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Center Tapped Mains Transformer as Parafeed Plate Choke

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Just out of curiosity, would the attached schematic work? Perhaps channel separation would be poor?

As the plate DC currents flow on opposite direction on the primary, they cancel the magnetic flux. There should be no core saturation and inductance would remain high.

Perhaps high enough VA rating is required to get the primary inductance for decent bass? How high? Assume say 5K:8 OPT.
 

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It could work but the mains transformer will have fairly low inductance. You really want your plate choke to have high inductance since the OT and choke effectively have their inductances in parallel which like resistance lowers their value at the critical bass frequencies.
I once used a Microwave Oven Transformer as a plate choke but it produced some nasty phase shifts in the bass area. You should expect the same with your proposal.

Shoog
 
Channel separation will be nonexistent with this setup, because the center tapped primary of the mains transformer consists of two magnetically coupled windings in series. An AC voltage on one anode will appear in reverse phase on the other. When both tubes are driven with the same signal, the tubes fight each other, and the net result will be no signal whatsoever. To put it down briefly: this setup does not work at all.
 
The idea behind parafeed is to relieve the tension between two conflicting demands on the output transformer: high primary inductance and low inter/intra-winding capacitance while at the same time exhibiting low leakage inductance (is equal to good coupling between primary and secondary). Especially with a single ended OPT, a high primary inductance requires a large core and a large amount of copper, at the cost of winding capacitance and coupling. Winding techniques to reduce intrawinding capacitance also tend to reduce coupling between windings, and so do not achieve the ideal.

With a parafeed, you can wind a plate choke for high inductance and low intrawinding capacitance, and not worry about coupling because there is no secondary. This is relatively easy to achieve. The output transformer carries no DC bias, and thus can be made without an air gap. That transformer now can be made with a sufficiently high primary inductance, while being physically a lot smaller than a SE transformer of similar power, so there is extra room in the design to reduce capacitance and leakage inductance.

This performance benefit can only be realized with a plate choke specifically made for parafeed use. A mains transformer is optimized for power transfer at low cost, not for low capacitance. It employs none of the winding techniques that make it a good plate choke. Using it in this role will flush all potential benefits of the parafeed concept down the toilet, no matter how good your output transformer is. This is because, from an AC point of view, the OPT primary and the plate choke are connected in parallel. A too low inductance of any of them will short out the bass, and a too high capacity in any of them will short out the treble. DC bias in the primary of an ungapped transformer will cause saturation of the core and the subsequent collapse of the primary inductance, so mains transformers are only good for limited fidelity push-pull output stages.
 
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timpert said:
Channel separation will be nonexistent with this setup, because the center tapped primary of the mains transformer consists of two magnetically coupled windings in series. An AC voltage on one anode will appear in reverse phase on the other. When both tubes are driven with the same signal, the tubes fight each other, and the net result will be no signal whatsoever. To put it down briefly: this setup does not work at all.
Separation will be poor, but not nonexistent. The 'PP' transformer is likely to have a higher impedance than the combination of anode and speaker load (otherwise you get LF rolloff) so although it will try to transfer signal across to the other channel (in antiphase) it will only have limited success. As I said, it all depends on transformer inductance.
 
Channel separation will be nonexistent for all practical purposes. The transfer of signal from the "left" channel to the "right" channel will depend on the coupling between both primary halves of the top transformer. Ironically, in a typical center-tapped mains transformer, the halves of the primary are probably the best coupled winding pair you'll find. Any channel separation between left and right will exist due to leakage inductance (i.e. imperfect coupling) between these two windings, so at best you can only get some channel separation at high frequencies. Al low frequencies, the transformer will force the stage to operate as a push-pull circuit, and succeed quite well in this.
 
Yes, but for this, you'll have to tie both cathodes together and lash it up as a long-tailed pair with one grid grounded. It has been done before (it saves parts), although I can't quickly recall an example. Of course, you'll have to use the top transformer as a conventional PP output transformer and leave the side transformers out.

Edit: here's an example:
http://www.ampbooks.com/mobile/amp-technology/6L6-phase-inverter/
 
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