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Old 1st October 2010, 02:56 PM   #1
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Default 12AU7 / 12AX7 push-pull

I'm taking a shot at a mini stereo amp. Per-channel it uses 2 dual triodes to make the PI (12AX7) and output (12AU7) stages. I'm using a cheap $5 PA matching transformer for the output transformer. A couple cheap 12vac transformers will make my power transformer. I'm stepping 120 down to 12 to grab heater power then stepping 12 back up to 120 with the second transformer. A voltage doubler provides me about 300vdc. My output transformer has an 8-ohm secondary and quite a few taps on the primary.

0
62.5
125
250
500
1000
2000
4000
8000

I'll use the 4000 for my center tap and 0 and 8000 for the 2 phases. B+ will be around 225~275v to the 12AU7 and 200v to the PI.

Here's a pic of the single-ended monstrosity I built just to try it. The power stage is a parallel 12AX7 and one side of another 12AX7 for a preamp. No Criticism! I'm still on my learning curve.

I've seen a design like this somewhere online but I can't find it. Anyone have it?
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Old 1st October 2010, 03:46 PM   #2
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Will it blend?

Click the image to open in full size.
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Old 1st October 2010, 03:58 PM   #3
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I'd suggest changing to "floating" paraphase inverter for better balance. Also you're throwing away 2/3 of your voltage swing at the 12AX7 (220K plate load and 100K grid resistor). And 2K is the center-tap... but 8K is pretty low for 12AU7s
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Old 1st October 2010, 04:42 PM   #4
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Quote:
Originally Posted by Tom Bavis View Post
I'd suggest changing to "floating" paraphase inverter for better balance.
If I knew what it was.

Quote:
Originally Posted by Tom Bavis View Post
Also you're throwing away 2/3 of your voltage swing at the 12AX7 (220K plate load and 100K grid resistor).
As far as I know the 100k on the output tube's grid is to match the PI's voltage divider (100k+4.7k) on the other output tube.

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Originally Posted by Tom Bavis View Post
And 2K is the center-tap...
So 2k center tap, 0 and 8k for phases? Why is that? Shouldn't they be equal impedance?

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Originally Posted by Tom Bavis View Post
but 8K is pretty low for 12AU7s
It's a cheap easy to find transformer.

Last edited by lazzer408; 1st October 2010 at 04:58 PM.
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Old 1st October 2010, 05:51 PM   #5
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12AU7 Amp?

http://webpages.charter.net/dawill/t...Hept%27AU7.gif
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Old 1st October 2010, 05:59 PM   #6
Gilgy is offline Gilgy  United Kingdom
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Quote:
Originally Posted by lazzer408 View Post
If I knew what it was.
So 2k center tap, 0 and 8k for phases? Why is that? Shouldn't they be equal impedance?
Long story short - the impedance ratio of a transformer is the square of the turns ratio
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Old 1st October 2010, 06:03 PM   #7
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Quote:
Originally Posted by DigitalJunkie View Post
Nope that's not it. The one I'm thinking of specifically had a 70v PA transformer as it's output transformer.
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Old 1st October 2010, 09:13 PM   #8
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by lazzer408 View Post
So 2k center tap, 0 and 8k for phases? Why is that? Shouldn't they be equal impedance?
Much info on line transformers as output transformers:
Cheap Output Transformers
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Old 1st October 2010, 11:34 PM   #9
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Much info on line transformers as output transformers:
Cheap Output Transformers
Thanks. Lots of usefull data there. It still doesn't answer why I'd want a difference impedance for the 'push' then I would for the 'pull'.

I built the thing. Sounds like @#$%. More for fun then anything. I have a feeling I wired something wrong though. I had some time to kill and figured what the heck why not but not enough time to debug it. It sounded much better single ended on the 8k tap. A battery powered version of this, as a portable mp3 amp, would make a nice conversation piece. I'd make it look just like a mini DIY hifi amp.

I might fiddle around with it after I get my stereo amp finished.

Last edited by lazzer408; 1st October 2010 at 11:40 PM.
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Old 2nd October 2010, 12:00 AM   #10
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by lazzer408 View Post
Thanks. Lots of usefull data there. It still doesn't answer why I'd want a difference impedance for the 'push' then I would for the 'pull'.
The impedances are the same (symmetrical), but by using the centre tap you end up with 4ka-a rather than the 8k you expected.

For small-signal valves you really want one of those really small line OPTs, the ones that offer you several primary taps up to about 1W (usually 0.1W, 0.25W, 0.5W, 1W). That way you can get impedances up to 10k, which are more appropriate.
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