Author Topic: OT: simple tube amp on a wallwart -> schematic inside!  (Read 17237 times)

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slajeune

OT: simple tube amp on a wallwart -> schematic inside!
« on: May 21, 2004, 10:38:48 PM »
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

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #1 on: May 21, 2004, 10:55:05 PM »
COOOL! I gotta try this!

Peter Snowberg

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #2 on: May 21, 2004, 11:39:11 PM »
That's VERY COOL!  8)  8)  8)

One question....

On your heater connections you say
Quote
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
Where are you? Add yourself to the DIY Map!

slajeune

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #3 on: May 22, 2004, 07:02:29 AM »
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

Old Caddy Radio
« Reply #4 on: May 22, 2004, 07:14:41 AM »
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.
Convention creates following, following creates convention.

slajeune

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #5 on: May 22, 2004, 12:32:33 PM »
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

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #6 on: May 22, 2004, 01:22:31 PM »
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

  • Guest
OT: simple tube amp on a wallwart -> schematic inside!
« Reply #7 on: May 22, 2004, 02:46:16 PM »
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
« Reply #8 on: May 22, 2004, 03:34:42 PM »
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

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #9 on: May 22, 2004, 09:43:37 PM »
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.

Quote from: Peter Snowberg

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

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #10 on: May 22, 2004, 11:57:59 PM »
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?
Alex Fiddes

Alpha579

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #11 on: May 23, 2004, 12:11:22 AM »
btw, does it distort at all?
Alex Fiddes

Peter Snowberg

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #12 on: May 23, 2004, 01:15:10 AM »
Quote from: 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?

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

I've been thinking about push-pull too.

Take care,
-Peter
Where are you? Add yourself to the DIY Map!

Johan

Re: Old Caddy Radio
« Reply #13 on: May 23, 2004, 04:10:00 AM »
Quote from: 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.


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

Johan
DON'T PANIC                                                                   www.myspace.com/dieseldownsweden
                                                                                      www.dieseldown.se

slajeune

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #14 on: May 23, 2004, 05:15:13 AM »
Hi Alpha,

Quote from: Alpha579
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

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #15 on: May 23, 2004, 05:19:15 AM »
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.

Quote from: Peter Snowberg
Quote from: 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?

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

I've been thinking about push-pull too.

Take care,
-Peter

slajeune

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #16 on: May 23, 2004, 10:40:49 AM »
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

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #17 on: May 23, 2004, 03:57:45 PM »
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. :D

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

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

Take care,
-Peter
Where are you? Add yourself to the DIY Map!

brett

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #18 on: May 23, 2004, 09:27:16 PM »
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.
Brett Robinson
Let a hundred flowers bloom, let a hundred schools of thought contend. (Mao Zedong)

slajeune

OT: simple tube amp on a wallwart -> schematic inside!
« Reply #19 on: May 24, 2004, 10:57:07 AM »
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.