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In Reply to: RE: Fi 2A3 mono amps v. this (see photo) posted by drlowmu on February 23, 2015 at 15:49:50
Thanks for the critique, Jeff.
First of all, I have no technical knowledge so please forgive my ignorance.
By DC, you presumably mean direct-coupled? Since there appears to be a capacitor (mounted on the gold-coloured metal bar) in the signal path, I assumed this to be a capacitor-coupled design.
What/where are the Rks you refer to?
Re the use of a 6J5, it's my understanding that a low gain driver will give better results if the loudspeaker being driven is sufficiently sensitive. (Audio Note UK, for example, use a 6J5 in their low gain version of the Conquest 300B amp.)
Re the sound - well, just 2 days ago I installed a pair of JJ 2A3-40s (replacing EML mesh plates) and I have to say I'm really impressed. They seem to be much better for low-level listening and, so far, appear to have all of the finesse of the EMLs.
Just last night I was listening to a Frank Zappa LP (the wonderful Uncle Meat) and, WOW, I don't think I've ever heard such fidelity of sound! The engineer (Dick Kunc) on this recording did an amazing job but I believe quite a lot of the credit for the quality of reproduction is due to the amps I'm using.
Makes me glad I didn't win the eBay auction for the Fi amps.
Follow Ups:
Yes, 6j5 drives 2a3 two stages work. I built one for my friend, just like the AN low gain version, probably similar to what you are using, it is a very sweet sounding amp. It doesn't play loud or has the dynamic of three stages design, however if the speaker is efficient enough, preamp has enough drive, such design actually works well; the transparency is very close to direct couple, easy to build, very little parts, power supply is simple.
I built a px4 amp base on similar design, I like it very much. It is for ensemble, light jazz, not for Stones or Led Zep though.
Thanks for the endorsement of the 6J5.
My speakers are 95dB, 16ohms (so an easy load), my preamp (same builder as amps) has a transformer-coupled output and no feedback (so plenty of drive) and my room is only 9ft by 11ft.
I'm not into rock music and always listen at lowish levels.
As I say, I'm liking what I'm hearing with the JJ 2A3s.
.
Hello,
Rk is "resistor cathode" and there seems to be electrolytic caps on yours. The gold colored metal bar appears to be an Rk for the 2A3 . It acts as a return to ground run from the hum pot which is connecting the two filaments pins of the 2A3. It is NOT in the tube's control grid circuit. That gold colored bar is a Rk, about 10 watts or more, a power resistor. It HAS what appears to be a electrolytic across it. That is an area where the cap is IN the signal path, can be improved. Looks like the input tube has a similar electrolytic, going to ground, from a casual lookie-see.
As I look closer, there is a large gray colored military looking flat cap in the left bottom corner, and that may be a coupling cap. (It would be IF it is attached from the plate of the input tube to the control grid of the 2A3 .)
Contrary to what Audio Note has written, a two stage 6C5 - 2A3 is too low of a gain for efficient speakers, needs to be "pushed" all the time to sound superb, which means low listening levels are not as vibrant sounding. That is not the end of the world. Enjoy !!
Jeff Medwin
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