MagneQuest Forum

parafeed the output stage?

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Hi Bas:

Just as a study excercise... to begin with... I'd like to see your schematic redrawn with a plate choke on top of each anode and a dc blocking cap in each leg of the OT...

the potential advantages as I see it are manifold;

1) gets the power supply out of the music business.... i.e., you have parallel paths for the ac and dc... and you get much, much greater PSRR.

2) gets rid of the problem of trying to balance the dc currents across the primary of the OT.

3) the OT will like the idea... once you remove the dc heating current from the primary winding... it is equivalent to increasing the wire size by a guage or two if we calculate out the copper current density without the dc current component.

4) you can dispense with that 40 mfd cap... which won't provide nearly the isolation that a parafeed arrangement would anyway... and use (typically) a 5 to 6 mfd cap in each leg of the OT.

5) conceptually... to me... I've never seen the elegance of dragging the ground path of the ac through the output trans and through the necessary blocking cap... and then tying it into the cathode ground path of the first tube... always struck me as a circuitous route to ground... and I've always thought that you should want a low impedance (sort of direct) path to ground.... i.e., that you want to get to ground as quickly and directly as you can... hence... my sense is always to take your ac ground from the primary of the OT (whether se or pp) and get to ground the immediately....

but i'm no circuit genius so perhaps I'm missing the beauty and elegance of the alternate method of grounding the ac.


Now.. back to parafeeding a PP output stage... biggest drawback.... is the obvious expense of getting two plate loading chokes... less expensive and perhaps as effective may be a beefy enough CCS on top of each plate.

but... my hunch is that there are (despite the increased cost) as many benefits to parafeeding a PP output stage as there is to parafeeding a SE output stage...

be interesting to see how this would look on paper and hear comments re: the idea.


Mike



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