These things are built like anvils. This one had 12AX7s throughout the preamp, but only half of the official Ampeg "12AX7 modification" had been performed on the circuit.
Ampeg released those modifications out of necessity when 12DW7s were discontinued. However, even with the circuit modifications, 12AX7s compress the signal way too much. So, I reverted the circuit back to stock and threw in some NOS 12DW7s - now it sounds like an SVT!
I also replaced all the electrolytic capacitors, and fixed a broken connection which prevented the channel one midrange control from functioning.
