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Old 31st July 2006, 05:45 PM   #1
sgerus is offline sgerus  United States
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Default Best Operating point for a 2A3?

What are the best Transfomer Size(K), Plate Voltage, Cathode Voltage and Current to run a 2A3 at?

I’m looking for the best sound, power is not an issue.

The output transformer will be a Magnequest DS-025.
2.5K (or 5K using the 4R output into 8R speakers)

2A3
2.5K 255V -45V 60mA 6.2% 2nd har 3.5W
5.0K 255V -45V 60mA 2.3% 2nd har 2.6W
5.0K 276 -55V 55mA 3.7% 2dn har 3.0W

2A3-40 (JJ)
2.5K 289V -50V 80mA 3.3% 2nd har 5.4W

Based in the info from the 2A3 load line, looks like the 5K 255 -45 60mA is a good choice.
The “Classis” 2A3 configuration is 2.5K 250V -45V 60mA
The 2.5K configuration yields just a little more power yet almost 3 times the distortion than the 5K?
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Old 31st July 2006, 06:01 PM   #2
kmtang is offline kmtang  Canada
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I tried both 5K and 2.5K OPT's from Hammond, i.e., 1627SE and 1628SE.

2A3 sounds much better with 1627SE. Highly recommended.

The THD doesn't mean much.

Johnny
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Old 31st July 2006, 06:59 PM   #3
Fuling is offline Fuling  Sweden
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If mismatching your OPT is your only way to try different impedances then I believe 2,5k will work best.
Connecting the load to other taps than the intended one works, but it´s not optimal and the response at the extremes of the frequency range might suffer from it.
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Old 31st July 2006, 07:13 PM   #4
sgerus is offline sgerus  United States
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My current amp has the 1627SE (2.5K), set up using the 4R output for my 8R speakers.

I will rewire to use the 8R out and listen how it sounds…

Thanks
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Old 31st July 2006, 07:46 PM   #5
Fuling is offline Fuling  Sweden
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I did exactly the same thing (by mistake) with 1627 and 6B4G once. If I remember correctly the bass response improved when I changed to the correct tap.
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Old 31st July 2006, 11:23 PM   #6
arnoldc is offline arnoldc  Philippines
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Hi,

I own a DS-025 which I use for my 2A3. Before finalizing everything I played with different operating points-

1. classic 2A3 with 750R cathode and DS-025 2.5K tap
2. 820R cathode at DS-025 2.5K tap
3. 750R cathode at DS-025 5K (via 4ohm tap)
4. 820R cathode at DS-025 5K (via 4ohm tap)
5. 880R cathode at real 5K using Tamura F-475
6. 45 mode 1k5 cathode using Tamura F-475

I don't like #1 and #3. It seems to me that the 2A3 became shouty.

I currently use #2, because if I remember right #4 sounds similar but that was brief listening.

I briefly tried #5 but it was exceeding the max current capability of the F-475 so I did not have it for long. It sounds good though. #6 is pretty much ok too, albeit at lower power.

By the way, I'm not sure if Mike changed the specs of the DS-025, but it has 9ohms tap instead of 8 (as if it matters )

DS-025 Hookup Wire Color Codes
Primary Red is B+ / Blue to plate
Secondary
* Black is ground/4 ohms/9 ohms/16 ohms
----------------------Brown/Orange/Yellow
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Old 1st August 2006, 12:50 AM   #7
SET12 is offline SET12  United States
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Default Re: Best Operating point for a 2A3?

Quote:
Originally posted by sgerus
What are the best Transfomer Size(K), Plate Voltage, Cathode Voltage and Current to run a 2A3 at?

I’m looking for the best sound, power is not an issue.

The output transformer will be a Magnequest DS-025.
2.5K (or 5K using the 4R output into 8R speakers)

2A3
2.5K 255V -45V 60mA 6.2% 2nd har 3.5W
5.0K 255V -45V 60mA 2.3% 2nd har 2.6W
5.0K 276 -55V 55mA 3.7% 2dn har 3.0W

2A3-40 (JJ)
2.5K 289V -50V 80mA 3.3% 2nd har 5.4W

Based in the info from the 2A3 load line, looks like the 5K 255 -45 60mA is a good choice.
The “Classis” 2A3 configuration is 2.5K 250V -45V 60mA
The 2.5K configuration yields just a little more power yet almost 3 times the distortion than the 5K?


Hello Sgerus,

Its nice to have a DIYer in my neck of the woods here in WI as I am just 2 hrs to the North in Appleton,WI just off of Hwy 41 on Appletons North side.

I have another alternative for you I also have both of these Hammonds the 5k 1628 is reputed to have a poor frequency response on the 8 ohm tap a big dip at 13k but not the 4 ohm tap, Since my Klipsch Forte's are 4 ohms I have never used the 8 ohm tap the primary taps have been known to of been mislabeled I would measure the DCR's to check them out!

I also use the Hammond 1642 SE it goes some 28 lbs and is 5k with a single secondary winding for 4,8.and 16 ohm taps I absolutely love this transformers sound.

I took a chance on purchasing these because of their size and always the the possability of poor bandwidth lurked in my mind although I haven't run a response through them subjectively the are very extended in the frequency extreams I recently had a Solid-State hybrid 100 watt Jollita intergrated here my 10 watt SET a 811/572 Class A2 at 500 volts and 100-120 ma. design just shocked me as it had more extension in both extreams and no comparison in the sheer imformation it was giving me.

What I found in a comparison between the 1628 SE and the 1642 SE and my amps are not of a classical design but one of which I feel express's great emotion and energy are that the 1642 SE conveys a great sense of ease, better space, and powerful tight rythmic bass lines many have told me that my bass is very solid state like one of my local club members a 30 yr veteran of audio told me he had never heard a low power SET with the Dynamics of my SET

I think a few designers like big as I do for instance Convergant Audio Technology has a 100 watt Push-Pull Mono Block with a 50 lb output tranny it doesn't take much off set to eliminate a bunch of a PP trany's inductance and a SET typicaly has even a lower inductance to work with but maybe the 1642's secret is the volume of its core.

I don't know! But I do know whether I'm building a 2A3 or a 300B these are what I am using I like the Magnequest line you get what you pay for!

As far as operating points I prefer the high value primarys for High Dynamic conductance ( Transconductance x load) for sheer dynamics all of my work is centered around this especially my front end stuff.

I belong to a club here in Northeastern Wisconsin called NEWAS

I am known by a screen name of SET under system profiles you can view some pics of my work!

http://www.newaudiosociety.com/index.htm

Some of our members live in the Milwaukee area as well!

We are having a round robin meeting in several peoples homes in the Fox Vally here on Sat. Aug 26th running from noon to when ever 3 out of the 4 of us have SET amps one is a 2A3 tulip and a Cary 300B kit amp and of course my 811's you could pick up more than a few ideas here!

Hope you drop me or the club a line!

SET12
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Old 1st August 2006, 06:43 PM   #8
sgerus is offline sgerus  United States
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Arnoldc,

It’s interesting you preferred the sound with 820R on the cathode vs. 750R

By using a larger cathode R you lowered the current and raised the voltage at the cathode,
So you shifted the operating point lower on the load line. As you move down on the loadline the curves get closer together, hence higher distortion.

I would have expected that to sound worse?

I’m no expert, but I have cone to realize that what you see on paper doesn’t always match what you hear.

Do you have the before and after voltages at the plate and cathode?
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Old 1st August 2006, 11:21 PM   #9
arnoldc is offline arnoldc  Philippines
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Hi sgerus,

I'll try to find my notes where I recorded the voltages. But if I remember right, the Vk isn't that far off, I think just 1V-2V difference. With the 820R, Vk at around 43-44V I think. The current about 4mA difference. Yes, I ran it at a little bit less current.

The MagneQuest drops around 5V.

ps.

B+ is around 300V
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Old 2nd August 2006, 12:07 AM   #10
sgerus is offline sgerus  United States
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Arnoldc,

Looks like you about as close to the Classis config as one can get.
295V -44v 55mA and 2.5K OPT

Thats about what I'm thinking.
Are you happy with the Magnequest Iron? Any personal experience with Hammonds.

I generaly feel that you get what you pay for... Is that true for the Magnequest Iron?
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