From a rock perspective, if there’s a “where it all began” amplifier, then Fender’s tweed Bassman of 1958-’60 is it.
The 5F6-A circuit was the pinnacle of the tweed Bassman’s with updates in output and presence control. Jim Marshall was so impressed with this circuit that he essentially copied it for his early Marshall JTM-45 amplifiers.
*This amplifier is from Michael DeTemple’s private collection. Not all caps and resistors are original