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In Reply to: RE: Fi 2A3 mono amps v. this (see photo) posted by soulvibrations on February 22, 2015 at 11:52:00
There are two problems I see with your present amp.(1) The 6J5 is a nice linear mu of 20 tube as I recall, but its gain is too low as used effectively in a two stage amp. It will pass a signal, but only sound fun and lively when pushed hard. A mu of 100 tube is usually used in a two stage DC 2A3 amp.
(2) Both the Rks seem to be bypassed with electrolytic capacitors. They should be film capacitors, usually multiple films, carefully chosen as to brand and values, to get the linear wideband playback response. As it is now, you are listening through ( and to ) the electrolytics. Not good enough. You can change this, write to me sometime.
I would NOT necessarily suggest switching to a Fi amp either, as any ultimate solution. Build your own !!
BTW, how do you like listening to this amp ??
Jeff Medwin
Edits: 02/23/15 02/23/15Follow Ups:
This is not true. It is usually used in an DC 2A3 amp owned by Jeff.
Assuming anybody who runs a 3W amp, already has efficient "enough" speakers,
then the choice is this:
Do you want to attenuate more than 50% of your source signal, accept a certain amount of HF rolloff and unnecessary distortion?
OR
attenuate just 0-15%, have zero HF rolloff and significantly less distortion ?
Some will no doubt "prefer" the former, for metaphysical reasons, OK.
Some will discover the later will be more satisfying for obvious reasons.
Try one of the smaller power pentodes. 6AQ5, 6V6, EL84, or 6CL6 for instance. Run a maximum of 10k plate loads. Enjoy the irony that the driver stage is capable of producing more power than the power tube...:) Match g2 voltage carefully to the load and idle point.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
nt
See his last sentence, URL below, two stage DC amp.
GSH just never did his as best as it should be done, with Rk bypassing. Thats OK.
MOST 2 stage DC amps use a mu of 70 or higher ( eg mu 100) tube, ie : 1947 Robin/Lipman, 1960s Asano, 1970s Shishido and 1990's Garber, Uthus, and Fraker.
Jeff Medwin
.
Read his last three paragraphs. He too never properly bypassed Rks, but he is talking about doing that now, in the referenced post. Thanks for looking. Regards.
Jeff Medwin
.
Sure, thats fine, Rage is discussing his very recent experience - of going back to a mu of 100 12AX7 as a driver in a two stage DC amp, and liking it.
This pertains to what GSH and I were discussing, I see no problem, its on-topic.
Jeff
the 12ax7 is better than the 6ej7 pentode (triode strapped) for me right now.
I took some measurements and distortion is slightly lower with 12ax7!
hf rolloff is more but not that much. I will post measurements on the diy forum here pretty soon..
I am actually very surprised by this as I was swayed by the technically correct... and have learned a lot there... but the 12ax7 works IMO. MUCH better than expected.
You would probably need to run the pentode driver as a pentode w/feedback and measure again to see if the distortion from the driver stage can be lowered.
FWIW, the 7B4/2A3 combo at the standard 60mA operating points produced the best 1W THD% I measured when I had that circuit on a breadboard.
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.
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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