# 12AU7 as a power output? Paralleling?

#### NameET

Joined Jun 4, 2018
12
Hi all,
I recently came across a guitar amp that used both sides of a 12AU7a as a 1.5 watt power output phase (connected to an audio transformer).
Where B+ is 270v DC.

I was wondering, is this a safe/practical way to design a low wattage amp? Also, would there be a way to parallel this schematic to add a little more clean headroom? I know the 12AU7 is primarily a preamp tube. I have 2 sitting in my drawer right now, and I've got the preamp thrown together on a breadboard upstairs. Any help would be greatly appreciated!

#### NameET

Joined Jun 4, 2018
12
D'oh, and one more thing. Since I don't know how to wind a center tap transformer, what would be the best way to create an artificial tap for the output? Thanks in advance.

#### MrChips

Joined Oct 2, 2009
19,728
You're planning on winding your own output transformer?

#### NameET

Joined Jun 4, 2018
12
Collin55... LOL. There have been times where I have considered doing something like this... unfortunately I really want to have that all-valve class A/AB tone in a little on-the-road package. The only issue is that I would like to get a higher power output to add a little more clean headroom... anyone have any ideas?

#### Colin55

Joined Aug 27, 2015
519
Don't forget you have DC going through the transformer so you need an air gap.

#### NameET

Joined Jun 4, 2018
12
Don't forget you have DC going through the transformer so you need an air gap.
I did gap the transformer, even so I have a 12H inductor from an old TV that I coukd easily use in parafeed.

#### Colin55

Joined Aug 27, 2015
519
Use a 6BM8 and transformer from an old valve TV

#### NameET

Joined Jun 4, 2018
12
Use a 6BM8 and transformer from an old valve TV
That might be a good option... I was messing around with AmpBooks' calculators and found that the 6922 offers some decent power too... seems like with some luck I could pull 4w from it in this configuration actually. Is there a known way to parallel cathode followers? Because in that case I could use the 12AU7a's to (maybe) get a 6-7w output.

#### Colin55

Joined Aug 27, 2015
519
I have an old steering wheel in the garage and an old starter motor from a Model T Ford. Can someone show me how to build a Maserati.

#### BR-549

Joined Sep 22, 2013
4,938
I don't look at many tube circuits anymore.....and I am no expert......but something looks cockeyed in that circuit.

Isn't a differential wanted at the input of that primary?

What does the other grid connect to?

They say this circuit will work with almost any triode.

#### recklessrog

Joined May 23, 2013
985
I built a similar amp 55 years ago and it still works, the design was in "practical Wireless" if I remember correctly, Mine has a 12AT7 as a a pre-amp and phase splitter.
The o/p transformer is a small mains heater transformer with two 110 volt primaries in series for the center tap, and it has a 5 and a six volt secondary in series that drive a 6" speaker from an old radio.
Ht is around 260 volts from a 6X4 tube rectifier, (great compression when amp is flat out)
Although theoretically all wrong, it actually sounds awesome when I crank it up with my 1959 Les Paul standard!

P.S. you need to ground the grid of the top triode in your diagram.

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#### NameET

Joined Jun 4, 2018
12
They say this circuit will work with almost any triode.

View attachment 154046
I tried this in a simulator. With a high-gain 12AX7a, I was given .7w of clean power and 2w of overdriven power. This seems to work pretty well. The transformer was 10H primary inductance at a 46:1 ratio. B+ was 260v. I was wondering what the ideal value for the cathode inductor would be? I used 2H in the simulation.

P.S. I'm sorry if I seem inexperienced, I really haven't done a whole lot of tube circuitry in my lifetime.

#### BR-549

Joined Sep 22, 2013
4,938
NameEt........sorry... I can not help you with your sim. But there are real experts here that will.

I design circuits like I type. Poke and hope for typing and mix and match for circuit construction.

For a real circuit, I would recommend using the highest voltage the tube can take.......and don't let the grid go positive. I would not try to build the transformer. Not without some experience.

The resistance of the choke should set the OP...and the inductance should shape the lower bandpass. Experiment with a pot.

Another tube could invert the phase and feed push pull. Many configurations are possible.

There are many tube audio experts here too. Their advice will result in a decent amp.