• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Need help picking a 2A3 SET design....

analog addict

analog addict

Glory or Death!
Subscriber
I'm finally going to cut bait. I'm going to order a pair of Transendar SET 3K OPT's with a 40% UL tap from Gery tomorrow....(Santa threw some cash in my stocking...:thmbsp:)

attachment.php



and will be looking to use this PT as the other hunk of iron.....

CIMG7166.jpg


I'm guessing the OPT's will be 8 ohm on the secondaries. Would anyone want to argue 5K on the primary? 50% verses 40% on the extra tap?

I am open to any suggestions for design and/or driver/rectifier tube choices. I suppose I will also need a choke to tame the B+ of the PT.

Anyone???....:scratch2:
 
Register to hide this ad
You'll prolly need a choke-input power supply with that PT. That 1080VCT will give you nearly 460VDC with one, and much more with a cap-input supply. 460VDC is already a lot, you'll have to burn some it away somehow for 2A3s.

Between the 40% and 50% taps, I don't think it'll make much of a difference. If used, they'll be for feedback, and that small 10% change won't make too much of a difference in performance.
 
Well... somebody chime in here if they think this is way off-base... but what would stop him from running 120V into the primary of the transformer, wired for 230v? That would cut the voltages in half. AFAIK, the transformer might actually run more efficiently that way... half the voltage means very LITTLE chance of getting ANYWHERE CLOSE to saturation.

With that, Duncan PSUDII gives about 285v B+, with around a 120ma-140ma total load (two 2A3s and input tubes). A little high, but within the 300v limit of the 2A3. You'd just have to adjust the cathode bias resistor, to get proper dissipation from the 2A3 (i.e, a little less current than usual)...

Naturally, he'd have to get a secondary transformer for the heaters on the 2A3s... but, he'd need that anyway, to get 2.5v. If you wanted to get away with not having to find a 2.5 amp transformer, you could always use 6A3s or 6B4s. Same plate structure, just with 6.3v filaments. You could use a simple 6.3v dual-secondary transformer for those... Antek has one for around $13, IIRC.

BTW: I modeled this using a 5R4 rectifier tube, which will need 5v heater. To do that, just series the existing 5v heater winding, with the 6.3v 2.6 amp heater winding, and add a little dropping resistance to get the voltage at the heater pins of the tube down to 5V. Shouldn't require much resistance... three 1 ohm 1 watt resistors in parallel (to give about 1/3 ohm) should do it. As for heaters for the input tubes- just series the 1.2 amp winding with the 4 amp winding, and you got 6.3v at up to 1.2 amps. That'd run a number of options. In fact, if you needed more, you could parallel the .6 amp and 1.2 amp windings, series the 4 amp winding with that, and have 1.8 amps...

Regards,
Gordon.
 
Last edited:
I've done that before Gordon and it works fine. That said, I think I would take advantage of the extra volts and go choke input. Seems like SET amps live or die by their power supplies, so I would go all out on it. The last 2A3 I built was a direct coupled modified Loftin White AA. The only trouble with the design I have is, the film caps for the PS can get a little costly.
 
Jay,

My buddy that owes me the 2A3's also has some 83 tubes. I have asked him before for my Hickok tester.

Do you have a schematic for your build?

Any comments about the OPT's before I order them?...:scratch2:
 
I do have a schematic (kinda hand drawn) that I can dig out. I used those same outputs even. This is choke input, then split in to seperate supplies with a choke on each one. I had some power to shed too, so I used it up for filtering. Makes for a heavy amp though.:drool: The third pic shows the film caps I was talking about. Micah sent me some FETs to try a mod on this one, but I haven't got around to it yet.
 
Last edited:
I went with the JEL SRPP input 2A3 circuit. I love it. As to your 3k vs 5k primary impedance question, I went with the 5k primary impedance. You will have less distortion and any loss in output power was not missed with the LaScala speakers.
 
Bit the bullet....

...and ordered the Transendars from Gery. 3K primary, 8 ohm secondary, and a 40%-50% UL tap.

Now I gotta pick a good design, and pull the rest of the parts together....:scratch2:

Anyone know why the 6SL7 seems to be such a popular driver tube for 2A3's?

I have a couple NOS 6SL7's of late manufacture, but I'm a lot better stocked with 6SN7's....

The 717A design is also tempting, but I only have two, and they're not an exact match IIRC....
 
Last edited:
I'm gonna get around to that pretty soon. Got all the parts gathered...

jaymanaa said:
I do have a schematic (kinda hand drawn) that I can dig out. I used those same outputs even. This is choke input, then split in to seperate supplies with a choke on each one. I had some power to shed too, so I used it up for filtering. Makes for a heavy amp though.:drool: The third pic shows the film caps I was talking about. Micah sent me some FETs to try a mod on this one, but I haven't got around to it yet.
Click to expand...
 
I like the SN7, gives you a lot more options later, and better bandwidth due to lower impedance. The only sacrifice is gain, which for this won't be a huge deal.
The SL7 is probably popular for the same reason as the AX7: high gain.

analog addict said:
Anyone know why the 6SL7 seems to be such a popular driver tube for 2A3's?

I have a couple NOS 6SL7's of late manufacture, but I'm a lot better stocked with 6SN7's....

The 717A design is also tempting, but I only have two, and they're not an exact match IIRC....
Click to expand...
 
I'm not planning on running anything less efficient than 102db/w/m with this amp. I currently have my 45 SET amp on in the other room, and it makes the model 19's plenty loud for a small room.

However, given the progress of my new mancave, I was looking to jump up to 3 watts or so for some more oomph on some of my other horn systems.....

Micah, might I request some guidance at modifying one of the SL designs to a SN design?
 
jaymanaa said:
That said, I think I would take advantage of the extra volts and go choke input. Seems like SET amps live or die by their power supplies, so I would go all out on it. The last 2A3 I built was a direct coupled modified Loftin White AA. The only trouble with the design I have is, the film caps for the PS can get a little costly.
Click to expand...
I've been kicking around ideas for the next SET amp build, and I'm getting hung up on the power supply thing. Choke input vs. cap input, low DCR vs low ripple and all that, I'm gettin' purty cornfused purty fast.

Jay, what would happen if you used big oil caps in that Loftin White, instead of the film caps? :scratch2:
 
You must log in or register to reply here.
Back
Top Bottom