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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 plate 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.
Sounds less woolly in the midrange. Love it!
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Post by EmRR on Aug 20, 2017 7:00:57 GMT -6
Less NFB, better sounding tube. Yes. I'd go so far as do it with octals to get it even better, but impractical unless you go point to point. You mean plate resistor, of course.
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Post by jimwilliams on Aug 27, 2017 17:03:32 GMT -6
12AY7 likes 90 volts at the plate. With the stock 220k plate resistors you will see 120 volts at the plate, too high. Lower those to 100k and it will be higher.
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Post by EmRR on Aug 27, 2017 18:43:37 GMT -6
Why 90V? RCA ran 12AY7's in broadcast equipment anywhere from 112V to 269V, Altec 75V at lowest and 260V at highest. They all work. This mod is longstanding and well documented.
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Post by rocinante on Aug 28, 2017 0:13:57 GMT -6
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."
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Post by EmRR on Aug 28, 2017 7:27:45 GMT -6
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.
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