Swapping Ge Diodes for Schottky

[bhcarpenter]

Any general guidelines for when it’s ok to swap a germanium transistor for schottky? I’ve heard that the forward voltage drop is similar (and the germanium’s are a bit all over the place anyway).

Secondly, what is the purpose of these D4/D5 diodes in the signal path of the Promethium, and is it necessary for them to actually be germanium?

 

[jubal81]

Forward voltage drop is not what needs to be considered when comparing Ge and Schottky diodes.
The factor that matters is the Ge leakage. For any Ge setup like that, you have to imagine a resistor in parallel with the two anti-parallel diodes. Leaving out this resistance will alter the frequency filter, changing the 'tone.'

Long story short, if you replace Ge with Schottky, put a 470K resistor in parallel with them.

 

[manfesto]

D4 and D5 on the Promethium are a “coring” circuit, which is basically a primitive noise gate (since that circuit is *super* noisey already).

Honestly I think when I’ve used Schottky’s it worked *better*

 

[iamjackslackof]

I've never heard of Ge diodes leaking before. How does that work exactly, they leak some current still when reversed biased maybe?

 

[bhcarpenter]

Forward voltage drop is not what needs to be considered when comparing Ge and Schottky diodes.
The factor that matters is the Ge leakage. For any Ge setup like that, you have to imagine a resistor in parallel with the two anti-parallel diodes. Leaving out this resistance will alter the frequency filter, changing the 'tone.'

Long story short, if you replace Ge with Schottky, put a 470K resistor in parallel with them.

D4 and D5 on the Promethium are a “coring” circuit, which is basically a primitive noise gate (since that circuit is *super* noisey already).

Honestly I think when I’ve used Schottky’s it worked *better*

Thanks for the detailed responses!

 

[spi]

I almost always socket Ge diodes these days. Gives you the option to experiment, and if your stash runs low, you can pull them out to use on other circuits.

 

[spi]

a “coring” circuit, which is basically a primitive noise gate

interesting, I was looking up how this works but my search queries are coming up empty. Are there any references you know of?

 

[manfesto]

interesting, I was looking up how this works but my search queries are coming up empty. Are there any references you know of?

back to back diode on signal path on BOSS HM-2 and AmpTweaker Tight Metal

back to back diode on signal path on BOSS HM-2 and AmpTweaker Tight Metal

www.diystompboxes.com

as I understand it signals with amplitudes below the diodes’ Fv don’t get passed, so if your noise floor is below the Fv of your diodes it’s a simple noise gate.

In my experience schottkys are better at battling noise in this circuit than germanium, probably for a bunch of reasons (e.g. consistency, lack of leakage, temperature invariability, etc)

 

[manfesto]

interesting, I was looking up how this works but my search queries are coming up empty. Are there any references you know of?

Also check out how the coring circuit is implemented in the AmpTweaker designs, it’s 1N4148 diodes with a pot in parallel, making it adjustable.