6AL5 for clipping diodes?

[ThSpeakerDude88]

Couldn't find much on the net about this, but could a 6AL5 be used in place of the two diodes in a distortion pedal, such as a TS or any "insert popular distortion box here" circuit? Might have different clipping charactoristics if so.. and wouldn't requier a HV source for operation, only fils. Its also available from EH now apparently, for what reason I am unsure...

[Miles Prower]

Originally posted by ThSpeakerDude88
Couldn't find much on the net about this, but could a 6AL5 be used in place of the two diodes in a distortion pedal, such as a TS or any "insert popular distortion box here" circuit? Might have different clipping charactoristics if so.. and wouldn't requier a HV source for operation, only fils. Its also available from EH now apparently, for what reason I am unsure...

Sure you can. The 6AL5 was designed as a high frequency, small signal dual diode (useful for making SB and DB passive ring mixers). No reason they won't work at AF. They will, however, require a volt or two of reverse bias. Silicon diodes don't turn on until the Vf rises to 0.7V. This gives you the clipping/limiting action.

[ThSpeakerDude88]

hey thanks! I was a little worried about that... I thought that it might just plain act like a short and pull down the signal without some sort of reverse bias... can I just put a resistor in the cathodes , or will that only act like a more resistive short?

[Miles Prower]

Originally posted by ThSpeakerDude88
hey thanks! I was a little worried about that... I thought that it might just plain act like a short and pull down the signal without some sort of reverse bias... can I just put a resistor in the cathodes , or will that only act like a more resistive short?

Your best bet would be a LED in series with each cathode for reverse bias. The AC impedance is quite low, and will hold off conduction better than a resistor. A voltage divider that puts a couple of volts on the cathodes might give a softer break. Depends on what kind of effect you're after here.

[jim]

If I'd build a tube-distortion-box with a 6AL5, where do I put the 6AL5 ?
In a feedback loop (like in opamp-stompboxes) or at the output of a gain-stage, wired in anti-parallel to ground ?

Thanks,
Jim

[kevinkr]

Originally Posted by jim

If I'd build a tube-distortion-box with a 6AL5, where do I put the 6AL5 ?
In a feedback loop (like in opamp-stompboxes) or at the output of a gain-stage, wired in anti-parallel to ground ?

Thanks,
Jim

You can actually do either, in the case of an output there should be a resistor between the clipper diodes and the op-amp..

You can also experiment with varying levels of reverse bias on the tube. Silver Oxide or alkaline watch batteries can be used in the cathode circuits to provide reverse bias in addition to the leds mentioned previously. More complex bias circuits would allow you to vary the point at which the 6AL5 starts to conduct. Leds can also make good clippers..

[jim]

Originally Posted by kevinkr

More complex bias circuits would allow you to vary the point at which the 6AL5 starts to conduct. Leds can also make good clippers..

How can this be done ? The bias circuit should have a high output impedance in order not to load the circuit, right ?

[djgibson51]

1. So would a watch battery have to be in series with each 6AL5 diode?
2. Would there be an advantage preceding the 6AL5 reverse parallel circuit with a cathode follower?
3. Has anyone out there made this work?
(Merlin has mentioned reducing filament voltage to change forward conduction point).

[djgibson51]

Soooo.... Nobody has actually made this work?

[jim]

Originally Posted by djgibson51

Soooo.... Nobody has actually made this work?

I tried it a few week ago, but without the reverse bias and it did not work very well. Put some led's in series with the cathodes but that didn't work either.
The 6AL5 was driven by an opamp, supply voltage 20V.
Will try this again later after I figured ou how to use the 6as6 as a distortion tube à la Culture Vulture.