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Old 8th May 2009, 09:11 AM   #1
3nity is offline 3nity  Austria
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Question 40W class A push pull amp with JJ 2A3-40w possible?

Hi,

Has anyone had the chance to test this kind of amp?
A JJ 2A3-40W push pull biased in Class A.

Is it possible to get 30/40 watts on a 4 ohm speaker?
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Old 8th May 2009, 11:28 AM   #2
coresta is offline coresta  France
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NE-VER .
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Old 8th May 2009, 11:54 AM   #3
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Maybe for 2-3 minutes

Cheers!
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Old 8th May 2009, 11:58 AM   #4
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Even though JJ-2A3 are 300B with 2,5V heaters, it is not doable.
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Old 8th May 2009, 02:03 PM   #5
kmtang is offline kmtang  Canada
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May be we can use 4 pairs of 2A3's to get 40Watts output. The output transformer should be rated for 250mA idle current.

Also, we need a 2 x 2.5V 10A for the 2A3's filament supply.

The price of 2A3 is not cheap at all.

I would rather just use 2A3 in singled end amplifier that gives me 3.5W sweet and beautiful sound...

Johnny
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Old 8th May 2009, 02:52 PM   #6
waltube is offline waltube  Italy
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The 2A3-40 by JJ is a very good tube.It is very stable and not so difficult to select; I have Sofia.
At this link:
http://www.multitask.it/diy/fabfour.JPG
you can see the first prototype of a pair of amps using these tubes.

This is a second version (for my friend):
http://www.multitask.it/diy/1000eniumab.jpg

The rms power is 45 w - 8 ohm and 60 w at 4 ohms; impulsive around 75 w.

It works for the 40% in class A.


Bye

Walter
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Old 8th May 2009, 03:03 PM   #7
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Once again:

JJ 2A3-40W has nothing to do with a 2A3.

JJ 2A3-40W IS A 300B WITH 2,5V HEATERS!

The "funny" guys att JJ are still trying to fool us by having the RCA Ua/Ia-curves in their data sheet.
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Old 8th May 2009, 03:10 PM   #8
waltube is offline waltube  Italy
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Correct, the 2A3-40 is a 300B with a 2,5 v filaments.
The good thing is that it can be powerd with ac on fil. and the hum is very low.
In my last amp the noise at the otuput was abou 0,45 mv without Feedback. Same project with 300B are 0,7-0,8 mv.

I am able to send, ASAP, the curves of this tube from my Sofia .

Bye

Walter
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Old 8th May 2009, 03:20 PM   #9
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Default RH300B

That is a good information: all those who wish to build the RH300B amplifier can do it with the JJ2A3 tubes obtaining the same power as with a "normal" 300B, but with less hum due to AC heating!

I did not know that -- I do not follow new tubes
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Old 8th May 2009, 04:00 PM   #10
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Default Re: 40W class A push pull amp with JJ 2A3-40w possible?

Quote:
Originally posted by 3nity
Hi,

Has anyone had the chance to test this kind of amp?
A JJ 2A3-40W push pull biased in Class A.

Is it possible to get 30/40 watts on a 4 ohm speaker?
There may be a way to get about 35 watts at the expense of
distortion and damping factor. Check out the SE op point at 450V
anode and 80 mA into 2000 ohms. Class A (more or less) push-pull
at this op point and 4000 ohms plate-plate would give you about
35W at the anodes and well over 30W out after losses. The anode
dissipation will be 36 watts at idle.

The chart is for a 300B but the JJ 2A3-40 has the same characteristics.

For a 4 ohm speaker you just need a 4000 ohm to 4 ohm OPT.

Michael
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