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Single Ended Triodes (SETs), the ultimate tube lovers dream.

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RE: Webster output transformers

Posted by dave slagle on December 2, 2019 at 06:56:15:
the whole small gap thing is actually hugely misunderstood when it comes to EI lams. The only way to get a gap "suitable" for any DC offset is to have a completely discrete gap with all of the E's on one side and the I's on the other. I think much of the misinformation about alternately stacking the E's and I's in groups to decrease the size of the airgap comes from Rueben Lee

I find that this graph will work well for predicting AC permeability at a given excitation level but has no relation to DC saturation characteristics. In other words a 0.001" gap with EI lams alternately stacked in groups of 4 will behave very differently with respect to DC than a well polished C-core with the same effective gap.

I have mentioned this before in the John Hogan amp discussions and essentially adding a substantial DC offset to a PP output transformer actually linearizes the inductance as can be seen in the plot below of a ST-70 output.

given that the inductance you end up with is not huge, the listening reports seem to support that it can still sound really good and the math says response down to 40 hz shouldn't be a problem.

dave