Tube chart · Guitar amp
Tubes for Marshall JTM45
Era: 1962–1966Power: 30 W
The original Marshall — Jim Marshall directly cloned the Fender 5F6-A Bassman circuit but substituted KT66 GEC power tubes for the 5881. That single swap is the founding moment of British rock guitar tone. Eric Clapton recorded "Bluesbreakers with John Mayall" through a JTM45.
Complete tube line-up
Every socket in the JTM45, with stock tubes, direct-swap alternatives, and rolling notes. Tube names link to dedicated specification pages.
Important: bias before you swap
Power tube swaps almost always require rebiasing — running a new quartet of KT66 cold or hot will shorten tube life dramatically and can damage the output transformer. See the biasing guide and the troubleshooting guide before rolling tubes.