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Old 10th January 2006, 03:17 PM   #1
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Default My 2A3 schematic, comments please.

Hello,

I have always been on a tight budget, but finally I got me some real triode output tubes. The famous 2A3.
If I don't remember wrong they are from EH.

So yesterday I started thinking of a new amp with these tube just to check that I'm on the right track. The attach'd schematic is what
I came up with. I like your comments so that if there is any errors, I can get rid of them before building the amp. Not that I am saying that I expect every thing to work perfect from day one.

Something to say: Power supply for the anode on 2a3 will be around 250v and a little lower for 6sn7. Ac heating on both tubes. The amp will be conected to a center in the beginning, until I have money to make a preamp and input selector with tubes so I really have pretty much drive to the input of the amp.

Best regards,
Magnus Kofoed
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Old 10th January 2006, 05:23 PM   #2
kevinkr is offline kevinkr  United States
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The design looks good except for the volume control error - the wiper should go the grid of your 6SN7 and the top of the element to your source.

I would also add a 1K grid stopper resistor right at the grid of the 6SN7 and another on the grid of the 2A3. These should be at the socket in series with the grid.

IMHO you should operate the 6SN7 SRPP at =>300V for good headroom and low distorsion.

The 2A3 bias voltage is going to be about 45V so for an effective plate voltage of 250V and a few volts dropped across the output transformer it seems that a supply of 300V here is just about right.

Some of my older amplifiers are quite similar in design, and this topology sounds very good. You should be happy with it.

Hope this helps.

Kevin
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Old 10th January 2006, 05:45 PM   #3
Giaime is offline Giaime  Italy
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Default Re: My 2A3 schematic, comments please.

Quote:
Originally posted by Makofoed-6sn7
The amp will be conected to a center in the beginning, until I have money to make a preamp and input selector with tubes so I really have pretty much drive to the input of the amp.
Hello, can you explain this part? What is a "center"?
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Old 10th January 2006, 06:14 PM   #4
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Hello,

Thanks for the tips kevinkr!
I have attached the new schematic.
I will use around 300v for both stages, forgot the voltage drop in the cathode resistor and transformer. I also didn't realise that a srpp stage need that high voltage to get the distortion down.
Thats is way I like diyAudio, so many great tips to snap up on the way.

Giamie:
Center is a preamplifier, inputswitch, eq and volume control. It has hell-o-many inputs and often also a video switch. The propose of the center is to use it with powerstage that has noting else than input and output. The reason I call'd it center is that what we call it in norway. Never heard any one call it anything else, sorry for the confusion.

Best regards,
Magnus Kofoed

Edit: I see that I forgot the value of the grid resistors, but after Kevinkr adivce they are all 1k.
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Old 10th January 2006, 06:18 PM   #5
Gluca is offline Gluca  Italy
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What OPT are you going to use there?

Ciao
Gianluca
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Old 10th January 2006, 06:21 PM   #6
kevinkr is offline kevinkr  United States
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Hi Magnus,
Just occured to me that you might want to reduce the value of R5 to say 220K or less, and increase C2 slightly to 0.15uF or so to compensate..

In English what you describe as a center would normally be referred to as a pre-amplifier if it has the functionality you describe. There are also music centers which are the cheap all in one music systems usually with a pair of included speakers - I assume this is not what you meant.
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Old 10th January 2006, 06:22 PM   #7
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Hello,

I am planning to use Hammond T1627SE.
Its 2k5 primary and 4, 8 and 16ohms primary.
25watt output. Yeah I know, its a little bit overdimensoned.

Best regards,

Magnus Kofoed
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Old 10th January 2006, 07:58 PM   #8
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It might be good to also add a grid leak resistor (between volume-pot wiper & ground) of say 470K on the input. This will help to avoid noise when changing the volume.

The reason for this is that if the volume-pot wiper ever make intermittent contact, the stage loses the grid to ground reference. With a good quality pot, this shouldn't really be an issue, but it's good practice just in case.
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Old 11th January 2006, 03:24 PM   #9
kevinkr is offline kevinkr  United States
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You ought to take a look at the Lundahl line, I assume they would be a lot less expensive in Norway than they are here, and are generally better transformers than the Hammond. Maybe the price difference is not so great.

Kevin
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Old 12th January 2006, 08:18 AM   #10
jane is offline jane  Norway
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What is your estimated voltage swing for driving the 2A3 in this setup (B+=300V) -- about 80-90Vp-p for full drive? And how much gain will the 6SN7 stage give you, ca 10x?
In that case you will need 9Vp-p or 3.2Vrms from your preamp. Is that what you are aiming for?
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