JackButler
Senior Member

VHT D60
Fryette D212
Memphis M30C
Fryette Valvulator GP/DI (Pending)

Much of the debate about the difference in tone between the plexi- and aluminium-panel Marshall amps comes from the fact that in 1974 Marshall's US distributor had them change all of the amps sold in the US and Japan over to the much more rugged 6550 instead of the EL34 output tube. This produced a much different sound from the EL34—a sound perceived as less smooth and more metallic when overdriven. This change was brought about due to reliability problems with the EL34's, and the 6550 generally allowed the amps to make it through warranty without problems. The circuit changes required to switch the amps were very minor, and it was easy to change from the 6550 to the EL34 or vice versa by changing a few resistor values, moving the tap for the feedback loop and rebiasing the amp