Introducing: the 12AL5 duodiode vacuum tube.

[brad]

I've started a bit of a stompbox related "journal" lately...mainly to keep track of my ideas because I can never remember half the things I've done!  I'd also like to help promote the creation of new stompbox ideas in the DIY community.  Anyhow...I've just posted some stuff about the 12AL5 duodiode tubes that I've been mucking around with this year.  This particular tube produces some really pleasing soft clipping that's -to my ears anyway- much nicer than germanium diodes.  They're also only 3cm tall...which makes them perfect for stompbox use!  I'll be writing entries showing some oscilloscope traces and a schematic for a distortion pedal that uses the tube over the next week or so.

You can find my entry here: http://lustfund.livejournal.com/

[Pedal love]

Great, I'll start on an all tube octavia.

[funkbass187]

so could you just substitute one of those for a pair of clipping diodes? or am i just completely wrong.

[Phorhas]

Cool!
Thanks for the note

[bwanasonic]

You can find my entry here: http://lustfund.livejournal.com/

Nice, and pretty tube boxes as well. Do the 12AL5 or 12AJ6 *light up* with 12V?  

Kerry M

[brad]

so could you just substitute one of those for a pair of clipping diodes?

Yes.  I don't really understand their capabilities, but they seem to work fine as clippers (don't expect silicon-like saturation though).  The problem with using obsolete technology is there's hardly any information about it out there!  Perhaps Ton would know more?  This picture is from the William Orr book I mentioned:

Nice, and pretty tube boxes as well. Do the 12AL5 or 12AJ6 *light up* with 12V?

Yep, they use 12v on the heaters.

[brett]

Hi.
I have some 6AL5/5726s.  They are similar except for 6.3V 0.3A heaters.
Such cute little valves - miniature 7 types - about 1 inch high.
Got them out of an old radio.  Lots of radios have similar diode tubes in them. 
Their datasheet is here http://frank.pocnet.net/sheets/093/6/6AL5.pdf.
cheers

[brad]

We must be on the same wavelength Brett!  Have you found any uses for them? There's a schematic using the 6AL5 in the William Orr book that looks interesting:



You can download the whole book from Pete Millett's website (warning: it's 61mb and 800+ pages long  ):
(actually, he might flip out if I link directly to it.  It's in the "Technical Books" section on www.pmillett.com )

[mpeg2]

You might want to take a wander over to http://www.sopht.ca/mambo/index.php - Stephane has been experimenting with the 12V tubes for guitar amps & has worked out some of their kinks. I built a few of his designs - 1 using a 12AL5 (didn't sound too good as an amp - altho Stephane commented that it sounds better as a pedal). Another using a 12K5 (sounded much better). The 12K5 is shown below (sitting on a 4x12 cab that its driving).

http://psip.home.comcast.net/MiniTube.jpg

Be aware that these tubes suck a large amount of heater current - I had to use a 12V power supply that could supply 1A of current for that tiny amp.

   Rich

[brett]

Hi.
I would use them for an octave pedal.
If their Vf is low enough, they might also make a good passive ring modulator.
cheers

[brad]

A passive tube ring modulator?  Sounds like an interesting idea 

I've read articles about old AM Detector circuits where it's been said that tube diodes are perfect because "ge diodes aren't as sensitive and won't respond to weaker signals" and that tube diodes are "perfect diodes with no voltage drop that rectify down to small voltages".  Considering ge diodes are used in passive ring mods for their sensitivity, the tube diode may work even better.

[Paul Perry (Frostwave)]

For  a passive ring modulator, if you don't want a lot of feedthru, you have to match the diodes. I doubt you could match 6AL5s terribly well, but the proof is inthe trying. Go for it, someone!

[brad]

I'm onto it 

All this tube-talk and Ton is nowhere to be found!  I bet he's already tried it, bought the t-shirt, and seen the off-Broadway play.

[brett]

Paul commented that:

For  a passive ring modulator, if you don't want a lot of feedthru, you have to match the diodes. I doubt you could match 6AL5s terribly well

Doh!

Maybe the octave idea will work better.  The Octavia gets a great octave effect without being particularly balanced.  It would be nice not to have any gating (Rambler/Bobtavia etc) or forward biasing issues (Octavia).
cheers

[Paul Perry (Frostwave)]

Don't on any account let me stop anyone trying this! Thaough as you say, an octaver might be easier than a ring modulator.
What you really want for the best ring modulation, is a diode that follows a square law for curent vs voltage. I don't know how a tube diode compares to others in this regard.

[brad]

I gave it a go today using two 3K:3k transformers.  I couldn't find a third 1/4" jack, so I just attached the carrier to the input.

First time I tried, I just got an overdriven signal...but it just turned out I'd mixed some connections up.  As soon as I fixed it...the circuit produced an octave up effect, but also needed to be driven by a boost to generate enough output volume.  I can't comment on how well it works as a ring mod until I find another jack to use.  At this point, it's safe to say that these tubes will at least work in this type of circuit!  I used the JAN 12AL5s I have because I figured they'd probably have the most consistant specs.

Who needs Tubescreamers when you can build low volatage vacuum tube ring modulators? 

[brett]

Very cool.  A ring mod with options for 1. modulating itself to make an octave up, or 2. modulating on a LFO to make Dalek sounds, has always seemed like a good idea.

Keep up the good work