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RAT and diodes

1/12/1998 2:18 PM
RAT and diodes
In a previous post I mentioned that I changed the silicon diodes in my RAT for germanium (3 germaniums in a row for each silicon). There was a noticeable change of tone - for the better.  
When reading R.G. excellent papers on the Fuzz Face and the TS 9/808 he says, “The Fuzz Face is a fortuitous combination of circuits that combine initial soft clipping with asymmetrical clipping that changes toward symmetrical clipping under drive”.  
This makes me wonder, would it be of any value to try different diodes (they seem to go in two directions) in the RAT to further improve the distortion qualities?  
1/12/1998 6:57 PM

Trying different diodes is a favorite trick of mine in getting better distortion. There's been a discussion of this in the Guitar Effects FAQ since the first draft. In general the following diodes have the following characteristics:  
germanium - lower output and more distortion per unit drive control than silicon, as they break over at about 1/2 of a silcon diodes Vf. Softer start of conduction, so the distortion is different in character  
Schottky - kind of like half a silicon diode. They have a VF intermediate between germanium and silicon and usually a lower resistance. Kind of sharp edged to my ears.  
silicon - Vf=0.6 to 0.7V, this is what's in most clippers these days. What's NOT commonly known is that different kinds of silicon may have different internal resistances, so a 1N4005 in anti-parallel with a 1N914 is different from two 1N914's.  
LED - different colors have different Vf's but they start at about 2X silicon, or about 1.2 to 1.4 V for old-technology red ones, and go up to 2.0 or 2.2V for green/yellow etc. There are silicon carbide blue ones that have Vf of 4V or over. The lower voltage ones are good for distortion as they have a large junction capacitance which softens the distortion and is current sensitive as well. Used in the Scholz Rockman. You may well need more "Drive" to break them over if you put them in a stock pedal.  
zener - zeners are usually very sharp edged reverse conduction, and need to be end to end for clipping rather than back to back.  
tubes - yes, you can spend a whole duotriode makeing two back to back tube diodes, per O'Connor in The Ultimate Tone. Lots of work there...
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1/12/1998 8:04 PM
John Kos

How about using duodiode vacuum tubes (I have a ton of them)? Have you ever played with them? What are their curves like (soft, sharp,...)?  
Just wondering,  
John Kos
1/13/1998 1:15 PM
I haven't played with the duodiode tubes, but I'd imagine that they are not too different from signal triodes used as diodes - conduction starts from essentially 0V but through a very high resistance, and the resistance goes down significantly as the voltage goes up.  
Do duodiodes have separate anodes and cathodes? The only ones I remember have separate anodes, common cathode.
1/13/1998 1:37 PM

You're right about the conduction at (very close to) zero volts. The 6AL5 is a miniature 7-pin dual diode, two complete separate diodes with common heater only, which is a small signal diode used as the rectifier in vacuum tube voltmeters when measuring AC voltages. It works better there than 1N----'s because of the extremely low forward voltage drop, probably in the range of 0.1v. I think there are millions of 6AL5's still around. They're about the smallest 7-pin tube made.  
There's a discussion & examples of using these tubes as clippers in TUT-2. Kevin says you have to put a reverse bias voltage (neg. wrt gnd.) to keep them from conducting until the desired voltage. A little more complex than using ss diodes with discrete drops across the crystal.  
1/13/1998 5:48 PM
Ah! That's what those things are. I must have half the 6AL5's ever made in my these-aren't-audio but I-hate-to-throw-them-away tube boxes.
1/15/1998 7:38 AM

They're easy to recognize, even if the number is rubbed off. The internals are always bright shiny metal, never gray or black like most other tubes. Also, since they're low power diodes, the plate strucure is rather thin almost flat.  
Occasionally you will find a 12v version (12AL5), but the bulk were made for 6.3v operation.
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