The two main transformers in an amp are the power and output transformers. The Power Transformer (PT) takes wall voltage of 115VAC and transforms it into a very high voltage tap, a low volt heater supply 6.3VAC and can other secondary outputs as well. We do not recommend replacing a PT if it's in good working order. A new PT may sound a bit better, but not much. The output transformer (OT) converts the high voltage low current signal to a low voltage high current signal that can drive a speaker. 100% of your tone goes through the OT and we highly recommend upgrading the OT.
Many amps also have filter chokes and reverb transformers. A reverb transformer converts the signal from the tube High Impedance to a Low Impedance signal that can drive the spring tank. If yours works leave it be. The choke is the second recommendation we can make, especially if your amp does not already have one. A filter choke reduces AC noise and ripple and can greatly increase the amps bass response.
We offer replacement transformers from the following companies.
SOURSOUND - If you have not heard of SOURSOUND before that is not surprising. The company owned by Bryan Sours has been around for a while making amps and effects and during the pandemic Bryan's focus shifted to transformers. He makes a line of reproduction transformers and also does custom design and winds iron for some of my favorite amp companies. Made in small batches of 25 by hand in Portland Oregon. Bryan is a true artist of the transformer. He is now making transformers for me, based on my specs. We are currently offering the OT-18 for the Blues Jr and Pro Jr, and offer in-shop install of his entire line. We will expand the line to include all the amps we have mod kits for. Our new amplifiers (coming soon) will all have custom designed SOURSOUND output transformers.
Mercury Magnetics - Mercury has been making quality transformers in sunny California for decades and they have a massive line of products available. We offer replacement transformers for the amps we have mod kits for and are a dealer of their entire catalog. That being said if we don't have it in stock we can order from Mercury. They also have a retail site where you can order direct. We have offered Mercury since I started the company in 2008. If you have ever played an amp I built before working with SourSound it has Mercury Inside!
Hammond Manufacturing - In addition to making the infamous 1590BB pedal enclosure they make really great transformers. We offer them in the Marshall Class 5 Z mod kit and others. Hammond is a great option for power transformers, chokes, and price conscious OT's. Made in Canada they are a Mass Produced quality product at a good price point for iron that is not as tone critical as the output transformer. For example the soon to be released JOEDAY will have a Hammond PT and Choke with OT by SourSound. We often use them in the shop for PT replacements as well.
If you really want to dive deep into the technology of transformers. Bryan Sours was interviewed on a pod cast on the Fretboard Journal that is really amazing.