12AU7 / 12AX7 push-pull

[lazzer408]

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?

[lazzer408]

Will it blend?

[Tom Bavis]

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

[lazzer408]

Originally Posted by Tom Bavis

I'd suggest changing to "floating" paraphase inverter for better balance.

If I knew what it was.

Originally Posted by Tom Bavis

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.

Originally Posted by Tom Bavis

And 2K is the center-tap...

So 2k center tap, 0 and 8k for phases? Why is that? Shouldn't they be equal impedance?

Originally Posted by Tom Bavis

but 8K is pretty low for 12AU7s

It's a cheap easy to find transformer.

[DigitalJunkie]

12AU7 Amp?

http://webpages.charter.net/dawill/t...Hept%27AU7.gif

[Gilgy]

Originally Posted by lazzer408

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

[lazzer408]

Originally Posted by DigitalJunkie

12AU7 Amp?

http://webpages.charter.net/dawill/t...Hept%27AU7.gif

Nope that's not it. The one I'm thinking of specifically had a 70v PA transformer as it's output transformer.

[Merlinb]

Originally Posted by lazzer408

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

[lazzer408]

Originally Posted by Merlinb

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.

[Merlinb]

Originally Posted by lazzer408

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.