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Old 29th October 2012, 11:52 AM   #1
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Join Date: Oct 2012
Location: Pyrenees
Default Modifying a J Sound Lab 2A3 amplifier

Hi to everyone,

First of all I would like to say hello to everybody here. It is my first post in this forum and I am not sure if it is placed correctly. I do not know much about electronic, but I want to learn in order to make other projects.

I have never tried a diy project but I want to start trying to modify a J Sound Lab 2A3 amplifier.

What I would like to modify is the rca and speaker sockets which are on the top of the amplifier and I want to make a new chassis with them in the back side.

The next modification would be, place the potentiometer in the front side with a switch which allow to disable it, I would like to change it by an alps one, but I do not know which is the suitable. I wonder if it is possible...

There is also a deep noise by the speakers when you switch on without music, once you play music it desappear I have read it could be a matter of a lack of direct and stabilized current in the 2A3 tubes, somebody could help me how can I eliminate.

If anybody has an idea to improve it please tell me, I would be very grateful.

I upload a picture of it.

Thanks for all, regards.
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File Type: jpg _MG_7429.jpg (970.2 KB, 220 views)

Last edited by fotronero; 30th October 2012 at 07:29 AM.
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Old 29th October 2012, 01:54 PM   #2
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Join Date: Sep 2003
Location: London
Hello! Welcome to the forum. The Pyrenees are a beautiful place - you are lucky to live there! the usual advice to a beginner is to buy a copy of Morgan Jones "Tube Amplifiers" and read it a few times. Edition 3 is in book form, but even if you see a cheap edition 2 that's good too for the basics. Edition 4 is electronic and expensive. Really, a tutorial in Spanish would be good for you. Do other Spanish speakers know of any?

When I look inside the chassis myself I would rip the whole circuit out and start again! But for quick modifications, replace the 2a3 electrolytic cathode bypasses with polypropylene film caps - whatever size goes in there. 100uf would be good but even 47uF in a film cap would sound better. And the cathode resistors are pretty horrible too - those white ceramic ones. Go for green 14W Welwyn W24 wirewound ones. I'd be wanting to put two interstage transformers in there somewhere - maybe Lundahl LL1635/20mA would fit. And then use an input tube like 5842 or otherwise D3A or EF184 or E280F or E180F in triode or something like it. You might need a preamp but that would be another good project.

This might all be a bit advanced for where you are now - do you have a friend who could help and knows tube stuff?

Last edited by andyjevans; 29th October 2012 at 02:01 PM.
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Old 29th October 2012, 07:53 PM   #3
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Join Date: Sep 2011
Hi,
It is good to see the actual build of J Labs amplifier as it definitely looks different than the pictures on their website. The quality of build looks quite horrible...
To address the problem of hum I would install humpots on 2a3 cathodes. I can see some kind of rectifier with bridge and one cap, I assume it is for signal tube fillaments - you need to replace that with better regulated dc board.

As Andy said, the whole thing looks bad, I woul keep the chassis but rip the guts out and build something decent at least.
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Old 29th October 2012, 08:38 PM   #4
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Join Date: Aug 2008
It looks like some resistors were pre used, too! Blech

It already has 2 places for octal (8-pin) valves, so you can only go with 8-pins at these positions, 9-pins like andyjevans recommended are smaller.....
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Old 29th October 2012, 10:15 PM   #5
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Join Date: Sep 2003
Location: London
Are those octal sockets on the input? Hard to see. There's a loctal C3G in triode, or make an octal-9 pin bodge to fill in the existing socket space. I'm sure there are options. Me, I'd use a 4P1L and then use a preamp with it. That would need a DC supply like Rod Coleman's boards. I'd then take the power supply outboard. But that's too much for a beginner.
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Old 29th October 2012, 10:32 PM   #6
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Join Date: Apr 2006
Location: Earth
hi, would there be a big difference if 20uF polypropylene film cap is used instead of 47uF or 100uF in cathode bypass?


Quote:
Originally Posted by andyjevans View Post
Hello! Welcome to the forum. The Pyrenees are a beautiful place - you are lucky to live there! the usual advice to a beginner is to buy a copy of Morgan Jones "Tube Amplifiers" and read it a few times. Edition 3 is in book form, but even if you see a cheap edition 2 that's good too for the basics. Edition 4 is electronic and expensive. Really, a tutorial in Spanish would be good for you. Do other Spanish speakers know of any?

When I look inside the chassis myself I would rip the whole circuit out and start again! But for quick modifications, replace the 2a3 electrolytic cathode bypasses with polypropylene film caps - whatever size goes in there. 100uf would be good but even 47uF in a film cap would sound better. And the cathode resistors are pretty horrible too - those white ceramic ones. Go for green 14W Welwyn W24 wirewound ones. I'd be wanting to put two interstage transformers in there somewhere - maybe Lundahl LL1635/20mA would fit. And then use an input tube like 5842 or otherwise D3A or EF184 or E280F or E180F in triode or something like it. You might need a preamp but that would be another good project.

This might all be a bit advanced for where you are now - do you have a friend who could help and knows tube stuff?
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Old 29th October 2012, 11:37 PM   #7
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Join Date: Sep 2003
Location: London
As you go below 100uf you're progressively losing bass. I find 47uf just about acceptable on outputs - never tried less than that. 20uF may be OK if you have a subwoofer, otherwise the loss of bass would probably be quite audible.
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Old 30th October 2012, 12:29 AM   #8
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Join Date: Apr 2006
Location: Earth
anyone can point the formula how to calculate the frequency using different capacitance?
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Old 30th October 2012, 07:28 AM   #9
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Join Date: Oct 2012
Location: Pyrenees
Thanks for the replies.

Now, I don't know what to do...I just change the sockets because there are many electronic devices to modify or keeping transformers and chokes try another design, I am in two minds!
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Old 30th October 2012, 07:37 AM   #10
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Join Date: Oct 2012
Location: Pyrenees
Quote:
Originally Posted by ClefChef View Post
Hi,
It is good to see the actual build of J Labs amplifier as it definitely looks different than the pictures on their website. The quality of build looks quite horrible...
To address the problem of hum I would install humpots on 2a3 cathodes. I can see some kind of rectifier with bridge and one cap, I assume it is for signal tube fillaments - you need to replace that with better regulated dc board.

As Andy said, the whole thing looks bad, I woul keep the chassis but rip the guts out and build something decent at least.
It is not the new one, it is the older design. I am not sure but the new one has a rectifier tube though, apart from that, it seems quite similar.
Anyway I tried twice to put in contact with them, about the deep noise, and I am still waiting for the reply...
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