.

That's good.   So it can only come from that stage.

So maybe the next step is to drive the input of the tone control directly with a signal generator.   That will isolate that transistor stage entirely.
 

Here is ltspice model of Acoustic 360 preamp, no idea what's going on.

Unfortunately I'm not running my engineering computer at the moment so I can do any spice sims.   I would be nice to see the spice sim.  If it shows up in the idealized environment of spice then it's a real effect.

Normally weird things like negative tip you are seeing are related to caps charging up.   What I'm saying here is the flipped peak is actually a cap charging up in the positive direction. There's only the input cap and the tone control caps. 
To charge the input cap it must come from:
- the transistor, which doesn't make sense under normal circumstances since the BE junction doesn't supply current.  If the BE junction broke down it could supply current but the voltages are too low.
- from the CB feedback resistor.  The CB resistor look too large to charge up a cap in that time and produce the  peak.

So the only think that makes sense is a tone control cap control is charging up.  That's where a spice sim would help.   That effect would show-up reliably in spice.

Another thought is  the flat-top on your signal generator going through the EQ of the tone control is promoting an odd-shaped waveform.   You could try smoothing the output of your generator with an RC filter with a 300Hz cut-off.   Alternatively just use your PC as a signal generator.  That will give a clean sine output.
(BTW.  you can often fixed distortion on sine-wave oscillators by tweaking the gain of the amplifier used in the oscillator.  Lowering the gain lowers the hardness of the clipping and that cleans up the waveform.)

[rankot]

But Rob, all of my caps are non-polarized, metal film, except those with 6u8. I disconnected this varitone by simply removing the inductor, but they are oriented just as they shall be.

And this is the distortion of generated signal. It's too complicated for me to make a good CRO shot while playing a guitar, so I used generator. However, it's distorted regardless signal source.

And in spice it simply works normal - at least I think it does.

That's good.   So it can only come from that stage.

Which one? The one before the tone stack or the one after it?

[Rob Strand]

But Rob, all of my caps are non-polarized, metal film, except those with 6u8. I disconnected this varitone by simply removing the inductor, but they are oriented just as they shall be.

Ideally you want to pull both the caps.  If you just pull the inductor part there is still a feedback circuit from the collector to the emitter via the two caps and the pot.   That could be doing *something* to the emitter voltage.
Pulling the inductor part does confirm the it's not cause by the gyrator or presence of the inductor.

When you get weird problems it's a good policy to strip everything back to the bare minimum so there's nothing else to blame - even if you think a lot of stuff can't possibly cause the issue.  If you strip a circuit back to two resistors and it's not doing what you expect then you have a very good chance of seeing what the problem is.

And in spice it simply works normal - at least I think it does.

That's very odd then.  Try different input levels.  You need to match the tone control pot settings is spice.
You could just replace transistor Q5. 

Unfortunately I don't have spice running otherwise I would have tried to break the circuit ages ago.

Which one? The one before the tone stack or the one after it?

Yes that one.  The stage with Q5.