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DUAL Output transformers


 :
9/15/1997 12:50 AM
Bruce
 DUAL Output transformers
Ok, here is what I have.  
 
2 10 watt Class A 4K output transformers from the  
same manufacturer with identical specs.  
 
All of the below is assuming I get the  
phase correct for pri/sec ... the wires  
are color coded...  
 
I want to run the primaries in series and  
feed that junction with the B+. And use the  
outside leads for each half of a push pull  
output stage. So each tube gets it's own  
output transformer, but, the two OT's are tied  
at the center and this common is where I feed  
the B+.  
In other words use them like a single PP OT.  
But the questions (other then am I missing  
something) are this:  
The secondary if these transformers are now at  
8 ohms stock. 4Kohm/8ohm  
Now if I tie the two secondaries in series  
do I have a 16 ohm output at 10 or 20 watts,  
divided the curren into both secondaries like 2  
resistors in series , each carrying half the load.  
I parallel the secondaries do I have a  
4 ohm output at 20 watts?  
Or do I series them and have a 10 watt secondary  
with an 8 ohm tap and a 16 ohm tap?  
 
By the way, I really do want to run each half of  
the push pull PA in Class A.  
I don't know why this is so difficult!  
This seems like it should be just a "solder it  
together and go" kinda project.  
But, something is bothering me about it...  
Am I thinking too hard?  
anyone?  
 
Bruce
  
9/15/1997 7:20 AM
R.G.
What you're making really IS two class A output stages run in parallel. With no magnetic coupling between the two half-primaries, there is no interaction on the secondaries, either. You have to run them Class A to keep from having distortion because they really are separate amplifiers. I'm not sure what happens if/when you try to use feedback from the secondary into the (presumably)common driver stage.  
 
So - on the secondaries, you have two 8 ohm outputs that you can connect in series to drive either two 8 ohm loads separately or one 16 ohm by placing the secondaries in series; the resulting power capability, given that you get the rest of the circuit right, is the sum from each transformer.  
 
If you try to parallel the two, you can get some interesting and possibly disturbing results. If the transformers really are IDENTICAL, then for equal primary drive, you get equal secondary voltages, and you could parallel them OK to drive  
a single load. If there is a difference between primaries, then the secondaries try to make different voltages, and fight it out. The differences are reflected into the primaries as a kind of push-back voltage on the output tube plates. Tubes being the forgiving things they are, this will probably not kill anything, but it will at least act like a different loading than you're expecting on a per-tube basis. I'm not certain exactly what effect this will have on linearity or life. If you were driving the primaries from a low impedance source, something would burn.
  

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