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Why no tiny output transformers?

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Good opt,low price,big flexiblity = audax tu101
s-l1600.jpg
 
Great link but no part numbers 😩

The 109L on page one is the closest suggestion yet relative to my hopes. I actually just bought a 109N to try as an interstage transformer.

I reached out to the author of that page and he was using a 70v transformer with 1/4W tap.

In all honesty, I didn't start this thread to try and find a work around. I was (am) genuinely curious why it seems like there are no options with a flat response below about 5 watts. I've learned a bit since then and I can see how a high enough inductance to maintain low frequency extension is in conflict with reduced size.
 
How about a 6VAC mains toroid as output transformer if you go push-pull.

I built a el84 amp using a 12VAC mains transformer as opt.
It sounds very good and the transformer was cheap too.

My last two builds use Toroids from Antek, a 10 volt secondary works great for ~4.2k:8 if you run the primaries in series, and 5 volts works for ~17k:8 for my 6SN7 builds.
 
My last two builds use Toroids from Antek, a 10 volt secondary works great for ~4.2k:8 if you run the primaries in series, and 5 volts works for ~17k:8 for my 6SN7 builds.

The transformer on the right is the 12 volt mains totoridal output transformer.

An externally hosted image should be here but it was not working when we last tested it.
 
Did you see Bill Whitlock's reference to "legendary wisdom" suggesting as a rough guide a core size of 0.34 pounds and 1.4 cubic inches per Watt for a "good fidelity output transformer" ?

Not that big for something under 5 Watts.

Mhhh..it's not clear if that rule is for PP amps or SE. He mentions that rule where he talks about PP amps and then the SE amp "section" is detached...
Should look into the reference (Langford book).

Anyway for PP it's overkill. For SE is ok.
According to that formula the core weight should be over 15Kg for 100W....it sounds ok to me only if SE.

For PP, if you specify the full power @28-30Hz for a power bandwidth (-3 dB) starting from 20-22 Hz you will save quite some weight. In my opinion is it's a more rational solution. This has nothing to do with frequency response that will typically be -3 db around 1-2Hz!
For example, 100W @30Hz with max AC induction of 0.85T and 1A/mm^2 for AC current density and approx. 0.2 dB loss will require an EI core of 5Kg. It could be the EI-135 (European metric) with 45x60 mm core area which is already a bit oversized. 4.5x55 would weight 5Kg....This would be an excellent transformer from any point of view. No compromise if properly wound. Granted!

If I want to exaggerate and get the same performance above at 20Hz then I would need a core of about 6.5Kg.


For SE, and same performance, I would need 4x weight because of DC current flowing.

Less weight for C cores.
 
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In all honesty it was just curiosity. However, the day the thought occurred to me I was thinking about a push pull amp using a dual triode (I’m partial to ECC99) for a little desktop amp. That Hammond 109 with a 10k:3.2 would probably work.

I think it's a better idea. I have myself as back-up amp PCL86 SE amp on my bench to listen to the radio. It's nice but and if one wants excellent performance the transformer is not can't be cheap. Not because of the materials but for the work needed. Yes, there are cheap SE transformers around but honestly one only gets what he is paying for in return.
For PP it's much better scenario in this respect.
 
Recently, I bought two Lundahl LL2766 SE OPT for building of the stereo 45 tube power amp. The size of the transformer is quite small in comparison with the LL1620 OPT that I used to use. Even though the low frequency can't match with the LL1620, the bass performance of the LL2766 is reasonably good and exceed my expectation.
 
Without going too much into details, C cores are more efficient because grains are 100% oriented. They can work up to 1.6 T induction if good grade (possibly 1.7T in best grades) without relevant distortion at low frequency, at least down to 30Hz in SE applications and lower in PP. In EI cores only about 2/3 of the grains are oriented and saturation doesn't surge uniformly. Saturation hots spots do have a visible influence on distortion and inductance. To get a performance close to that of C cores one has to stop at about 1T or accept some more distortion below 35-40 Hz for running up to 1.2T. So for the same core area and turns one can apply 30-60% more volts resulting in more headroom or can reduce the size of the core for the same headroom. Toroidals too are like C cores.
 
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