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Old 21st July 2007, 07:53 PM   #1
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Default Multiple primary windings


Is there anyone there have tried "multiple primary windings" in single ended amplifiers with more than one output tube? Instead of paralleled tubes, each tubes has it's own primary winding and output power is summed at the secondary output.

How much dc power is it possible to run though a transformer without sound problems? this is ofcourse depended of the transformer size and airgap.

Best regards Kim
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Old 23rd July 2007, 03:08 AM   #2
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I don't see where this approach offers any particular advantage as far as DC current is concerned. If you intend to operate some tube windings with reversed DC current to cancel others, you will have to do the same for the AC polarity, ending up with ordinary P-P mode.

An approach that has been used succesfully is to put a DC opposing current source (high impedance) on one winding side of a P-P transformer, although the power efficiency is poor compared to using an inductor for the same purpose.

As far as how much DC current an xfmr will take, the usual scheme is to use up half of the core DC saturation flux for DC idle current and the remaining half for AC zero to peak signal. How much current causes half the flux depends on the air gap and number of turns. A bigger air gap allows more current for a given number of turns, but also lowers the resulting inductance.

More exotic schemes have proposed using permanent magnets embedded in the core to cancel DC flux.

And using a complementary controlled current source in P-P with the driving triode tube voltage source for increased power and efficiency (ie. use a triode on one side and a high impedance pentode on the other). A further refinement being to use a precision complementing current mirror for eliminating any added pentode sonics.

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Old 23rd July 2007, 09:58 AM   #3
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This is not for PP operation nor DC canceling, it is just another way to make a PSE. Instead of directly paralleled tubes, there have to be matched, you can run each output tube in its own setup.
My current PSE has two primary windings in each transformer each connected to a 6C33C output tube.
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