Post by bluegrassdan on Aug 20, 2017 0:40:16 GMT -6
I did the "CJ Mod" to my ptp La2a the other night. Swapped out the 12ax7 for a GE 12ay7 and changed the cathode bias resistors from 220k to 100k. Also, added a small bypass cap to C4 which helped straighten out the standard 10k dip in frequency response. It's just about flat in frequency response now.
I switched mine on my D-la2a to 12ay7 and it definitely opened it up and made it more 3D and overall even more usable. It's also easier to find good 12ay7s. Oh and to quote Kingston at GDIY:
"Changing those 220k plate resistors to something lower is indeed a good idea. The 220k are only good for the really high mu 12AX7's. You could even go and check the tube datasheet if you really want to optimise them (and the cathode bias resistors) to a certain tube. Making the distortion more symmetrical is not automatically better of course."
A lot of those old broadcast and recording implementations did stick with 220K-240K R's and still went much higher in voltage. In some cases first stage plate R's were as low as 39K, operating with fairly high current.