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Thursday, March 22, 2012

New Lundahl Line Output / Interstage Transformer


In Lundahls line output transformers I always missed one which offers a ratio close to 6:1 step down. There is the excellent LL1660 with 4.5:1, the LL1692A offering 3.5:1 and the LL1689 with a high 9:1 step down ratio. After some email exchange Per Lundahl agreed to develop a transformer which fills that gap. Here is the new LL2745:

It has the same primary windings as the LL1689, only the secondary is wound such that it offers a 2.8+2.8:1+1+1+1 ratio. The transformer can be used for other purposes besides 5.6:1 step down. As with the other types, the winding arrangement can be combined in different ways, for example to get a 2.8:1 step down. It can also be used as interstage transformer, for example offering a 1:1.4 or 1:2.8 step up or step down.

I asked for this transformer as an ideal line output to go with tubes like the 26 or UX201A. For these triodes with a higher rp, the step down ratio gives a lower output impedance as the LL1660, but does not loose as much gain as the LL1689. Especially when a real 600 Ohm impedance should be driven for example wtih the 26, the LL1660 would not provide enough step down.

I got my first pair of the LL2745 gapped for 8mA, to be used with the 26. I will write about this project when it is getting built. Stay tuned.

Best regards


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