OT: simple tube amp on a wallwart -> schematic inside!

[slajeune]

Hi All,

I have recently been playing with low voltage tubes.  I am not talking about using tubes that were ment for high voltage with low voltage.  I am talking about tubes that were made to be used with low voltages.  In the end of the tube life cycle, a bunch of tubes were designed to run on a car's 12V battery.  These tubes were used in car radios.  I have made a simple single ended tube amp running on a single 12V wallwart!

The key to producing a somewhat decent volume is to use the following:

- 12U7, it's a medium dual triode very similar to a 12AU7
- 12K5 power tetrode as the power tube.  This can apparently yield close to a 1/4 watt on 12V
- A small output transformer (i.e. 600 ohm to 8 ohm Hammond's 119DA is the one that I am using, I think that the cheap radio shack 1k to 8ohm might be allright, although, it will not be as loud)
- A wallwart with about 1.5 Amps output @ 12V (the tubes heaters take .75 Amps!)

All of these things are easily available at http://www.tubesandmore.com

This produces a surprisingly loud volume for tubes running on 12V!!!  Don't expect ear piercing volumes or mega gain like a soldano.  This is simply a start of a way to safely experiment with tubes without fearing to be killed because I touched the wrong wire.

My goal was to produce a quality tube amp that can be used in a home setting (low volume) with low voltage (makes it safe to experiment with).

I want to experiment with another tube at 12V (a pentode, 12CX6) before moving up to 24V (the tubes have a maximum plate voltage of 30V)!  There are a lot more designs that I want to try out!  Since it's low voltage, it's very easy to play around with it, we can simply breadboard it (unlike high voltage stuff, I wouldn't want to breadboard stuff running at 300V!!!!!!).

Simply amazing little tubes they are.

Here is the schematic:

http://www.geocities.com/slajeunesse/low-volume.html

To pins for the heaters aren't showed on the schematic.  Here is how the heaters are connected:

This was edited:

For the 12U7:

- Pin 4 gets 12V
- Pin 5 goes to ground
- Pin 9 is not connected to anything


For the 12K5:

- Pin 3 gets 12V
- Pin 4 goes to ground

Make sure that the power supply is well filtered and regulated (see the ruby tuby schematic for a power supply with filtering and regulation http://www.geocities.com/slajeunesse/ruby_tuby.html ).

Here are the links for the pinouts for the two tubes that I am using:

http://tdsl.duncanamps.com/show.php?des=12U7
http://tdsl.duncanamps.com/show.php?des=12K5

Cheers,
Stephane.

[aron]

COOOL! I gotta try this!

[Peter Snowberg]

That's VERY COOL!      

One question....

On your heater connections you say

For the 12U7:

- Pins 4 and 5 get 12V
- Pin 9 goes to ground

From the data sheet it looks like it should be +12 to pin 4 and ground to pin 5 with pin 9 unconnected. Pin 9 is the center tap for 6.3V heater operation like in a 12AX7.

Take care,
-Peter

[slajeune]

Hi Peter,

the tubes are meant to have 12V on the heaters and on the plate.  They weren't ment to use 6.3V on the heaters and I haven't tried it.  The tubes were meant to be used with a single voltage power supply.  This makes it easier to play with them!!!

This diagram indicates that the 12V goes to pin 4 and 5:

http://www.mif.pg.gda.pl/homepages/frank/short/001/1/12U7.gif

We unfortunately don't have a lot of information on these tubes....  I am amazed by these tubes and by the fact that there isn't alot of information about them.

Enjoy,
Stephane.

[petemoore]

I had a '67 Caddilac Sedan Devil, with a tube radio worth writing home about.
 A Really excellent to listen to radio.
 It was hard for people to believe there's was a tube in my car radio.

[slajeune]

Hi All,

I have found some 'cheap' alternatives to the OT tranformer:

- The 1k to 8ohm RadioShack audio transformer.  As stated, this might yield a lower volume
- A 70V line transformer.  This is used, if I am not mistaking, in PA systems.  It is supposed to have a primary close to 600ohm.  Secondary is usally either 4ohm or 8ohm

Both of these transformers should work and they are under 5$.  This sould keep the cost of the tube amp low.  As a side note, here is the guestimated price for this project:

- Tubes:
    - 12U7 : 5$
    - 12K5 : 3.60$
- Transformer:
    - Hammond 119AD : 18.32$
- Sockets:
    - 9 pin : 3.50$
    - 7 pin : 0.35$
- Chassis:
    - Hammond 1444-16 : 10.43$

Total: 41.20

you can replace the hammond transformer for one of the above ones for roughly 5$.  The chassis is also optional if you already have a chassis.

Cheers,
Stephane.

[Lonestarjohnny]

Stephane, Maybe you started a thing here and it'll drive the price's up on my 12 V. tube supply's, Thank's Buddy, LOL !,
Just Joken of Course but some of the first guitar amps that I saw wnen I was a Kid were built outa Car and home Radio's that ran on DC voltage,
What a trip to see somebody Building a 12 V. tube amp again,
JD

[MartyB]

The Real McTube uses two 12v wall warts and a 12a_7 tube.  The build instructions describe how one of the wall warts can be external as well, i.e. no permanent power cord.

[erix]

Car radio tubes seem a natural for small amp and stompbox use - they're cheap, they're robust, and they operate over a wide voltage range. Here is a nice little page about this kind of tube with links to others:
http://www.pmillett.addr.com/low-voltage_tubes.htm

I've built the headphone amp from the same site and it sounds really nice. It uses the 12AE6A (which is a triode + two diodes in one envelope) driving a solid state output stage (the AWESOME BUF634 - don't know why stompbox guys haven't played with this one yet).  The link:
http://www.pmillett.addr.com/hybrid_head.htm

This amp is designed to drive a 32ohm load - you might have to adjust the input stage for guitar input as it is designed for audio-input levels.

You could also use this tube as a booster/clipper perhaps?

ok,
erix

[slajeune]

Hi Peter,

I rechecked and you are absolutely right.  I had miswired the heaters.  Man, these tubes are very resistant.  The correct pinout is the one you pointed out.  BTW, I can now get a nice crunch outta this little amp.  VERY cool indeed.  All of this at a very nice level (actually, very amazing for 12V).  If the 12CX6 is a bit louder, I won't have to go to 24V at all!!!

Cheers,
Steph.

From the data sheet it looks like it should be +12 to pin 4 and ground to pin 5 with pin 9 unconnected. Pin 9 is the center tap for 6.3V heater operation like in a 12AX7.

Take care,
-Peter

[Alpha579]

wow, thats awesome. I might try a push pull output, how many watts do you think you could get out of it if you used more gain in the preamp, and a push pull power amp?

[Alpha579]

btw, does it distort at all?

[Peter Snowberg]

wow, thats awesome. I might try a push pull output, how many watts do you think you could get out of it if you used more gain in the preamp, and a push pull power amp?

The 12K5 will give 35-40 milliwatts @ 12V in single ended configuration.

I've been thinking about push-pull too.

Take care,
-Peter

[Johan]

I had a '67 Caddilac Sedan Devil, with a tube radio worth writing home about.
 A Really excellent to listen to radio.
 It was hard for people to believe there's was a tube in my car radio.

My 1965 Volvo Amazon ( P121 for you americans ) had a "Blaupunkt" tube radio...best car I ever had...I miss her.. :cry:

Johan

[slajeune]

Hi Alpha,

btw, does it distort at all?

Right now, I'm getting a slight breakup.  It's very nice, not death metal, but a nice distortion.  I'll try to post some clips today.

Cheers,
Steph.

[slajeune]

I have read anywhere between 35-40mW to .5 watt for the output of the 12K5.  We don't have a lot of data on these tubes (which is a shame).  My guess is that the power output is higher than 40mW but less than .5 watt.  Maybe 100-150mW for a single ended design.

BTW, 12CX6 (which is a low voltage pentode) is supposed to be about twice as loud as the 12K5.  I will try it either today or tomorrow.

Cheers,
Steph.

wow, thats awesome. I might try a push pull output, how many watts do you think you could get out of it if you used more gain in the preamp, and a push pull power amp?

The 12K5 will give 35-40 milliwatts @ 12V in single ended configuration.

I've been thinking about push-pull too.

Take care,
-Peter

[slajeune]

I have updated the schematic to correct the fact that the 12K5 was misbiased causing it to run a bit too hot.

I also played a bit with the 12CX6 this morning.  It does have a slightly mellower sound but it also has less power than the 12K5.  So, for the 12V version, the 12K5 looks to be a clear winner.

Cheers,
Steph.

[Peter Snowberg]

For more data on the 12K5 including plate curves clear to 60 volts, goto: http://www.mif.pg.gda.pl/homepages/frank/sheets/137/1/12K5.pdf

I hadn't seen anything suggesting that you can get any more than 40mW out at 12V untill seeing this diagram showing 12.6V and 40mA being drawn. :? I dunno. I would tend to trust the sylvania datasheets a bit more in any case.

I found another old radio schematic that had 22 ohms as the cathode resistor, but no 0.68uF bypass.

Other tubes worth looking at are the 12AL8 and 12EA6.

The 12DK7 is another cool one....10mW @12V

Take care,
-Peter

[brett]

45mW into my 12" 94dB/W speaker should give about 83dB.  Not bad!  A lot can be achieved with efficient speakers.  

This would really fly with something up around 98dB/W.

[slajeune]

Hi All,

just a quick note, I have added a schematic for the 12CX6 pentode amp.  It's not as loud as the 12K5 but it is more efficient (heater and current wise) and therefore, runs a lot cooler.  The major difference between both tubes, treble.  The 12K5 is more treblish than the 12CX6.  Both of them sound very nice and I encourage people to play with both versions to see which they like.

Here is the URL: http://www.geocities.com/slajeunesse/low-volume.html

Cheers,
Steph.