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Fi 2A3 mono amps v. this (see photo)

95.144.115.51
Posted on February 22, 2015 at 11:07:33
soulvibrations
Audiophile

Posts: 65
Joined: December 10, 2003



An opportunity has arisen for me to pick up a pair of Fi 2A3 mono amps and I'm wondering whether anyone here would care to comment on the design/construction of my current 2A3 monos (apologies for the poor quality of the photo) and the likely differences in sound quality v. the Fi amps.

I'd rather not reveal who built the amp (at least not in public) just in case some of the comments are not too complimentary!

Many thanks.

 

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RE: Fi 2A3 mono amps v. this (see photo), posted on February 22, 2015 at 11:52:00
soulvibrations
Audiophile

Posts: 65
Joined: December 10, 2003
 Should have mentioned that the driver is a 6J5GT, rectifier a 5Z3.

 

RE: Fi 2A3 mono amps v. this (see photo), posted on February 22, 2015 at 13:16:11
GSH
 The mystery amp will be much easier to work on if nothing else.
Don't "Fi" amps use a high gain 6SF5 triode driver? That would be another reason for the homebrew amp, 6J5 will be wider band and lower distortion,
with less gain, of course.

 

RE: Fi 2A3 mono amps v. this (see photo), posted on February 22, 2015 at 15:38:40
Alpha Al
Industry Professional

Posts: 2958
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since: December 3, 2015
 Looks like good build quality, but I see one issue that needs addressing. The cathode bypass cap is very close to the cathode resistor. I assume the resistor runs hot, and heat is the enemy of capacitors. The cap should be moved to prevent premature failure.

 

the hum pot ..............., posted on February 22, 2015 at 17:03:16
bouncy ball
Audiophile

Posts: 1221
Location: British Columbia
Joined: July 26, 2003
 I prefer the hum pot mount differently so that I can access it without flipping the amp over.

 

RE: Fi 2A3 mono amps v. this (see photo), posted on February 22, 2015 at 19:15:56
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 I've had a lot of issues with that particular type of ceramic tube socket failing.

Other than that, it would be hard for anyone to judge what you have. You'd need a schematic and a parts list to have any chance of actually understanding the design and performance potential.

 

RE: Fi 2A3 mono amps v. this (see photo), posted on February 23, 2015 at 15:49:50
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 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

 

RE: Fi 2A3 mono amps v. this (see photo), posted on February 24, 2015 at 05:53:05
soulvibrations
Audiophile

Posts: 65
Joined: December 10, 2003
 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.



 

Electrolytics across Rks....., posted on February 24, 2015 at 07:47:48
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 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

 

RE: Fi 2A3 mono amps v. this (see photo), posted on February 24, 2015 at 08:19:56
bouncy ball
Audiophile

Posts: 1221
Location: British Columbia
Joined: July 26, 2003
 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.

 

" A mu of 100 tube is usually used in a two stage DC 2A3 amp", posted on February 24, 2015 at 09:21:06
GSH
 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.

 

RE: Fi 2A3 mono amps v. this (see photo), posted on February 24, 2015 at 09:58:37
soulvibrations
Audiophile

Posts: 65
Joined: December 10, 2003
 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.

 

base on your music preference, 6j5 drives 2a3 will work very well. Enjoy!(nt), posted on February 24, 2015 at 10:01:04
bouncy ball
Audiophile

Posts: 1221
Location: British Columbia
Joined: July 26, 2003
 .

 

Thanks for the different perspective, GSH. nt, posted on February 24, 2015 at 10:26:08
soulvibrations
Audiophile

Posts: 65
Joined: December 10, 2003
 nt

 

RE: Thanks for the different perspective, GSH. nt, posted on February 24, 2015 at 10:53:29
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005

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

 

Wrong link. nt, posted on February 24, 2015 at 10:56:48
kyle
Audiophile

Posts: 1839
Location: London Ontario
Joined: September 29, 1999
 .

 

Its correct, posted on February 24, 2015 at 14:19:46
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 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

 

The link is to a post by Rage not GSH nt, posted on February 24, 2015 at 14:21:50
kyle
Audiophile

Posts: 1839
Location: London Ontario
Joined: September 29, 1999
 .

 

RE: Fi 2A3 mono amps v. this (see photo), posted on February 24, 2015 at 22:48:50
Triode_Kingdom
Audiophile

Posts: 10047
Location: Central Texas
Joined: September 24, 2006
 "My speakers are 95dB... 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."

In that case, I wouldn't change a thing. All of the concerns raised so far in this thread are very minor.


 

RE: The link is to a post by Rage not GSH nt, posted on February 25, 2015 at 06:30:30
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 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

 

RE: the hum pot ..............., posted on February 25, 2015 at 06:47:43
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 My own preferred technique, for now, is no hum pot.

Lately, I have been experimenting with "balanced" Ef XFR windings for DHTs, (which we've never had), balanced as to DCR, Inductance, and Capacitance. Its a suggestion I was given from Dennis Fraker.

JM

 

RE: the hum pot ..............., posted on February 25, 2015 at 09:00:58
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 I'm sure it's a lot of fun to reject a high percentage of your 2A3's because they will hum without a hum pot.

Hopefully you've found a vendor that doesn't mind all the returns.

 

RE: the hum pot ..............., posted on February 25, 2015 at 11:30:50
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 CB,

I am a big boy now, so seldom do I return tubes to my vendor.

I just buy enough, say ten at a time, do my ear finger-tapping tests to get a pair or two, maybe test them for hum levels, and sell-off all the ones I don't want on eBay, to the many very wise people who use hum pots in their amps. Everyone is happy that way, and no one gets hurt.

Jeff Medwin

 

RE: Fi 2A3 mono amps v. this (see photo), posted on February 26, 2015 at 15:21:26
Todd Krieger
Audiophile

Posts: 37333
Location: SW United States
Joined: November 2, 2000
 Layout looks simple and clean.... But I have no clue how two amps I've never heard would compare.

 

RE: The link is to a post by Rage not GSH nt, posted on February 28, 2015 at 02:56:52
rage
Audiophile

Posts: 793
Joined: December 17, 2010
 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.

 

RE: distortion ..., posted on March 10, 2015 at 03:42:04
deathtube 667
 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.



 

RE: " A mu of 100 tube is usually used in a two stage DC 2A3 amp", posted on March 17, 2015 at 17:37:26
PakProtector
Audiophile

Posts: 12365
Joined: May 14, 2002
 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.

 

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