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 zilch99 13th January 2013 10:05 PM

tube output question

Single ended, push pull outputs , I have a pretty good idea how these work.
In a single ended amp an output transformer with an air gap is used so the core doesn't saturate from the DC current through it. The air gap reduces the tranny's efficiency.
In a push pull output, the magnetism[flux] in the core from the idling current of 1 output tube is cancelled out by the opposite flux from the idling current of the other tube. Hence an air gapped output transformer is not needed.
My question:
Can you use a push pull type output transformer in a singe ended output IF:
you run a DC current through the other winding adjusted so that the flux from the DC powered winding cancels out the flux from the idling current of the output tube?
At idle, the flux in the transformer would be near zero and the flux that creates the output signal swings above and below zero rather than 1 direction only. Since we are talking about 10s of ma. through the winding across the[fairly low] DC resistance of the winding, the extra dissipation should not be very high.

 TonyTecson 13th January 2013 10:09 PM

 DUG 14th January 2013 12:27 AM

If the push-pull transformer is rated for class A operation it could be done.

However, the output tube would be trying to develop signal into the balancing resistor as well as the output load. (speaker)

If the push-pull transformer were not rated for class A then you could increase the bias slightly as long as the DC component did not saturate the core.

If you did not increase the bias to make it a class A operation then you would get one half of the AC waveform. (Class AB with one side working)

IMHO

:)

 Dr-Joned 14th January 2013 12:37 AM

Look up some of the Patents for Randall smith with mesa/Boogie amps. He did something very similar.

 Wavebourn 14th January 2013 01:05 AM

We discussed this topic before, and were several projects developed, like triode on one side VS pentode on another, triode on one side VS modulated CCS on another, and so on, including just CCS on another side to compensate the current.

 zilch99 14th January 2013 08:13 AM

Thanx
I think I will stick to tried and true designs.

 palmas 14th January 2013 08:30 AM

Quote:
 Originally Posted by zilch99 (http://www.diyaudio.com/forums/tubes-valves/227737-tube-output-question-post3325056.html#post3325056) Thanx I think I will stick to tried and true designs.
The fun part is to boldly go where no man has gone before!
To have the same everyone has, just buy from the shelf!

 DF96 14th January 2013 10:01 AM

Quote:
 Originally Posted by zilch99 The air gap reduces the tranny's efficiency.
The air gap reduces the inductance. This may indirectly reduce the efficiency a little as more turns are needed to regain the lost inductance so more copper losses. Even gapped transformers remain quite efficient.

Quote:
 Since we are talking about 10s of ma. through the winding across the[fairly low] DC resistance of the winding, the extra dissipation should not be very high.
It depends where you get this extra DC current from. The efficient method would be to use a DC supply attached to the main supply rail. The grossly inefficient method would be to just use a resistor to ground, which will get hot.

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