I just checked some websites for info, and found that C-Core transformers are ideally used with negative source impedance when using mixed-feedback circuit designs. If you are using a no feedback design, this may not be the best choice for your application. Tamura makes some really great iron. They have a Model F5004, that uses an "Amorphous" core. Don't ask me what that is. It is advertised as the "ultimate" transformer for 2A3 and 300B. Costs $1600/pair. Magnequest makes some great iron too. They recommend their air-gapped DS-025 model for the 2A3. Price is $360/pair. You can get'em at Bottlehead. If you can find an old Citation amp, those trannies were recycled by some high end companies in some pretty expensive gear during the 80s and 90s. Notably, MFA. Good luck.
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There is no "Best"
Is this your first amp overall or your first 2A3?
If this is your first project I suggest that you don't try to build the amp of your dreams with really expensive iron the first time around. You can get great sound w/o the latest ultimate iron and learn enough to really know what you want the next time around. What exact circuit are you using?
http://www.angela.com/ link to Joe Esmilla 2A3 work with a "simple" and inexpensive Hammond transformer. It is really good too. See Joe's other site too
I'm not an old hand at his. Something that worked for me was to use a circuit that is well known and tweakable. That way there is lots of advice out there and you can pick a lot of brains and experiment with how sound changes with a given tweak. I use 300bs just to give me a little bigger speaker selection..
You can get Hammonds under $50. The Hammond 272BX power transformer has an HV 600VCT winding @ 100ma., 5V @ 2A and 6.3V @ 3A filament windings. If your circuit draws about 75ma. of current there will be a safety margin of 25ma from the HV winding.
Hammond's 125ESE is a great value for the money airgapped and multi-tap [2.5K, 5K and 10K] OPT, Highly recommended for experimentation with 2A3 circuits and DHTs.
If this is your first amp and you insist on using exotic iron I suggest get iron that has already been used and set up in the circuit (all the numbers are available) by someone with a lot of hands on experience with the circuit..
I'm really cautious only becasue i'm clueless.
Esmilla's review of a cheap T.
There is nothing wrong, and quite a bit right with a gapped C core tranny for a SE amp...
Amorphous core transformers are made from the relatively new metal alloy known as Amorphous Steel, alternately known as "Metglass" because of the way it is made and the lack of crystaline structure. It happens that it has extremely excellent magnetic qualities. They are very expensive cores and the primary effect in an audio transformer is fantastic HF response.
You need to get a transformer that is gapped properly for the current you are going to send through it. Ungapped transformers are unsuitable for most SE use. (the so-called parafeed design is an exception - I think that's what they call it)
The other thing, is that if you make the chassis layout somewhat flexible, you can put any transformer(s) in that you want. So, it may make sense to start with an inexpensive transformer.
The other option is to take the laminations apart on an otherwise non-gapped transformer, and turn it into a gapped transformer for SE use. That will take a little research, but it is a worthwhile project.
The Lundahl people make some nice iron... there are others too.