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Author Topic: 12v 12U7/12AL8 amp, have a few Q's  (Read 33450 times)

joecool85

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12v 12U7/12AL8 amp, have a few Q's
« on: August 11, 2007, 12:30:04 PM »
I'd like to build this tube amp using a 12U7 and a 12AL8 tube.


More info here:
http://www.sophtamps.ca/mambo/index.php?option=com_content&task=view&id=25&Itemid=37

Now, I've never built a tube amp before and I've always wanted to.  This little fella seems pretty cool but I have some questions.  What do I use for a PSU?  Could I basically just use a 12v regulated source and a 5000uF cap for filtering?

Where does the volume pot go on this thing?  Could I have a gain control on this?

I've priced out parts and I can get everything minus an enclosure and a speaker for around $50.  I'm going to do a lot more reading on tube stuff before setting out to build it, as I always research heavily before getting into a new project.

Also, what about biasing the tubes?  Hows that work?

Yes, I am a tube newb.
« Last Edit: February 17, 2008, 07:36:26 PM by joecool85 »
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teemuk

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #1 on: August 11, 2007, 01:42:22 PM »
5000 uF seems awful a lot. You need that kind of capacitance for high power amps; I think that circuit would work perfectly with less than that. 12V supply should work, just make sure it can supply enough current for the filaments.

The volume pot is in your guitar but I think you can convert either R5 or R6 to a potentiometer to get a volume/gain control to the amp itself.

Biasing tubes is pretty similar process to biasing JFETs. Why worry; it looks like the circuit has pretty fixed bias already. Anyway, if you look at the page you provided the link to more carefully you will find a "Theory" section with some information about biasing the gain stages. I assume AX84 site (and several other tube sites) should contain plenty of info about biasing.

joecool85

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #2 on: August 11, 2007, 06:30:19 PM »
5000 uF seems awful a lot. You need that kind of capacitance for high power amps; I think that circuit would work perfectly with less than that. 12V supply should work, just make sure it can supply enough current for the filaments.

The volume pot is in your guitar but I think you can convert either R5 or R6 to a potentiometer to get a volume/gain control to the amp itself.

Biasing tubes is pretty similar process to biasing JFETs. Why worry; it looks like the circuit has pretty fixed bias already. Anyway, if you look at the page you provided the link to more carefully you will find a "Theory" section with some information about biasing the gain stages. I assume AX84 site (and several other tube sites) should contain plenty of info about biasing.

Thanks for the reply teemuk.  I mentioned a 5000uF cap because I have a 25v 4600uF cap sitting around doing nothing, I think I might put it to use.  Also I asked about the pot because on the page it mentions "Please note that the pots should be log or audio pots."
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joecool85

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #3 on: February 17, 2008, 07:40:54 PM »
I'm back to thinking about this project again, I'll probably be ordering parts soon.  I read at the bottom of the page (http://www.sophtamps.ca/mambo/index.php?option=com_content&task=view&id=25&Itemid=37) that you can "add a volume control between the first two gain stages."  Would that be R5?  If so, what value pot should I use to replace R5 with?  If I am correct, R5 would work more like a master volume control and then if I put a pot at the beginning of the circuit replacing R1 that would be like a gain control?  I'd like to have both a gain control and a master volume on this amp, that'd be pretty cool.
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teemuk

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #4 on: February 17, 2008, 08:03:45 PM »
The way I see it is like this: The closer to power amp you put the potentiometer the more “master volume control” it becomes. If the potentiometer is in the very first stages it pretty much controls how much signal you send to the following stages, which in turn determines how much they will get overdriven. I guess this is what people refer to as “gain control” although it does not really control gain. The master volume control on the other hand, is often pretty much the last control determing the signal amplitude feeding the power amp (and in some very rare cases it may even follow the power amp or control its gain). So, “master volume” control stereotypically should have little effect on gain of the preamp.

I think there is not much difference between R5 and R1 because both are located in quite early stages. I think they suggested that the control should follow the first gain stage because after introducing some gain the SNR remains way better. (I think you can see the difference of R5 in comparison to R1 on that part). I would put the “master” on R8 (if it doesn’t affect bias) or perhaps to R6. “Gain” to R5. I think you can just substitute R5 with a 1Meg potentiometer and connect its wiper to the tube’s grid.

joecool85

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #5 on: February 17, 2008, 08:44:07 PM »
Alright, so R5 for the "gain" control.  Now how do I find out if switching out R6 or R8 would effect the bias?
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joecool85

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #6 on: February 18, 2008, 04:24:09 PM »
Alright, so R5 for the "gain" control.  Now how do I find out if switching out R6 or R8 would effect the bias?

Also, if anyone has information on biasing (tubes, transistors, anything) that'd be great.  I don't even know exactly what "biasing" something means lol.
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numpsha

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #7 on: February 18, 2008, 10:38:23 PM »

Warning, learning curve ahead!

"Biasing in electronics is the method of establishing predetermined voltages and/or currents at various points of a circuit to set an appropriate operating point."

http://en.wikipedia.org/wiki/Voltage_bias

Google: biasing transistors or biasing tubes or read Teemu's book for more in depth info

The 'Biasing' of tube amps commonly refers to "... setting the idling current in the Power Tubes in PushPull output stages" *

*Long definition:
http://www.lynx.bc.ca/~jc/bias.html

As per Teemu's advice:

http://ax84.com/

Take 1 faq, articles as tolerated, digest well and post results in SS,Tube/Hybrid and Hampster Wheel Guitar Amp sub-forum in the morning.

Caution: May be habit forming! Support groups (tube forums) work best for most.

Dive in, dude, the water's fine!

teemuk

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #8 on: February 19, 2008, 03:13:42 AM »
Take a look at project P1 theory document from AX84.com.

joecool85

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #9 on: February 19, 2008, 12:06:38 PM »
Thanks for the help guys, looks like I'll be doing some reading.
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joecool85

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #10 on: March 03, 2008, 11:15:05 AM »
Alright, my parts came in :-)

My 70v line transformer may be an issue though.  I tested the resistance and the highest I got was on the 0.62w tap @ 259 ohms.  I've read that the 12AL8 needs ~ 800 ohms to function properly, but I've also read on other schematics that as a Class A amp it can drive 480 ohms.  Would it kill the tube to try it at 259 ohms and then add resistors until it sounds "just right"?

**edit**
I emailed Stephane (sophtamps.ca) and this is what he wrote back:

Quote
Hi Joseph, is the secondary of the transformer connected to an 8 ohm
speaker / load?  Not sure that you can judge a transformer by measuring
resistance.  What you are seeing is that the 10W has less windings than
the 0.62W and therefore, an 8 ohm on the secondary will reflect less load
(i.e. 500 ohm on the 10W) than the 0.62W (probably close that 5000 ohm).

I would start with the 10W and see how it sounds.

I tried shorting the 8 ohm connector so that it would see a "load" and it didn't change anything for the resistance of the primaries.  Is he right that I should try the 10w winding even though it showed up as only 30 ohms resistance?
« Last Edit: March 03, 2008, 07:52:01 PM by joecool85 »
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joecool85

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #11 on: March 06, 2008, 09:34:51 AM »
Turns out he was right, I need the 10w tap.  You can't just measure the resistance of the tranny, you need the impendance.  To get that you need to figure out the windings ratio (12:1, 30:1 etc) then multiply out for what the tube "sees."

Example:

14:1 ratio
14 x 14 x 8ohm load = 1.568k that the tube "sees"
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numpsha

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #12 on: March 06, 2008, 05:12:03 PM »
Quote
How do I know what pins go to what wire?  I'm working from this schematic: http://www.sophtamps.ca/schematics/sopht_12al8-v2.jpg
It says about how to wire the heaters, but that's it.

Major downloadable tube database with specs (including pinouts) plus links to several tech sites for additional info (application notes, etc.) for just about any tube you're likely to run across.

http://duncanamps.com/tdslpe/index.html

Have you been doing your homework, Beaver?
Oh, Ward ... leave the poor kid alone!

 ;D

Rob

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joecool85

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #13 on: March 13, 2008, 07:49:58 PM »
Alright, I got it built and it worked first try!  I originally used the 70v 10w tap, then tried the 5w and later 0.62w and found the latter along with 400 ohms of resistors to be the best sound, I may try another 200 ohms and see what it's like.  It is pretty quiet, but is what I would expect for only 40mw.  I did some quick recording and it sounded pretty good, just loud enough for my mic to be happy about it.  It needs more gain, so I'm going to tweak it in a few days when I get the chance and see what I can do about that.  I ran a few pedals in front of it and it takes quite well to them, even a metal distortion pedal no less.

All that said, I'm not hooked on tubes - yet anyway.  I'm still happier with my SS amp setups.  I am thinking that a high gain tube preamp (maybe two 12u7's) hooked to a SS poweramp might be a good idea still though (I've always felt this way).
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joecool85

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Re: 12v 12U7/12AL8 amp, have a few Q's
« Reply #14 on: March 18, 2008, 04:09:40 PM »
What's the big deal with ohms and tubes anyway?  Everyone always worries about having a 4ohm, 8ohm and 16ohm jack.  I've hooked this thing up to 4ohm, 8ohm and 32ohm (headphone) as well as straight into the input of my K-20X with no issues or noticeable change in sound quality.  I also tried all the different taps on the primary of the transformer with little/no change.  Maybe the 12AL8 is pretty flexable though.

Is there any reason it would be bad to run the 12AL8 through a transformer and use the 4 or 8ohm out for a line out to an amp?  It seemed to work fine, but what do I know  ::)
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