Backwards wired germanium diodes in a tubescreamer?

M

mnemonic

Well-known member
Hi guys, quick question for those of you who know about germanium diodes, leakage, etc, I have an old modded tubescreamer clone I bought maybe 15 years ago when the boutique craze first got going, it’s been sat in the closet for years and I pulled it out recently. I was poking around inside to see what was going on and noticed that some of the diodes may be backwards, just wondering how that would work (or not work!).

It has a 1n4001 + GE diode in one direction, and a 1n4001 + GE + LED in the other direction. The germaniums look to be Russian ones (green stripe) and I believe (all?) Russian germaniums have the stripe on the anode. As you can hopefully see in the pic, that would mean the GE diodes are in backwards.

B4739ED6-1D73-45AD-982F-00CD0A8ADAC1.jpeg

Normally I think this would just nullify the clipping diodes, as if they weren’t there, however I understand that the old Russian germaniums tend to be quite leaky. I have a few D9E diodes on hand and I measured a few, mostly in the 70k to 300k range when measuring reversed resistance.

Does this mean that the clipping section here is essentially “SI diode -> large resistor”?

The pedal is loud, louder than my other tubescreamers, but doesn’t sound like no clipping diodes. Also, the LED does light up when I play, so it’s certainly doing something.

It’s built on a BYOC board dated 2006. This is the bodenhamer ‘bloody murder’ overdrive, if anyone wants to know.

Are there any germanium diodes that look like this with the green stripe that would have the stripe on the cathode?

Thanks guys
 
If I'm not mistaken the diodes are always in parallel in a clipping section and always in opposite directions. So it doesn't matter which way you put them as long as they are not both in the same direction. Below is an extract of the TS808 - Little Green Screaming Machine.

1621874523950.png

If you reverse D1 and D2 it is essentially the same circuit...
 
EGRENIER said:
If I'm not mistaken the diodes are always in parallel in a clipping section and always in opposite directions. So it doesn't matter which way you put them as long as they are not both in the same direction. Below is an extract of the TS808 - Little Green Screaming Machine.

View attachment 12104

If you reverse D1 and D2 it is essentially the same circuit...
Click to expand...

that I understand, but the SI and GE diodes are connected in series on each side (one side with an additional LED for asymmetry) and if these green-striped GE diodes are Russian then the stripe should signify the anode.

the picture may be hard to tell, but the diodes on the top left of the pcb are arranged as follows:

569F7269-6EA4-4716-861D-3DD0021C29E9.jpeg

unless there is some other GE diode out there that isn’t Russian and the stripe is on the cathode (but I haven’t managed to find one online).

However, it doesn’t sound like opamp-only clipping with no diodes (plus the LED does light when I play) so they must still work. Just wondering how GE diode leakage would play into this working, and how different it would be with the GE diodes reversed.
 
The diodes look like the generic ones tayda used to sell. Since the diode is clear glass you can ignore the stripe and just look inside for the anodes and cathodes.
 
I have not seen that arrangement before, and it seems counter-intuitive. Maybe one diode only lets signal above its breakdown voltage pass, and the signal that passes is then clipped by the other diode.
 
Sorry I missed the fact that there was 4 diodes in the circuit each paired in series... I can't understand how this works of the diodes in series face each other...
 
Betty Wont said:
The diodes look like the generic ones tayda used to sell. Since the diode is clear glass you can ignore the stripe and just look inside for the anodes and cathodes.
Click to expand...
Good point, I hadn’t considered that you can tell cathode and anode from looking inside. Maybe hard to tell from the pic but it’s clear in real life:

675C18C0-B618-43FC-9317-76D02C1A75AA.jpeg

I compared it to a 1n34a and Russian D9E, and these diodes in the pedal do have the stripe on the cathode, so they are in fact wired correctly!

I guess they’re not Russian diodes then, something western with a green stripe.

thanks for everyone’s help!
 
Looks like some clipping circuits I've seen involving zener diodes.

Have you measured those diodes out of circuit?
 
BurntFingers said:
Looks like some clipping circuits I've seen involving zener diodes.

Have you measured those diodes out of circuit?
Click to expand...

I have seen amp schematics with zeners wired back-to-back to ground but I’ve never looked into it further and also I’ve never seen it in pedals so I’m not sure how it works or sounds.

these ones it turns out are wired correct though, as I posted above.

glad I didn’t need to do any desoldering, I’m not very good at it.
 
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