For the DC-3 or DC-5, you could pretty much look at the same specs. I think the DC-3 (all Mesa 4xEL84 amps) has a smaller OT for a couple of reasons:
1. Keep costs down
2. Keep size down
3. Make the amp easier to overdrive. The smaller the OT, the quicker it will go into saturation, meaning it can't possibly absorb anymore energy and it starts distorting.
Hammond - is a great brand. The thing I don't like about them is that they rust a little bit quickly out here, and the extra wires for the Ultra-Linear selection are a bit useless to me. They sound phenomenal though!
- Also, you have to use their impedance selector or figure out which wires to use for the output jacks
- 1645 would probably be a great choice for an amp with an immense amount of headroom.
- 1620 might be a good choice for an amp that likes to be overdriven a lot.
This guy has a reputation of being a "Hack," among Fender Vintage die hards. I don't know him, but used to have his book. His descriptions of the Hammond transformers is pretty accurate: http://torresengineering.stores.yahoo.net/hambeauttubo.html
I think they are cheaper at www.tubesandmore.com though.
Heyboer - Another great brand. Really affordable, really well built. Most of their products are built to "Vintage" specs, which means they use paper spacers, wooden bobbins and build them the way the original Schumacher (Fender) transformers were built.
- Pro Reverb or Bassman or AC-30 would all be great choices for replacement transformers, but they are way too huge for a DC-3 chassis.
- Allen Amplification has a few models that are great for Mesa amps, you might drop him an email
Mercury - These guys care way more about quality than they do price. This can get in the way of most people's budgets, but if you want the best transformers, I think there are only a few companies that can top them. Soldano uses Mercury transformers on all of their production model amps. They use DeYoung on the SLO.
- Mercury will work with you to find exactly the OT you are looking for. When I call them and say, "I have this amp, and I want it to sound this way," they tell me my options and have me email photos of the chassis (with measuring tape in pictures) in to them. They try as hard as they can to make sure that the new transformers will fit before they sell them to you.
- I have never installed one and had the customer tell me that it sounded better before.
- A little expensive, but rock solid and it seems like they are the only transformer company that will actually increase your resale value.
Another great thing to do for a 4xEL84 Mesa amp is to find a used DC-5 (Just about any 50 watt Mesa amp) (Mesa part # 562051) output transformer and install it in your amp. It's way cheaper, if you can find one, they run about $40 or so. You won't believe the difference in tone and headroom!
Just keep in mind that almost any transformer will be physically larger than the one in the DC-3.
I have had great luck with the DC-5 transformer (barely fits, you have to re-drill 1 hole).
I have also had really great luck with the Mercury direct replacement for the DC-3.