Tube DIY Asylum

RE: But have you seen the price of 211's lately?

151.197.218.65

In Reply to: RE: But have you seen the price of 211's lately? posted by caffeinator on May 11, 2008 at 14:48:31

caffeinator asked:

"Can you parafeed an air-gapped SE transformer like the FS-100?"

I give you a quick yes and no answer to the above.

Yes. And it will work electrically--- you'll get sound out the other end.
In blocking the DC you will decrease the flux density by a whole lot (roughly speaking by say 50 to 60 percent). You will probably see some very small increase in the primary L. But not a huge increase (like if it had been designed as a parafeed unit)--- because the presence of the airgap will still murder the perm vis-a-vis if it had been stacked with no intentional airgap. You will get some increase in PSRR if you parafeed this trans.

Here is the no part. Say you use a 125 henry plate choke--- remember that this plate choke is in parallel with the L of the output trans' primary L.
The output trans will have the smaller L--- so that the paralleled L's will actually yield you less L then using it as a series fed airgapped unit (which is what it was designed for).

In the case of using a parafeed output trans and a plate choke--- the output trans will usually have a much, much greater amount of L than the plate choke itself. So here the plate choke dominates. You want the primary L of the output to be much greater than the L of the plate choke.

and--- perhaps most importantly--- throw all of the above out the window. From a practical point of view--- you already have the FS-100. Using it with a 211 at the Stereo Sound operating points (i'd bet money) will produce a good sounding amp assuming you do all the other things well in sourcing of parts, layout, and etc.

what I am trying to say is--- you already have the FS-100's--- if you went to sell them even if they are unused your still going to sell at the "used" price--- so why not actually use them--- build the amp--- listen to it--- optimize it--- and get some ears on with the good iron you already have.

After some time--- if your curious about parafeed (and I prefer parafeed if you were starting anew)---- then make the switch--- plus you won't have to spring for plate chokes right away if you build a conventional series fed amp as a starting point.

I'm a little tired--- I probably could have done a better job in explaining the technical points--- but I think the more salient point may bee that you have the iron--- why not use it as advantageously as you can.

And here I would say if you have an airgapped FS-100--- then build a circuit for which the trans was moreso optimized for--- which is a conventional series fed not parafeed.

MSL

Builder of MagneQuest™ & Peerless™ transformers since 1989

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