How much gain can 2 tubes and 12v get you? Answer inside!

[slajeune]

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.

[Peter Snowberg]

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

I've been thinking of a similar 12U7 + 12AL8 amp since about an hour after seeing your first Sopht post. 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. 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.

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!

Take care,
-Peter

[slajeune]

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.

[Peter Snowberg]

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

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

Take care,.
-Peter

[Alpha579]

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?

[petemoore]

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?

[slajeune]

Hi Peter,

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

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!

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

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

Interesting tubes.

Steph.

[slajeune]

Hi Alex,

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?

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.

[slajeune]

Hi Pete,

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.

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!!!

 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.

[petemoore]

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...

[Peter Snowberg]

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.

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

Take care,
-Peter

[Alpha579]

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

[slajeune]

Hi Peter,

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.

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

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.

[petemoore]

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!

[Eb7+9]

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

[slajeune]

Hi,

i 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.

[slajeune]

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

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'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).

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.

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.

[Peter Snowberg]

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!

Take care,
-Peter

[slajeune]

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.

[Peter Snowberg]

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.

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