12AU7 as a power output? Paralleling?

Thread Starter

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).Capture.PNG
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!
 

Thread Starter

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.
 

Thread Starter

NameET

Joined Jun 4, 2018
12
I'm really bent on building my own all-tube amp. Just as a fun little project. And yes, I have already started winding up my output transformer with 3400 turns on the primary and 75 on the secondary.
 

Thread Starter

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?
 

Thread Starter

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.

GROUNDED GRID differential SINGLE ENDED.jpg
 

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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Thread Starter

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.
 
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