TO20B Premium Low-Profile Output Transformer
Introducing… the TO20B
David Allen and I have been working on a premium quality output transformer for well over a year. We have developed a new bifilar wound unit (the center tap is not only the center of the number of turns but also exactly one half of the total resistance) with improved bass response. We’re calling it the TO20B because its specs are the same as the TO20, but it is a different design internally. It is a more “hi-fi” approach than our old school/vintage style TO20. Like the TO20, it uses the more-efficient, grain-oriented M-6 lamination steel, has pretinned leads, and, of course, is made in the U.S.A.
The TO20B features an electro tin plated finish more common to vintage OTs and chokes. Rated 6,600 ohms primary to 8 ohms secondary, it is ideal for a 2xEL84 or a 2x6V6 amplifier using an 8 ohm speaker load. It’s the same physical size and weight as the current TO20 unit. With 2-13/16″ mounting centers it is a perfect upgrade for many popular amps and drops right into the Blues Junior, Princeton Reverb, Pro Junior, the Excelsior, some old Gibsons and others. $69.
We’re keeping the TO20 in the lineup; we have no plans to discontinue it. So why would you want to spend the additional amount for the TO20B?
Here are the similarities and differences between the TO20 and the TO20B:
Both are “deep stack” low-profile transformers with considerably more core than the transformers they replace.
Both have 2-3/16 inch mounting centers so they are drop-in replacements.
Both use M-6 grain-oriented steel cores. We tested other core materials and M-6 delivers the richest, deepest bass, and more of it. It magnetizes and demagnetizes faster and more completely than other steels, so the transformer is more efficient at every frequency, although it’s easiest to hear in the bass.
Both have 6,600 ohm impedance into 8 ohms, a good match for EL84 and 6V6 tubes. Both can easily handle 20 watts with essentially no thermal rise.
The difference is in the windings. The TO20 is wound in layers. This is the vintage, or traditional way that output transformers are made. The two halves of the primary side, which connect to the output tubes, are interleaved with the secondary, or speaker side. One of the primary sides winds up as the outermost layer. The circumference of the wrap is greater on the outside, of course, since there is more wire. Thus the DC resistance of one side is greater than the other. It is also farther from the core, so the magnetic field is slightly weaker. The extra wire and weaker field balance one another out, so the impedance, or AC resistance is identical on both halves, even though the DC resistance is different.
The TO20B is bifilar-wound, which means that the two halves of the input side are wound simultaneously. They have identical DC resistance and AC impedance. This is a more difficult, more expensive process that uses different machinery than interleaved transformers.
Either the TO20 or TO20B will perform beautifully in your amp, but the TO20B has a slight edge in bass performance and overall efficiency.