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Hi All,

I've received a new tube to play with.  The 12AL8.  Basically, it's a power tetrode AND a single meidum gain triode inside one small tube!  So, we can now easily get 3 gain stages with only 2 tubes!

After playing a bit with the tube, the effect is awesome!  At 12V, it's still low volume (about the same as the 12K5), but talk about nice distortion!

I have a new site to host all of these neat things:

http://www.sopht.ca/

Here is the link to the 12v tubing page with the 12AL8 design, the sound clip can also be found on that page:

http://www.sopht.ca/index.php?s=26

Cheers,
Steph.

Your site looks fantastic and 12AL8 sounds GREAT! 8)  8)  8)  :!:

I've been thinking of a similar 12U7 + 12AL8 amp since about an hour after seeing your first Sopht post. :D My design is very similar with a 3+1 topology and no tone control, but I do have volume of sorts. To replace the tone control, there is a 6 position rotary with different coupling caps between stages 1+2 and 2+3. My plate resistances are much higher though and I'm using bypassed cathode resistance for the triodes. Output is into 800 ohms. Now where sdid I see those 800 to 8 ohm transformers recently??? It made me wonder what the 800 ohms was about. :o Now I know!  I have not built it yet, but I'm getting ready very soon.

I also have a push-pull 12K5 design waiting with a transformer phase splitter. :D

Here's a list of tubes "made for storage batteries". Not all of these tubes are made for 12V only operation. Some have filaments that work from 10 to 15.9 volts but still want 250V B+. I think I have all those marked with #. (all 2 of them)

12AB5      # 250v B+  NOT LV
12AC6      - Pentode, 7 pin
12AD6      - heptode, 7 pin
12AE6      - triode mu=~15 + dual diode
12AE7      - dual triode mu1=13 mu2=6.4 !
12AF6 - RF & IF pentode
12AJ6 - triode mu=55 + dual diode, 7 pin
12AL8      - triode/SC tetrode !
12BL6 - RF & IF pentode, 7 pin
12CN5 - pentode, 7 pin
12CX6 - rf or if amplifier subs with  12DZ6, 12EA6, & 12EK6
12DE8 - diode + pentode
12DK7 - twin diode, power tetrode, 10mW
12DL8 - like a 12K5 + dual diode
12DS7 # HV pentode
12DV8 - dual diode + AF tetrode 5 milliwatts! :)
12DZ6 - rf or if amplifier subs with  12CX6, 12EA6, & 12EK6
12EA6 - rf or if amplifier subs with  12CX6, 12DZ6, & 12EK6
12EC8 - triode/pentode made for VHF
12EG6 - pentagrid, 7 pin
12EK6 - rf or if amplifier subs with  12CX6, 12DZ6, & 12EA6
12EL6 - triode mu=55 + dual diode, 7 pin
12EM6 - tetrode + diode, 10mW !
12F8 - diode pentode
12FK6 - triode + dual diode, mu=7.4
12FM6 - triode + dual diode, mu=10
12FR8 - triode + diode + pentode
12FX8 - triode mu=10 + pentagrid
12GA6 - heptode for pentagrid converter apps.
12J8 - dual diode + power tetrode, 20mW!
12K5      - 35-40 mW SC tetrode !
12U7      - dual triode mu=20 !

Thank you so much again for turning me onto Auto tubes! :D

Take care,
-Peter

Hi Peter,

thanks for the list of 12V tubes!!!  This will definately come in handy.  I love the idea of the 6 position switch with different coupling caps.  The matchbox had that configuration.  It's a very nice way to change the tone of the amp without loosing too much gain!

If I do add a tone control, I might end up with this one.  I will probably move on to 24V in my next series of experiments.  The sound is loud enough for apartment playing, but, I want to see how loud in can get with 'safe' voltages (I know, 24V can still 'bite' you).

Let me know how your design works out!

Cheers,
Steph.

8)

Now my next question is what happens if you add another 12AL8 and use the triode as a phase inverter for push pull? :D

The triodes with a mu of 55 also look interesting for a preamp. :D

Take care,.
-Peter

I found a data sheet on the 12AL8, and it says it should drive a load of 1K. would it be more effective to use a 1K:8ohm output transformer than the 600ohm one your using? And would you lose much power if you used a 2.5K?

What a great site for me to see, a great site indeed !!! Very interesting work!!!!!!!
 I just got My First Tube project: Shaka Tube working, and I'm very excited with the results!!!
 I now have a list of tubes to look for in my wallet ! ! !
 I don't feel ready to tackle a High V. project, I need more experience working with and getting the right transformers first.
 I had A Cadillac with a superb sounding tube radio in it...wish I had that now...
 The clips are very enticing ! Great sounding amps!
 What are you using for speaker?

Hi Peter,

Quote from: Peter Snowberg8)
Now my next question is what happens if you add another 12AL8 and use the triode as a phase inverter for push pull? :D

I was thinking about that myself.  With only 3 tubes, you could get a nice push pull amp.  Once I get the proper transformer, I will try it!

Quote
The triodes with a mu of 55 also look interesting for a preamp. :D

I hadn't noticed that one.  55!!! wow, that's high for such a small voltage.

Interesting tubes.

Steph.

Hi Alex,

Quote from: Alpha579I found a data sheet on the 12AL8, and it says it should drive a load of 1K. would it be more effective to use a 1K:8ohm output transformer than the 600ohm one your using? And would you lose much power if you used a 2.5K?

Intersting, do you have a link for the datasheet?  The one that I was reading specified a load between 480-800 but not 1k.  At 800ohm, it gave roughly 40mW.

If a tube was designed to drive 1k, it is always more efficient to use a 1k load.  Over 1k might make it less loud and lower than this might generate more distortion (I think).

Cheers,
Steph.

Hi Pete,

Quote from: petemooreWhat a great site for me to see, a great site indeed !!! Very interesting work!!!!!!!
 I just got My First Tube project: Shaka Tube working, and I'm very excited with the results!!!
 I now have a list of tubes to look for in my wallet ! ! !
 I don't feel ready to tackle a High V. project, I need more experience working with and getting the right transformers first.

We followed similar paths!  I played with a starved plate tube preamp (ruby tuby).  This was my training ground.  I wanted to learn how tubes worked.  I still haven't fully mastered that aspect, that's why I wanted to play with an all tube amp.  Now, just like you, I wasn't ready to use high voltage.  I had two choices, starved plate design or find weird tubes.  Thank god for those little 12V tubes!  They are so easy to play with and all of this at a safe voltage level.  You can easily breadboard an entire design in less than 1 hour!!!

Quote
 I had A Cadillac with a superb sounding tube radio in it...wish I had that now...
 The clips are very enticing ! Great sounding amps!
 What are you using for speaker?

I heard thar Blaupunkt also made tube radios.  That must have been nice...  Right now, the amp goes into a jensen mod 10" 30watt at 8ohm which is in my Gibson GA-15RVT amp.

When I have found the right design, I plan on making a small combo out of it.  I have a jensen 8" waiting for it....

Cheers,
Steph.

A Jensen 8''...should be perfect!
 I'll make some calls/stops when the next working day arrives, at local junk yards, they have barrels of old radios...who knows until you try looking what you may find.
 The old Caddy radio, was mentioned long ago as a good starting point, [if you could find one]  however since this ground work has been done [Thank You], it would seen the obvious choice now for an initiation point into tube wirings.
 Question: Has anyone found any tube tuners useful parts wise which could be used toward a tube-pre or amp?...because they're there...

Hi Steph,

I was curious about your choice of resistors. In the car radio diagrams I have come across that use these tubes and the datasheets I've seen, the grid and the plate resistors are many times larger. These changes could make the amp less sopht. :D

I can't wait to hear how a 12U5 + 2* 12AL8 amp works! 8)

Take care,
-Peter

i wonder how these tubes would do in a cascaded high gain, mesa type circuit...

Hi Peter,

Quote from: Peter SnowbergI was curious about your choice of resistors. In the car radio diagrams I have come across that use these tubes and the datasheets I've seen, the grid and the plate resistors are many times larger. These changes could make the amp less sopht. :D

I can't wait to hear how a 12U5 + 2* 12AL8 amp works! 8)

Take care,
-Peter

This is interesting.  You are right about the plate resistors.  I have to play with it a bit.  The 12AL8 datasheet contains a schematic with the triode portion that has a 330k plate resistor (gasp!).  When I decided on the plate resistor, I simply use the following: 2 * plate resistance (which is 12500 ohm for the 12U7).  This gives me 25k.  I didn't have a 25k, but I had a 22k.  So, I tried the 22k and it seemed to sound nice!

Having said this, this is something that I can easily try.  That's the advantage of being able to breadboard stuff!!!  Where did you see these car radio diagrams?!?!?!  What typical values did you see?

Thanks,
Stephane.

Anyone try a Tube Emulator Rectirier?
 I read about a product that is all Transistor, plugs directly into a GZ34 socket, and supposedly is a workalike to that tube to the point that the only noticable difference is that it doesn't wear out like a tube.
 Anyone try these, or know anything about them?
 Something like this, if it could be adapted to small signal, or small voltage tube designs, might be worth messing with!

Stephane ... awesome clips, love the biasing in the 12AL8 circuit - what're the DC voltages in that circuit at idle ? ... total DC current draw ?

I wonder what these circuits sounds like if you terminate them with a 8-ohm/5-watt load and then wire in a variable Line-Out circuit to feed a bigger amp ?! Then you can focus more on tone ... otherwise, remember that you can always tie a few Tetrodes in parallel to get more output from your circuit ...

I was also wondering if it's possible to get transformers made with 2/3 of the laminations removed ?!  Same winding specs but with lower core saturation limits to add some compression in there ... I know you seek more output - it's just a thought ...

... excellent !
jc

Hi,

Quote from: Alpha579i wonder how these tubes would do in a cascaded high gain, mesa type circuit...

My guess would be kinda depends.  If you are looking at the higher gain mesa's, probably won't happen.  But, if you are looking at the lower gain versions, I'd say it could be very interesting!

Any specific Mesa schematic you're looking at?

Cheers,
Steph.

Hi Eb7+9 (cool name be the way),

Quote from: Eb7+9Stephane ... awesome clips, love the biasing in the 12AL8 circuit - what're the DC voltages in that circuit at idle ? ... total DC current draw ?

I'll have some spear time tomorrow night, I'll draw up a voltage chart for the 12AL8 amp.  I was trying to get the tetrode and not the triodes to distort.  Not sure if that is what is happening, but, I like it!  Altough it is better than my first shot with the 12AL8 (was a bit more harsh).  So I'd say I'm pretty close to my goal.  For the total DC current, I'd say a bit below 1A.  .7 for the heaters alone + misc current from the tubes (they pull quite a bit).

Quote
I wonder what these circuits sounds like if you terminate them with a 8-ohm/5-watt load and then wire in a variable Line-Out circuit to feed a bigger amp ?! Then you can focus more on tone ... otherwise, remember that you can always tie a few Tetrodes in parallel to get more output from your circuit ...

Interesting.  Something like a amp pre-amp....  Using two tetrodes is a design in the pipeline.  I'm just waiting for the proper output transformer.  Since the 12AL8 also has a triode in it, we can easily do a push-pull with 3 tubes.

Quote
I was also wondering if it's possible to get transformers made with 2/3 of the laminations removed ?!  Same winding specs but with lower core saturation limits to add some compression in there ... I know you seek more output - it's just a thought ...

... excellent !
jc

I haven't investigated custom transformers.  I would like this amp to be easily built using currently available parts.

Cheers,
Steph.

Steph,

Here are some 12V tube resources (of course they are all in Japanese ;)):

http://radiomann.hp.infoseek.co.jp/HomePageVT/AMBtube.html
http://www5.ocn.ne.jp/~portable/home.files/automobile-tubes.html (good list here!)
http://www5.ocn.ne.jp/~portable/ (check out the items in menu section 3!!!)

One of the circuits in the last URL actually uses a 12AU7 for the final IF stage and for an AF amp, with a 470K plate resistor (!) running from 12V  :shock:.

The radio in section 3-5 uses a 12AL8 into a 1.2K:8ohm OT.

All kinds of neat stuff in there!

I'm still looking for the schematic that shows the push-pull 12K5. All I remember about it now is that it was transformer phase split with a 7K:15K CT transformer, used 22 ohm cathode resistors, and dumped into 800 ohms for output. I'll see if I can find it again for you.

This is easily my favorite thread here now! 8)

Take care,
-Peter

Hi Peter,

thanks for the links!!!  Here is the link with the push pull 12K5:

http://www.geocities.com/wa2ise/radios/12ba7.html

Cheers,
Steph.

I think that may be a different one actually. I remember independant 22 ohm cathode resistors for some reason. The splitting transformer looks the same though.

Thanks for that link in any case!

Now picture that section being driven by a mosfet, being driven by a Runoffgroove or similar JFET preamp. :D

Or for the (semi) purists... picture the power stage being driven by three of those triodes with a mu of 55 ;) and maybe a MOSFET follower too. For extra credit, use 12AL8s and turn the triodes into a long tailed pair!

Take care,
-Peter

Here's another link I found on the same topic (it's in german, beware! but nice pics):
http://www.jogis-roehrenbude.de/Roehren-Geschichtliches/Niedrigspannungs-Roehren/Niedrigspannungs-Roehren.htm

These vintage 12V tubes are really hard to find, even on ebay, so I started looking for alternatives, anyone ever tried russian subminiature tubes, for example the 1SH24B? (more here (http://www.jogis-roehrenbude.de/Russian/Russische_Roehren.htm))

(http://www.jogis-roehrenbude.de/Russian/1SH24B.jpg)
This (http://home.arcor.de/rhapidophyllum/1j24b.gif) seems to be the datasheet, but it's in russian! :wink:

These tubes (pentodes?) are available from pollin.de (//www.pollin.de) for 26 cent, is there anything I could build with them? I have no experience with tubes whatsoever, and that would be an easy (and very inexpensive) start!

Well thanks, ... you did it.
You have ruined my life with your "toobs".
It was bad enough with "Mr Run-off-to-the-groovy" and his sparkly jfets.
I now have another obsession I didn't have before.

Fetzer.
Professor Tweed.
Matchbox.
Sopht tube amp (12AL8)

And a bunch more stuff, but Im going to start with a Ruby-Tuby.
Hey, life just got fun again  :lol:

I had some old tube Radios, I turned into guitar amps, and A Westinghouse tube powered extension cabinet...also tube.
 Although these amps didn't distort by themselves heavily at all, they were superb sounding late night/low level jamming amps.
 The reports of the tone from casual passers by, and users of the amps was that they sound very good!!!
 I didn't like the high voltage...and finding a suitable input point with the thing on...ya know the old 'touch' method with an insulated screwdriver...sure did make some fine low level jamming amps.
 I'm still looking for that 'gold bag' of tubes around here....
 12 volt tubes?...yup too cool !!!
 Breadboarding, and modding tube ccts. without the threat of high voltage, that's having the good, Without the bad ! ! !

Hi Spongebob,

I have a few of these "pencil tubes".  A few things worth pointing out:

- The output is very low (i.e. below 10mW)
- They require 'weird' heater voltages (i.e. 1-1.5V).  This can easily be supplied by a AA battery.

Fred Nachbaur as a 'baby design' built using tiny tubes:

http://www.dogstar.dantimax.dk/tubestuf/gallery4.htm

This site as LOTS of information on pencil tube:

http://pw2.netcom.com/~wa2ise/

Cheers,
Steph.

Just found 2 more VERY interesting tubes:

- 5797 power pentode
- 5798 dual medium triode

Interesting facts:

heater and plate voltage = 26.5 Volts (again, thinking about single power supply design).  The 5797 is rated at .8w max plate dissipation!!!  Hum, I smell another cool experiment.....

Steph.

That 5797 looks wonderful! Too bad the mu is only 24 on the dual triode, but three stages of that should be able to do just fine.

Hmmmm..... an oscillator and one of those little RF/IF pentodes would make for a 12V tube tremolo ;). I need some of these tubes! :D

Now there's also the four stage 12V tube overdrive pedal to be made here. ;)

I wonder if Sovtek/NewSensor/EH will ever explore this area?

Take care,
-Peter

This seems like a FUN and safe amp to explore. I need to try this. It already sounds great!

Are there any tubes designed to work at a mid point to the car battery ones and full blown 330v jobs? Something like a 50-100watt valve? Would you be able to use these(if there are any) for 5-10 watt amps?

And also, are there any valve rectifiers made to be used at these 12v levels?

Hi,

Quote from: Alpha579Are there any tubes designed to work at a mid point to the car battery ones and full blown 330v jobs? Something like a 50-100watt valve? Would you be able to use these(if there are any) for 5-10 watt amps?

Most regular tubes can be operated at roughly 100v.  So, if you want to go to 100v, you can use regular tubes.

QuoteAnd also, are there any valve rectifiers made to be used at these 12v levels?

In this thread, Peter posted a very good list of 12v tubes and characteristics.  A few of them have either 1 or 2 diodes.  This could be usefull as a rectifier.

Thanks,
Steph.

Steph,

Who is a good US source for these tubes?  What price can I expect to pay per tube?  This project looks really cool and do-able.   :D  

Thanks!

Derek

i just ordered some 12U7s and 12AL8s from Antique electronics
www.tubesandmore.com

they were about 3-5 dollars - the most expensive item was the transformer which was 18 dollars.

Can't wait to get them and start breadboarding - by the way can anybody tell me do these tubes glow when powered?

/Taz

Hi Tax,

Quote from: tazwolfCan't wait to get them and start breadboarding - by the way can anybody tell me do these tubes glow when powered?

/Taz

Yes, they do glow and the 12AL8 gets HOT (hotter than the 12U7).

Cheers,
Steph.

Tazwolf, which transformer did you get?

QuoteIn this thread, Peter posted a very good list of 12v tubes and characteristics. A few of them have either 1 or 2 diodes. This could be usefull as a rectifier.

Thanks,
Steph.

Looking at the data sheets, it looks like the cathodes of the two diodes/triode are connected :? . If this is right, how can you use them as rectifiers?

There are several 12V tubes with diodes in them, but these are small signal diodes and not power rectifiers. The 12DL8 is a good example... it's a space-charge tetrode with two diodes. The diodes are rated at 5mA MAX (each) and the space-charge grid wants 75mA on top of the 40mA that you can pull from the plate. They're good for detector use, but unuseable for supply.

I was looking at one low(er) voltage diode tube and the forward voltage drop was about 10 volts.

Take care,
-Peter

Hi Alpha,

Quote from: Alpha579Tazwolf, which transformer did you get?

At 18$, I guess he went for the Hammond 119DA which is what I am using for the low voltage tubes.

Cheers,
Steph.

Cheers Steph, didnt see that...btw, have you actually built a finished version of the sopht? If so, any picys?

Hi,

Quote from: Alpha579Cheers Steph, didnt see that...btw, have you actually built a finished version of the sopht? If so, any picys?

No, not yet.  It's still on the breadboard.  I might take a picture of my breaboard connected to my gibson cabinet just for fun!!!

Cheers,
Steph.

Alpha579: The transformer I bought was the Hammond 119DA.

/Taz

Hi All,

I had a bit of time to play with the amp tonight and I slightly revised the design.  It is now a bit more balanced in the output (highs versus lows) and the distortion sounds just right.  You can see the latest design at:

http://www.sopht.ca/index.php?s=26

I'll try to post a sound clip and voltage readings this weekend.  I might try to sligthly rebias the output tube as it is a bit below spec'd readings.

Cheers,
Steph.

Steph, how do you go about rebiasing the output tube?

That looks GREAT Steph! 8)

I'm really looking forward to hearing the new clip, but even more to getting some of these tubes and building a Sopht! :D

Take care,
-Peter

Quote from: Peter SnowbergI need some of these tubes! :D

Now there's also the four stage 12V tube overdrive pedal to be made here. ;)

I wonder if Sovtek/NewSensor/EH will ever explore this area?

Take care,
-Peter

I heard that! (back from a little Turkey-vacation...)

If s.o. got a stock of tens of thousands of those tubes, it might be interesting to start thinking about such pedals...

If there is s.o. to have such stock, it is probably the company
you mentioned...

for me, (feeling addressed...) when I started to re-vive my old tube-passion in the eighties, my intention was to do anything possible FX-wise with only the tube-type which I was sure would gonna survive for the next 30 to 50  years, and that is the 12AX7.

Although I`m using some less common HQ-tubes (some of them still/again in current production) in a couple of 19" rack studio projects, I don`t wanna bother too much with "exotic" types, which do not offer the features of the better known types, and bear the danger of becoming obsolete or over-expensive within a short time period.

Hi All,

I'm learning so much that it isn't funny :).  Anyways, played a bit with the design today and found one thing that was anoying me.  Bascially, the amp had too much 'bite'.  It was very sensitive to pick attack and it would really be anoying how sensitive it was.  Someone on the AX84 did build a sopht amp and came to the same conclusion.  After a bit of searching, I saw that the grid voltage for the first section of the triode was a bit too high (i.e. very close to 0v).  From the loadlines in the spec sheets, I knew this had to be below 0v.  These 12v tubes seem a bit harder to bias (or maybe I don't know how to bias a tube, which is a very good possibility also).  Anyways, adding a cathode resistor didn't help in briging down the voltage on the grid.  So, I increased the grid resistor (R2).  This droped the voltage of the grid.  I settled on 470k and it seemed like a good choice (grid now reads -0.2v).  The bite is now gone and we are left with a better sounding amp.  Please note that increasing the grid resistor will roll off some highs.  I might try adding a 'brightness' cap in case someone needs to have a 'brighter' input.

Cheers,
Steph.

wouldnt that lower the gain somewhat?

I've created new sound files which a voice like a different amp.  I used the original sound files posted the other day.  I started this poll because I believe the rig can have a large effect


http://www.diystompboxes.com/sboxforum/viewtopic.php?t=22392

Hi Alpha,

Quote from: Alpha579Steph, how do you go about rebiasing the output tube?

There are very good biasing information at:

http://www.aikenamps.com/Why70percent.html
http://www.aikenamps.com/Biasing.html
http://www.aikenamps.com/CrossoverNotchBiasing.html

Basically, everything in http://www.aikenamps.com/TechInfo_2.htm is a good read!

Cheers,
Steph.

Hi Peter,

Quote from: Peter SnowbergI think that may be a different one actually. I remember independant 22 ohm cathode resistors for some reason. The splitting transformer looks the same though.

Is this the link that you were looking for:

http://www.jogis-roehrenbude.de/Leserbriefe/ECC86-Audion-Diskussion/ECC86-Audion.htm

I have been playing with the 12AL8, but, I'm not sure I totally like it.  I will go back to a 12K5 design.  The 12AL8 has the luxury of having an extra triode, but, we don't have any loadline information making it hard to find the 'sweet spot'.

Cheers,
Steph.

Thanks. That's got to be the site :D. I was mixing up two schematics in my head I think.

Take care,
-Peter

I was just looking at the Hamond 125s. :)

http://www.hammondmfg.com/125.htm

It looks like using terminals 1&6 or 2&6 with a 4 ohm load would be a nice match for a pair of 12K5s or 12AL8s. :D

$29 for a 125A at Angela (3 watt transformer)
$20 for a 125C at Angela (8 watt transformer)

Take care,
-Peter

QuoteI have been playing with the 12AL8, but, I'm not sure I totally like it. I will go back to a 12K5 design. The 12AL8 has the luxury of having an extra triode, but, we don't have any loadline information making it hard to find the 'sweet spot'.

Rats. I was ready to order. This project has got me interested.

First of all, what transformer should I get? Secondly if 1K is possibly a better match, would that tiny Radio shack transformer work? It's 1K:8 ohms I think.

I'd like to build this amp. How loud is it exactly?

Hi Aron,

well, 40mw isn't a lot but, when looking straight into the speaker, it's loud enough (sorry, I can't really compare to anything...).  No feedback, though.  I will go back to the 12K5 design and see if I can make it a bit better sounding distortion wise.  I am learning, so, I am not always sure about what I do...  Having said this, buy a 12K5 and an 12AL8 and try both!!!  Both are rated for 800ohm load, so, the radio shack 1k:8 ohm is allright.

Cheers,
Steph.

Steph,

How big is the Hammond transformer??? The Radio Shack one is so tiny. I would imagine bigger iron would be better???? Not sure here.

Hi Aron,

I just recently got a few 1.2k ct : 8 ohm transformers and they are really small (made for transistors).  The hammond is about 5 times bigger.

Cheers,
Stephane.

Do you think the size makes any difference? Or would make a difference?

Hi Aron,

hard to say.  This is my first shot at making a tube amp!  If I do remember somewhere, they did mention that the size of the output transformer does have an impact on sound.  But, more importantly, it's the frequency range of the OT.  Even if it's HUGE, if it doesn't have a good frequency range, it might suck.

Cheers,
Stephane.

I've breadboarded my first attempt at an amp on breadboard using just the 12Al8S and a hammond output transformer. I used the schematic from the datasheet. i must say it sounds very good i drive it with a Vox valvetone or a supreaux.

It is a bit low in volume - I wonder if it would be an idea to to remove the transformer and iuse a solid state amplifier stage to bring the output to 1-2w maybe using an LM386 or one of those TDA chips.

would this be viable?

It's amazing seieng those little tubes glow.

/Taz

Hi Tazwolf,

glad you tried the 12AL8!  I didn't have much success with this tube, but, I am probably doing something wrong.  Do you have another triode infront of the 12AL8 or are you just using the 12AL8?  If you are only using the 12AL8, this could be one of the reasons why it is so quiet.  Typical low volume single ended tube amps have 2 triode stages followed by a pentode or tetrode stage.

You might want to try adding a single triode stage prior to the 12AL8 triode stage.

As for using the output of the 12AL8 to drive some solid state, this is possible as this was the initial entent behind these tubes.  They were meant to drive transistors.  Having said this, I am unsure how to go about setting it up that way.  Some potention issues:

1- Impedance mismatch between the tetrode section and the LM386 section
2- Voltage coming out of the tetrode might be to high for the LM386 (could be easily fixed with a voltage divider)

Issue one could be a show stopper.

Cheers,
Stephane.

I'm using the triode thats already in the 12AL8.
I have also got some 12U7 so I'll try putting one in front as per your schematic.
One thing I noticed though your capacitors between the tubes are smaller than the ones in the example schematic from the 12AL8 datasheet (0.1uF).

I suspected that the impedance mismatch would be an issue.
Oh well I'll keep experimenting - the smell of warm 12v tubes is rather addictive.

Stephane, why don't you take like the Fetzer, Jfet's my friend, exactly like wireing in a half of a preamp tube, just remember it'll take 2 Jfet's to equal  1 dual stage preamp tube, and this would give you a nice place to park a good tone stack and it should drive your tube the same as the jfet preamp on a PV drive's the tubed power section, all you have to remember with tube's is keeping the signal in phase with the output or if not sometimes all you'll get is oscillation. :idea:  :idea:  :idea:
JD

tu-be or not to-be  8)

Hi all,

Taz, thanks for pointing out the caps between tubes.  I will have to try it.  I think that an extra triode stage will definately increase output volume.  You are right, it can be pretty exciting to play with low voltage tubes!!!

JD, yes, a jfet would definately increase the output.  But, as puretube points it, I was looking for an all tube solution.  A jfet would be a good solution to drive the 12U7 into clipping, which could be an interesting sound also.

Thanks,
Stephane.

it`s good to know, that all is happening @ low voltages in this thread  8) ;

A certain Mr. Barkhausen had interesting findings about in-phase/outa-phase relationships many decades ago   :?  ; .....

It makes me smile/grin/happy, to see that there are still people,
who don`t believe in the sayings of >40 years ago, like:
"we don`t need that old hot glass-crap anymore, because that new germanium stuff does everything better"  :D

Quote from: puretube"we don`t need that old hot glass-crap anymore, because that new germanium stuff does everything better"  :D

:shock:  :shock:  :shock:

Huh??? How the hell is it supposed to work if it doesn't glow? ;)

Take care,
-Peter

PS: When I worked for the U.S. Gov't, my e-mail account name was "tubetech". :D

it did glow (for a moment), when you gave it 5% above specs..  :lol:

(grin)

stefane - the thanks is all to you for sharing this 12v tube thing with the forum.

One thing I did was to put a 470k potentiometer in place of the (330k data sheet schematic or 10k your schematic ) resistor going to the anode of the tetrode - ths allows me to vary the gain in the output stage I don't know if this is the correct way to do it but it seems to work.

Another idea I had was to take a small crummy old 8 ohm speaker and tear off the cone and connect it on to the secondary so that the transformer would still have a load (but without sound) and then to take a serial feed off the speaker as a line out kind of thing.

Sorry - correction my previous message should read potentiometer connected to the anode of the triode!

riode dode bonode bonometer - time to get some sleep.

I tried the RS 1k-8 transformer today vs. a generic c. 10W fenderish transformer.  The RS transformer cuts out way too much of the sound for my taste, killing pretty much all bass below 300hz (if I recall the specs correctly).  The bass is important to the perception of round PT distortion, at least for me.  With non-guitar specific speakers, the tranny matters much less.  

The 12AL8 (driven by a 12U7) responds very well to effects.  Boosters, etc, increase volume dramatically.  I drove it with a McTube and a Big Muff last night, and the results were impressive.  I need to get some of the ROG stuff perfed to see how it reacts.

I mic'd the speaker just off center, as close to the cone as possible.  It gets very good tubes/speaker about to blow sounds.  For recording, I've been running the mic into a nearby amp, giving nice, controllable feedback.  The feedback further improves the perception of volume.

I'm finishing a .04 Watt with an EA trem in front of it (nice sounding boost, always on), kind of a precursor to 6GW8 (13W) or 6BM8 (9W) 18 Watt adaptations.

:?: A couple of questions:
Is there any reason to have a bypass in this thing?
How large a signal did the designers of these 12V tubes expect to feed them?

QuoteHow large a signal did the designers of these 12V tubes expect to feed them?

maximum pk/pk = grid bias voltage...

Hi Travis,

I am really interested in your 12U7/12AL8 design.  I had some issues with my design and am currious in seeing this work!  I just got an Oscilloscope this week and I will debug the circuit when I have some free time.

Thanks,
Stephane.

Right now it isn't much of a design, just pots connecting everything together.  It sounds good enough for me to show to other people, however.  I have three other projects (probably about a week and a half) that I need to finish before I can pull the 12U7/12AL8 back apart and take all of my measurements for a final copy.  It was initially built to specs, but was unspectacular as such. I still need to play with som eof the cap values, hopefully opening it up a bit.

I've had a few pros play it over the last couple of weeks, and they were really impressed.  Anyone who has spent time in a recording studio can appreciate the ridiculous low volumes of this thing.  They enjoyed it more than the Filter S/H that I left out for them.

The 12V tubes are an insane amount of fun.  I like being able to touch the circuit I'm working on without fear of immediate death.  A paralyzing sleep deficit keeps me too scared to work on high voltage tube stuff very often.

I'm hoping that a couple of the 55mu triodes will give me enough gain for a 12V tube wah.  Can you roll your own Darlington tube (without anything exploding)?  Also, is there any real use for the included tube diodes?  If there is, I may work them in.

Anyway, I should have a variety shipment of the 12V's coming in sometime next week.

problems with Tube-Wah (TM) is not so much depending on gain, but on impedance issues...  8)

Nice, thanks.  Guess I need to buy one of your envelope filters now.  Fortunately, I've been looking for an excuse to do just that.

I was thinking of building along the lines of the GEO tube wah (or valve wah, if it makes you more comfortable :) ), so that I have a simple established design (inductor/Vox) and rely on the tubes for distortion behind the sweep.  I really don't have the time to do anything fancier.  

How low-gain can you go?  Would an amplification of 20 be too low?  How about 10?  I suppose that if I can get a sound I like out of it, I can always tack on a gain recovery stage.

http://www.geofex.com/Article_Folders/tube-wah/tube-wah.htm

Stephane, I'll see if I can get the 12AL8 sorted soon.  What problems did you have with yours?

Hi Travis,

the issue with the 12AL8 is that the distortion just seems to harsh.  It happens when I use the triode of the 12AL8.  Using the circuit in the spec sheet, I get ugly distortion.  If I raise the grid voltage just a bit (from -.9 to say -.75), some of that harsh distortion goes away.  If I try to bring it higher, I get some sort of oscillation...

I still need to wrap my head around all of that.  Maybe I can start learning how to use an oscilloscope to see some of this, I am fairly visual at these things.

If you have any ideas, please let us know!

Thanks,
Stephane.

Slajeune: I connected up a 12U7 (according to your schematic ) on the fron of the 12AL8 and I am getting the same results as you. A harsh and "blatty" distortion as well as oscillation - although the volume is quite a bit louder like you said. Could it be that the 12U7 triodes are providing too high a signal to the 12AL8 triode? I'll keep on experimenting.

Travis: When you write down your schematic would it be possible for me too have a look at it?

Taz

Slajeune: great thing you got a scope now !  8)

From now on, you`ll be able to take bigger steps, faster and better...
:D

Hi Puretube,

yes, yeah are right, I will be able to advance faster, once I learn how to use the oscilloscope!!!

Taz, thanks for confirming my problem.  With old tubes, the possibility of having a bad tube is always there.  I need a few components to start using the oscilloscope.  I am getting these today and hopefully, I will be able to debug a lot of these things next week.

Right now, I have a very clean low volume amp with one 12u7 and one 12k5.  I did play with a distortion pedal infront (a tube distortion pedal) that goes from somewhat clean boost to a good heavy distortion and it was very sweet!  With the clean boost, the volume can get pretty loud whitout breaking up!

Cheers,
Stephane.