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2A3 amplifier - SET Fever

68.148.189.116
Posted on November 22, 2014 at 19:57:10
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004



All my recent interest in the WE91A has motivated me to dig out an amplifier I built back in 2006, and was never satisfied with it.

The amplifier is a single ended 2A3 design, using 5000/4-8-16 ohm Sowter SP02 output transformers. I used a power transformer recycled from a 1950s RCA console hi-fi. The circuit is simple - a 6J7 driving a 2A3, I used a schematic I found on the internet. More or less a generic design not unlike the sound practices we91a schematic but using a 2A3 and a totally different power supply. High voltage is provided by a 6CA4 rectifier, going through a PI filter with two capacitors and an ancient choke of unknown origin.

The two 2A3 tube filaments are connected in series and powered by 5VDC, with the middle point where the two 2A3 filaments are connected togther being connected to a common self bias resistor, which is in turn bypassed by a large electrolytic capacitor. This is clearly a compromise, but was intended to be cheap and give me an idea what 2A3s sound like without involving more parts than absolutely necessary.

Back in 2006, the amplifier had some hum issues I couldn't figure out, and I ended up shelving it. I then converted it to a single ended 6AQ5 amplifier, but it continued to hum, so it has been moved around and generally ignored.

Anyway armed with 8 years of additional reading and fixing things, I tackled it tonight. First converted it back to 2A3s. Next, used the cathode bias voltage to raise the heaters of the 6J7s above ground by 45V, and then I noticed the shielded wire I used for the plate caps had an aluminum shield, and the solder never stuck properly, so used a crimp connector.

I also noticed that I had originally used the wrong value of self bias resistor, which was running the output tubes way too hot.

I now have them with 260V on the plate, and 43V of bias. Around 60mA per tube - maybe some slight difference as they are not matched.

Anyway long story short, WOW! first off, this thing absolutely fills the room with sound... I remembered it as humming and having a pathetic amount of power, but I can only hear any hum right up close to the speaker, and it gets way more than loud enough for normal listening. Detail is just jumping out of the speakers in a way I am not used to. I can't wait to see what a larger scale (300B) SET brings forth!





 

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Clean and simple!, posted on November 22, 2014 at 20:09:51
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 I've got a couple OT that are for a 2A3.

Could you post a schematic?






First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: Clean and simple!, posted on November 22, 2014 at 20:57:45
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004



I just drew it out - I lost the original schematic but the power supply is my design. Don't copy how I used diodes to drop voltage to the 2A3 heaters, it would be better to add another RC filter but I had no low value resistors on hand. I can add component values later, for now am enjoying listening to it too much :)

 

RE: Clean and simple!, posted on November 22, 2014 at 21:45:00
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 edit: the diodes I used to drop voltage to the 2A3 filaments are drawn backwards... I should proof read before posting!

 

Yeah, there is something nice about these designs. nt., posted on November 22, 2014 at 21:49:44
RC Daniel
Audiophile

Posts: 1922
Location: Brisbane
Joined: November 3, 2002
 .
"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki

 

RE: 2A3 amplifier - SET Fever, posted on November 23, 2014 at 05:27:04
hifipaul
Audiophile

Posts: 735
Location: NY
Joined: December 22, 2008
Consider using separate dedicated filament supplies for the 2a3s.

 

that's not just any front-end tube..., posted on November 23, 2014 at 07:10:26
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 It's a coke bottle Mullard EF37A - in my estimation, the very best of the 6J7 equivalents!

 

HA! I've done that once or twice!, posted on November 23, 2014 at 07:14:14
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 Thanks. Nice clean design.




First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: that's not just any front-end tube..., posted on November 23, 2014 at 08:28:59
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 Yes this is true! Although to be honest I am now running it with 1940s metal RCA 6J7 tubes and prefer their sound slightly.

I am glad I dusted this thing off and gave it some attention, I had no idea that such good sound was right under my nose all along. Now I finally see how you "crazy" SET guys can be perfectly satisfied with such low power! I don't understand the why it works though, objectively this shouldn't be happening.

 

RE: that's not just any front-end tube..., posted on November 23, 2014 at 09:07:34
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 "Now I finally see how you "crazy" SET guys can be perfectly satisfied with such low power! I don't understand the why it works though, objectively this shouldn't be happening. "

Well, let's see.

Because you are most likely within the reverberant field and that field starts no more than 3 feet from the speaker, with 2 watts into a 90db speaker that speaker will give 93db spl at the listening position (or anywhere in the room beyond where the reverberant field starts).

There are 2 speakers in the room each receiving 2 watts and each producing 93db spl, that gives you 96db.

96db is really loud.

I would say, objectively, it should be happening.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: that's not just any front-end tube..., posted on November 23, 2014 at 09:31:37
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 I didn't define "it" :)

What I am referring to is the quantity of information my system is now delivering to my ears, it's like I went from 480p to full HD, using an amplifier which has higher distortion, almost no headroom to speak of, and a rolled off top end.

 

RE: that's not just any front-end tube..., posted on November 23, 2014 at 10:17:37
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 I'm sorry. I thought the "it" was the SPL's you are able to get.

Never mind.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Clean and simple!, posted on November 23, 2014 at 10:47:05
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Kiss rules !! Nice thread.

I wonder if it would be a good idea to use a grid choke to ground, on the 2A3's grid, so as to avoid a high value resistor there ?

My original audio mentor, Bob Fulton, HATED the usually-used 100K Ohms and above Ohmic value at that spot, preferred to have 20 K as grid-to-ground resistance on a 2A3's grid, to "put the grid in control" or to " center " the grid and the tube's operation.

Of course, this took a tremendously powerful driver stage to pull a 20 K grid to ground. I built it!! The sonic results of lowering this grid-to-ground resistance was a more engaging music presentation. I recall "sensing the musicians in the room, playing for ME" more-so, as I went lower in value there.

It was better to hear !!

Now a well-wound quality grid choke will allow you to have, what the EEs call a high AC impedance I believe, and a superbly-low DCR to ground, that the 2A3 grid will "love" to respond to, musically.

FWIW : Fulton would use terms such as " Puts the music grid in control of the tube, or CENTERS it on the music signal ".

No flames requested, this is a serious post by me. Good input by well meaning people is welcomed, encouraged. Open to discussion, positive discussion.

Jeff Medwin

 

RE: Clean and simple!, posted on November 23, 2014 at 11:11:45
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 The Heathkit W6M drives the output tubes with a pair of cathode followers, achieving essentially what you are saying, driving power tybe grids with a very low impedance. I have always seen that as a fantastic idea and would like to try it some day.

So far as this amplifier goes, I will add a little more filtering to the filament supply for the 2A3s, and maybe to the B+ too, beyond that I am calling it done, as I would like to concentrate my hobby expenditures (time and money) on building a WE91a replica (see post below). I think I also need some more efficient speakers!

 

RE: Clean and simple!, posted on November 23, 2014 at 11:44:38
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Yes, as a by-product, Low Z drive ( from the anode, not from the cathode ) was there, but the REAL goal was 20K or less grid-to-ground 2A3 resistance.

I spotted a high resistance Finals grid-to-ground in Thorsten's regulated amp also, and I thought a grid choke would excel there too.

Pentodes sound best with a separate supply for G2, ala Paoli 60M. What everyone else does is too much of a compromise ...a drop down resistor and cap.

Speakers .....Altec is possibly the best bang for the buck. I am partial to 604 MLTLs because they are a point source, or, on a budget ( I am! ) A7-800s with their partial front horn load of the woofer. Fun to hear. Hard to lose value, almost an investment.


Jeff

 

RE: Clean and simple!, posted on November 23, 2014 at 11:49:42
GEO
Audiophile

Posts: 4749
Joined: April 7, 2000
Contributor
  Since: September 9, 2000
 These are nice drivers.

 

RE: Clean and simple!, posted on November 23, 2014 at 12:06:08
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 Have you actually heard these? on paper they look great... It's a shame the second page is included, with te $57/ea banana plugs, it makes me doubt if the speakers are worth their asking price, although going by specs and build quality they do look like a fair value.

 

RE: Clean and simple!, posted on November 23, 2014 at 12:27:34
GEO
Audiophile

Posts: 4749
Joined: April 7, 2000
Contributor
  Since: September 9, 2000
 I own the H21LB15 and the super tweeter. I stop thinking about speakers after I bought these yeas ago. YMMV.

 

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