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RE: Maybe we need to define "little"?

Are we talking about two amps that use the same tubes and have the same impedance transformers? Or are we talking about a transformer for EL84's compared to one for 6550's? Very different thing.

I'll assume we are talking about two transformers rated for the same tubes, the same loads, and same output power. Lets say a 10K to 8 ohm 15 watter for EL84's.

What does a physically larger transformer typically give us? More inductance....which lets us play lower bass at higher power levels. Most units will do the quoted 20-20khz at one watt but few will do it at full rated output power. One could say the more inductance the more the high end has to suffer as a gross general comment.

So perhaps if you don't intend use more than a small fraction of it's rated power....and you don't need low bass....then a smaller transformer will suffice....and might even do better on the highs.

But there are some other very important factors. Some to do with the transformer and some to do with the audio circuit.

Feeding a 5 hertz signal into any OPT at any reasonable power level is going to saturate the core. One needs to design the circuits so there is an absolute sharp cutoff at some point. Best to have one on the high end too.

Now lets think a bit about how a transformer is made/wound. You are going to have something like a 2,300 turns of small gauge wire in the primary and maybe 98 turns of heavier gauge wire on the secondary (I am not going to do the math to make this realistic but it is close enough). This wire is going to wound on a single bobbin. It is push pull so you have to have an even amount of primary windings. You need the same number of turns for each tube and the same DCR. You also want to completley fill the window in the EI core.

So you wind the first primary (P1) on the bobbin. Then you wind the secondary (S1). Then you wind the second primary (P2). All of these windings go on top of one another and in a vintage transformer there is paper insulation between them. Now the astute reader will see that the length of wire for winding P2 has to be longer then P1. Therefore DCR can't be equal. Maybe we don't care because we have made no provision to balance current in the output tubes. Maybe we don't have a phase splitter that always provides equal and opposite drive to the output tubes? Maybe those issuses are a far greater error than the difference in DCR in the transformer. So why pay for a better transformer?

Now what could we do to make matters better? Suppose that we had two different bobbins so that P1 and P2 could be laid down first with equal turns and DCR. Then S1 was split in half between the two bobbins (S1 and S2).

Bottom line, good transformers might have better material for the core (grain orientated high silicon), might have a different core shape (C-core), might have several primary and secondary windings (P9 and S7), might lay down primary and secondary windings together rather than on top of each other (bifilar). Etc, etc, etc:) Way more stuff than I know about. Rest assured the really good stuff is way top secert!

But again, if you aren't going to make the amp circuit good enough to make the transformer the weak link why pay for a better transformer? Besides, sometimes it is the imperfections that make something special and more appealing. And that might be why you are here instead of enjoying some solid state amp:)

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  • RE: Maybe we need to define "little"? - Russ57 09:48:16 06/04/17 (1)


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