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JJ 2A3-40 for Lynn Olson Amity
JJ 2A3-40 for Lynn Olson Amity
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Old 15th October 2019, 01:28 PM   #1
Tubenstein is offline Tubenstein
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Default JJ 2A3-40 for Lynn Olson Amity

Hello!
My friend has sold me all parts for building the Lynn Olson Amity, and it has the power transformer but not choke and output transformer.

Here is the circuit.

http://www.nutshellhifi.com/IT-Triode-Amp.gif

I have questions for you guys about substitute JJ 2A3-40 for the output tubes.

1) If I want to use the JJ 2A3-40, is the operating point (310v) still applied?

JJ Electronic - 2A3 - 40

2) As the suggested primary impedance of output transformer is between 6 to 8K, Which impedance should I go for?? 6.6K or 8K??

Many thanks!
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Old 15th October 2019, 01:59 PM   #2
indra1 is online now indra1  Indonesia
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JJ 2A3-40 seems to have a 300B electrical charateristics and physical form factor fitted with a 2A3 filament. It should be safe to opt for 450V @ 60mA bias. The kind of difference of OT impedance you are concerned with is perhaps secondary to winding technique, materials used and other differences in specifications.
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Old 15th October 2019, 02:37 PM   #3
Tubenstein is offline Tubenstein
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Quote:
Originally Posted by indra1 View Post
JJ 2A3-40 seems to have a 300B electrical charateristics and physical form factor fitted with a 2A3 filament. It should be safe to opt for 450V @ 60mA bias. The kind of difference of OT impedance you are concerned with is perhaps secondary to winding technique, materials used and other differences in specifications.
Which of the following would you suggest for Amity with JJ 2A3-40??

TTG-EL34PP - Tube output transformer [6,6kOhm] 2xEL34 / 2x6L6 Push-pull or similar - Shop Toroidy.pl

TTG-EL84PP - Tube output transformer [8kOhm] 2xEL84 Push-pull or similar - Shop Toroidy.pl
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Old 15th October 2019, 02:45 PM   #4
indra1 is online now indra1  Indonesia
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Quote:
Originally Posted by Tubenstein View Post
Which of the following would you suggest for Amity with JJ 2A3-40??
This one.
Check the specification, has ~50% higher primary inductance, better bass response.
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Old 15th October 2019, 04:22 PM   #5
Palustris is offline Palustris  United States
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Just to be clear: the JJ 2A3-40 is a 2A3 NOT a 300B. It biases up at the classic 2A3 operating point e.g., Ep= 250V, Eg= -45V, Ip= 60mA. And it follows the plate curves for a 2A3. The difference is that the 300B plate allows for a much more aggressive operating point. I have used 300V @ 75mA with no issues.
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Old 15th October 2019, 04:40 PM   #6
Tubenstein is offline Tubenstein
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Quote:
Originally Posted by Palustris View Post
Just to be clear: the JJ 2A3-40 is a 2A3 NOT a 300B. It biases up at the classic 2A3 operating point e.g., Ep= 250V, Eg= -45V, Ip= 60mA. And it follows the plate curves for a 2A3. The difference is that the 300B plate allows for a much more aggressive operating point. I have used 300V @ 75mA with no issues.
So I should operate the JJ 2A3-40 at 310v as the circuit diagram indicated that is supposed to have conventional 2A3, right?
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Old 15th October 2019, 05:37 PM   #7
indra1 is online now indra1  Indonesia
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Palustris gave you a nice and tried operating point within the safe operating area. Should be OK.
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Old 15th October 2019, 05:58 PM   #8
analog_sa is offline analog_sa  Europe
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JJ 2A3-40 for Lynn Olson Amity
Quote:
Originally Posted by Palustris View Post
Just to be clear: the JJ 2A3-40 is a 2A3 NOT a 300B.

Interestingly, it sounds much closer to 300B than to 2A3.

Not convinced that a toroidal output is a great idea.
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Old 16th October 2019, 11:04 AM   #9
Tubenstein is offline Tubenstein
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Originally Posted by analog_sa View Post
Interestingly, it sounds much closer to 300B than to 2A3.

Not convinced that a toroidal output is a great idea.
The toroid opt doesn't cost arms and legs for a trial, so why not?

Also, I have listened one of the finest 300B pp amplifier back in early 90's that is called LA Audio from Denmark, and it has toroid irons.

A bad EI or C type opt can sound bad.
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Old 16th October 2019, 07:56 PM   #10
6A3sUMMER is offline 6A3sUMMER  United States
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I have JJ 300B tubes, and I also have JJ 2A3-40 tubes.

The plates look identical (size, shape, color).
The glass looks identical (size, shape).
The 2A3-40 has a black header that holds the pins (painted black).
The 300B has a white header that holds the pons.
The bottom of both 2A3-40 and 300B headers are white.

The 300B 5V filament connections are from far end to far end
(and the center is un-connected).
The 2A3-40 2.5V filament connects the far ends together, the other connection is to the center.

I would bet the filaments are identical except for the connections.
2.5V @ 2.5A, and 5V @ ~ 1.25A

The JJ 300B is a Mono-Plate tube.
The JJ 2A3-40 is a Mono-Plate tube.
These are both 40 Watt maximum plate dissipation.

What I can not tell is if the grids are the same or different on the JJ 2A3-40 and JJ 300B.
I bet they are the same, but I am not going to take excellent, well made, working tubes apart just to satisfy anyone's curiosity.
The JJ 2A3-40 and JJ 300B sound good.

There are other 2A3 Mono-Plate tubes, but they are either very old original RCA, or are made by some newer tube manufacturers (Electro Harmonix and Sovtek are examples).
But all of these are 15 Watt maximum plate dissipation.

There are very old stock and new manufactured dual plate 2A3 tubes. Valve Art is an example of a newer manufactured 2A3. But these also have 15 Watt maximum plate dissipation.

As to how a 2A3, 2A3-40, or 300B sounds in your system, I leave that to you.
You can run your own scientific, or non-scientific listening tests.
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