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Old 15th April 2003, 08:51 PM   #1
mig-ru is offline mig-ru  Russian Federation
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Default Finally built my 6SL7, 2A3/6A3 amp

Well I finally got all my parts in last week, redesigned the power supply, and changed the 2A3 B+ and bias. I finished soldering last nite at around 11:30, plugged in my test tubes, hooked up a speaker and a portable cd player and.... Wam!!! sound!!! I was really surprised that it would work at all on the first attempt. So now its working very well, although the hum is a little higher than the PSUDII predicted 8.4mV; this problem will soon be corrected when i change the 5H choke to a 15H. In addition, after hooking it up with the proper components (Cambridge Audio D500-SE, and Boston Acoustic VR-975 Lynnfield Speakers) and plugging in the right tubes (all NOS, JAN Phillips 5691, Sylvania 6A3's, and JAN Sylvania 5U4G) I was blown away by the sound, so transparent, so smooth, now I know what all the hype is about DHT triodes. Although I was in a state of mania while soldering and didnt take pictures of the progress, here are some pics of the final result, in addition to the schematic.
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Old 15th April 2003, 08:53 PM   #2
mig-ru is offline mig-ru  Russian Federation
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Old 15th April 2003, 08:57 PM   #3
mig-ru is offline mig-ru  Russian Federation
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schem:
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Old 15th April 2003, 09:16 PM   #4
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That looks really good! I'm thinking of tackling a 2A3 amp soon, and this is a schematic that I hadn't seen before. I have a few questions:

* Is that a standard Hammond aluminum chassis? What finish did you put on it? It looks pretty nice. Also, what tools did you use to for the metalwork?

* What are the two knobs on top - the hum trimpots?

* What do all the switches do?

* Could you post some pictures of the wiring inside?

Thanks,
Saurav
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Old 15th April 2003, 09:34 PM   #5
mig-ru is offline mig-ru  Russian Federation
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Quote:
Originally posted by Saurav
That looks really good! I'm thinking of tackling a 2A3 amp soon, and this is a schematic that I hadn't seen before. I have a few questions:

* Is that a standard Hammond aluminum chassis? What finish did you put on it? It looks pretty nice. Also, what tools did you use to for the metalwork?

* What are the two knobs on top - the hum trimpots?

* What do all the switches do?

* Could you post some pictures of the wiring inside?

Thanks,
Saurav

The chassis is Hammond, but steel, I didn't think the aluminium version would hold the transformers without sagging. The finish is automotive pearl silver spray paint. For tools i used a drill press, hand drill, drill bits including hole drills (1 3/8" and 1") for the socket holes, i used a rounded file to finish the holes of and to bring the 1" hole to the 1 1/16" needed.
You're right about the knobs, they're hum trimpots. The switches left to right: filament voltage selector (2.5V/6.3V) to be able to run 2A3's and 6A3's, HV winding to rectifier, primary winding switch. Pics of the insides might be added later, check previous posting about manic soldering rush, hehe.
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Old 15th April 2003, 09:35 PM   #6
mig-ru is offline mig-ru  Russian Federation
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Old 15th April 2003, 09:36 PM   #7
mig-ru is offline mig-ru  Russian Federation
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Old 15th April 2003, 09:54 PM   #8
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Thanks.
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Old 15th April 2003, 11:01 PM   #9
PassFan is offline PassFan  United States
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Default Ceramics

Dmitry:
Those look like ceramic sockets in there That puppy looks nice man. You do excellent work. Thanks for sharing the schematic. I may rewire mine one day and try yours out. You've got more guts than me. I have to walk away for a while and recheck later before I hit the switch. On second thought, perhaps I have more patience than you
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Old 15th April 2003, 11:25 PM   #10
Morse is offline Morse  United States
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Good show, Dmitry!!

That's a nice, professional looking amp!

>>>...I was blown away by the sound, so transparent, so smooth, now I know what all the hype is about DHT triodes...<<<

Yep, the same thing happened with me when I fired up my 2A3!! That's why I started stocking up on 2A3's as spares.....

>>>...although the hum is a little higher than the PSUDII predicted 8.4mV...<<<

Hmmm, you might try SLIGHTLY adjusting the position of some of the wires inside, particularly the heater wires for the input valves. I had 4mV of hum in mine in one channel when I started, and have since dropped it to somewhere between 1-2mV by doing just that, along with a couple of other things. You might also make sure that your trafo's cores are in good electrical contact with the chassis, and make sure that your signal leads are well clear of even the B+ line, in case the residual ripple is coupling to it. Have you an oscilloscope so that you can see if the ripple is 60 cycles per second or 120 cps? That will tell you a lot about the origin of the unwanted noise.

Good luck and all the best,
Morse

PS: You might consider fullrange speakers (like one of the 6.5" drivers in the Fostex line) next - that was the next big step up in terms of sound quality with my system after the 2A3.
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