How much difference do output transformers do?

[duhvoodooman***]

See, I told ya Tung would have the straight poop! :

I can definitely vouch for the impact of a good quality/well-matched OT on the amp's tone. When I swapped out the stock OT from my Epi Valve Jr. with a Hammond 125ESE, it made a very noticeable tonal improvement, particularly in the bottom-end output. There's more bass now and it's punchier and better defined. Of all the many mods I did to my Valve Jr., this one had the single biggest benefit for the amp's tone. The Hammond OT was not only much better quality and rated for higher max. wattage (15W), but was also a better match to the VJr's EL84 output tube--another important factor, as Tung points out. When I upgraded a second VJr--the one Strummy bought from me--I used the 10W 125DSE model instead (still 2x the VJr's 5W rating), and it sounded just as good. Check out the difference in size and construction between the 125DSE and the stock OT:

[tunghaichuan***]

To add to what DVM said:

The Hammond 125ESE is about 4X the size of the stock VJ OT. The Hammond is made better than the stock VJ OT using better materials. Since the VJ is a single ended amp, the OT is made slightly different than a push pull OT to prevent core saturation. Physical size is also important: the larger the OT, the better the bass response.

tung

See, I told ya Tung would have the straight poop! :

I can definitely vouch for the impact of a good quality/well-matched OT on the amp's tone. When I swapped out the stock OT from my Epi Valve Jr. with a Hammond 125ESE, it made a very noticeable tonal improvement, particularly in the bottom-end output. There's more bass now and it's punchier and better defined. Of all the many mods I did to my Valve Jr., this one had the single biggest benefit for the amps tone. The Hammond OT was not only much better quality and rated for higher max. wattage (15W), but was also a better match to the VJr's EL84 output tube--another important factor, as Tung points out. When I upgraded a second VJr--the one Strummy bought--I used the 10W 125DSE instead (still 2x the VJr's 5W rating), and it sounded just as good.