Best Answer radatats , 06 April 2014 - 11:53 AM
while they MAY be compatible, they may also be different. apparently original 6CA7's were identical to EL-3s but today's 6CA7 version is a beam tetrode whereas the EL-34 is a pentode.
Bottom line, I would not swap them out as too many other things can go wrong in that dark hole of mystery known as a DT amp... Stick with EL-34s...
Did you swap out the 12AX7s first? It may just be the Phase Inverter tube... worth a shot and no biasing involved...Go to the full post