Help understanding DIY Sparkle Drive that hates Germanium diodes

[markypizz]

Hey all, I'm building a sparkle drive clone for my friend. I am using the board from effectslayouts of their clone called the Glitter Bomb: http://effectslayouts.com/wp-content/uploads/2022/10/GLITTER-BOMB-build-doc.pdf

The schematic includes a switch to change between two clipping modes, or no clipping. I put in sockets for the 4 diode pads. I built it without issue and it sounds great when using 1N4148s or LEDs. However, I can't get germanium diodes to cooperate. This is the first time I've ran into this weird issue. I have D9Bs and D18s. All of them measure between .26 and .35 Vf, and sound just fine in my Klone.

With either of the germaniums, the sound is quieter for sure, as expected, but far too quiet, and does not clip at all. The gain knob seems to have no effect.

I also tried two back-to-back germaniums in each direction, measuring .6 Vf, just shy of my .65 Vf of my 1n4148s. To my surprise, this made no difference at all. The behavior is the same as before.

This video outlines what I'm hearing: https://youtu.be/lPyk9UWtiTA

Here's two images of the board (as best as I can get with the pots in the way):
https://www.diyschematics.com/schematics/i.php?/upload/2023/03/25/20230325222934-51267db0-xl.jpg
https://www.diyschematics.com/schematics/i.php?/upload/2023/03/25/20230325222945-f57968bb-xx.jpg

For context, I skipped their relay bypass circuitry, and soldered right to the input and output pads for use with a 3PDT. Frankly, I feel that the build is fine. I think what might be going on here is some lack of understanding I must have of diodes, and some characteristic of these germaniums that is causing this. I would love to hear your guys's thoughts and expertise on why I'm observing this, or if it's more likely that it's a mistake on my part in the build, I'll start taking the required measurements and add them to this post.

Thank you so much for your time and expertise!

Mark

[ElectricDruid]

With either of the germaniums, the sound is quieter for sure, as expected, but far too quiet, and does not clip at all. The gain knob seems to have no effect.

This part makes no sense! Not your fault, I'm not saying that - it's just *weird* is all!

You'd expect the sound to be quieter *because* of the lower clipping threshold. "Does not clip at all" shouldn't make it quieter...unless one of the diodes was actually shorted? Then it might reduce the gain, stop the gain knob working, and prevent clipping.

You said they work ok in your Klone, which I can't explain, but "diode shorted" is my best guess right now.

[markypizz]

Thanks ElectricDruid! That was my thought too. It sure seems to act like it's shorted. From my reading online, I wonder if leakage current is playing a role here. There was a great post on pedalpcb about the subject: https://forum.pedalpcb.com/threads/whats-all-this-stuff-about-germanium-diode-leakage.7185/

Given that we're working with a 500k gain pot, I wonder if the expected high leakage and approximate "resistance" across the diode as a result of the leakage is low enough to provide a new path through the feedback loop, resulting in the low gain and apparent disregard for the gain pot. This makes a lot of sense, but I don't want to get too certain without looking at other options.

The pedalpcb post also mentions how this is less of an issue in hard clippers, which makes sense. I'm wondering if this might explain why I haven't seen many germanium soft clippers.

Bold ideas to test this theory: maybe a silicon in series (schottky?) with each germanium could counteract the leakage current. I'm not sure if I'm still getting much germanium clipping at that point though (though I know a lot of the tonal difference is placebo). Alternatively, a high value resistor in series with the germanium diodes could be an interesting experiment. Might result in a weird gain structure though. I hope I'm on the right track. I appreciate any other thoughts!

This is a lot of fun to learn!!

[Rob Strand]

Make a test jig with a 9V battery, a series resistor then the diode under test to ground.
Measure the voltage drop across the diode with the DMM with different resistor values,
10M, 1M, 100k, 10k.  It should give you an idea of the forward characteristic of the diode.

You might also do the same test with the diode in reverse.   If there is significant reverse current
then the reverse diode could be screwing up the clipper.   You don't know until you measure something.

[PRR]

....with a 500k gain pot, I wonder if the expected high leakage and approximate "resistance" across the diode as a result of the leakage is low enough to....

You may be right. Try (approximately) these values, which will swallow 100 times the leakage of the for-Silicon design.


Odd thought--- put it in the freezer for a few hours, then try it quick. Freezing will reduce leakage by several orders of magnitude. Not a practical fix, just a quick theory test.

[Rob Strand]

This guy gets 0.5uA leakage at 9V for the DB9,

https://hackaday.io/project/10698-clockwork-germanium/log/35373-germanium-diodes

Since the leakage is relatively low he can measure the leakage across the 10k series resistor, not the diode.
A low voltage measurement across the diode would require a lot of leakage.

A few examples here as well,
https://forum.pedalpcb.com/threads/whats-all-this-stuff-about-germanium-diode-leakage.7185/