newbie plunges into DIY 2A3 SET, challenges abound

[Tom Mobley]

I posted this deal over at the Tube DIY Asylum, don't know if I'll get much response, I'm new over there.

Yeah, I know there's better iron out there, but for a first effort I feel the need to moderate.

I'm inviting comment, especially in the areas mentioned below. Jeff, Erik, Mark, Kelly, Craig, anybody, feel free to bomb away. This deal will be done mostly online, I'll doc it up , post pics, whatnot. Among other things I need help slecting a schematic, there's literally a ton of SET and 2A3 plans out there.

Here's what I put up over there:

Well, I'm about to take the plunge into DIY SET. No kit, just build it. I want to build something pretty conventional, I'll probably have enough issues debugging something reasonably normal. Hammond has this 302AX PS, has these outputs:

198VA

300-0-300

200mA DC

50V bias tap

2.5V CT @ 2.5A

5V CT @ 3A

6.3V CT @ 6A

I can get something similar from Electraprint, I spent some time on the phone with Jack yesterday.

Now, I was thinking to run a 5AR4 rectifier, mostly because it's what's in my Scotts. Would there be any advantage to running a 5U4? according to the tube data I found at http://hereford.ampr.org/cgi-bin/tube?tube=5U4G it wants 3A at 5V, that all that tranny is rated for. The 5AR4 is stated as needing only 1.9A, seems it would be easier on the PS. Looks like either will provide the needed voltage.

Apparently the 5AR4 has built a reputation as slow to build output voltage, easy on the other tubes and stuff. But the 5U4G is available in the shouldered bottle, cool looking.

Any caveats? warnings? good advice?

while we're at it, if I plan to use these monoblocs with a preamp, do I need an input tube? I was thinking to use a 6SN7 and 2A3, standard cap coupled deal. there's a lot of schematics out there, some have an input tube, some don't.

any suggestions on OPT? I'm looking at Electraprint TM3KB or less expensive Hammond 1627SE, both of these claim to be good for 2A3, but have quite a bit different primary Z. 3400 for the Electraprint, 2500for the Hammond.

I'll be driving a set of Klipschorns with constant impedance crossovers, friendly load.

Tom

[Erik Mandaville]

Tom, this is a GREAT plan for a project!

Sounds to me like your headed in the right direction. I think a slightly larager power transformer would be better for this, though. The PT302AX is designed for 2A3s, but this will probably require a fairly large input capacitor to nudge the voltage up. Some companies use that approach, which of course works. What's also an advantage of 302AX is that it has, as you noted, 2.5 volt filament winding for the 2A3. The Classic Hammond series are really nice, and I used one for the Horus monoblocks I made this summer. I can't remember exactly what he said about them (classic series), but I believe Jeff's opinion on them was positive.

For the Horus, I used PT374BX, which has a higher voltage rating. However, it DOES NOT have a 2.5V filament secondary. I used a separate 2.5 volt filament transformer, with primaries paralleled with those of the 374BX. This has worked out very, very well, and the Horus are very quiet amplifiers -- even much more so now (more on that in another post). I am using a small value input capacitor (1mfd) into an LCLC filter. This will provide B+ of about 330 volts. Remember that actual plate voltages on the 2A3 are taken between the plate and cathode, so the voltage will be maybe 45-50 volts lower. Perfect for 2A3s.

The Horus schematic has been on the forum, and would be several pages back. If I were to build this amp again (which I actually may be doing for a friend in the near future!), I might use a single 10-15 H filter choke, but the dual chokes above have helped make for a very quiet amp. Depending on the input/driver you use, you'll also need full-wave bridge rectifier for DC heaters. You could get by with simply grounding the 6.3V secondary center tap, but depending on the tube, DC might be better. You can build a full-wave bridge out of four diodes. The rectifier for this power supply is a 5AR4.

Jeff's choice of the AC coupled 5687 works extremely well (IMO), and would also be a great choice for this amplifier. You could actually use the input driver and power supply circuits of the Horus, and very easily incorporate the Ultrapath connection (pleace consider that!)down the road sometime. Moreover, you would be just about perfectly set to take the parallel or shunt feed route if you wanted to. I would actually consider that right now, if possible!

If you can get an Electra-print OPT that would by great. They are very high quality OPTs, and I have them on the Moondogs, and just worked with the latest edition on Kudret's DRD. Some say that the OPT is among the most important, sound influencing aspects of an amplifier, and there are lots of choices -- Magnequest, Electra-Print, Toroidal (from Plitron).

Also, I think you might choose a rectifier rather on it's function and performance over looks...which I know can be important, too! Edmond sent me a new pair of Phillips 5AR4s, which truly seem to be a step up from the JJs that I had been using. The JJs have a really neat shape and cool-looking writing, but the Phillips are honestly the better tube, I think. But I was using them with the new Golden Dragons we just got.

Ok -- I've blabbed on enough. All the guys you mentioned should be a BIG help, and LeoK has very professional experience with all of this, too.

I think it's great you want to do this, Tom. I have also seen some schematic using direct-coupled input/driver stages, which I think may have some advantages. Since this is going from the ground up, you might want to experiment using a choke on the 2A3 grid instead of the usual resistor to ground. (see the BCP-16Ni from MagneQuest).

Have fun, and please let us know how things go! Welborne Labs sells chassis separately, too. Maybe one of those or one from IAG (tough chassis to punch!) would be nice if there is a way to do that.

Erik

[Erik Mandaville]

What constant impedance networks are you using? Are you thinking of an autoformer coupled approach, or something -- like the 'Zero' autoformer? I'm curious about this, Tom.

[Erik Mandaville]

The 374BX transformers I'm using are absolutely quiet -- no mechancial ANYTHING. No buzz, hum....very queit and ruggedly built. They do tend to run a little on the warm side, I've noticed, and I'm even using separate filament transformers for the 2A3.

[Painful Reality]

I'd second the 374BX choice too if you want to go with a tube rectifier. Using a 300-0-300V will require a large input capacitor to get you around a 350 B+ (quite typical B+ if you wnat to build a 2A3 in self bias mode). The catch is that a large input cap will limit the rectifier you'll be able to use.

I would stay in the Hammond 300 serie. They are fairly cheap, pretty robust and extremely quiet compared to their 200 serie.

If you do your homework well, you'll find a lot of nice 2A3 schematics availabe on the web.

[Erik Mandaville]

Tom: I was going to use a less expensive Hammond PT for cost savings on my Horus, and it was Jeff's idea to just make the greater expense. I'm so glad I did that, despite the hassle of sending the others back to AES (Antique Electronic Supply). I've used cheap power transformers on other projects and they can just be a paint. But you're in the Classic series, anyway.

[mike stehr]

"be careful in selecting orientation and placement of this power trans."

There is a trick to this. Power up the transformer, run the output through a scope, and orient the transformer/transformers around the chassis watching the scope for noise levels. When the scope shows it's lowest noise levels, then you may have found the right orientation.

Now I'm not sure about power supply transformers, but I've seen it done with filament transformers(on my amp), and I would guess OPT's would work too.

This is with a metal chassis, BTW.

[Deang]

Well now, that's a neat idea.

[NOSValves]

Mike,

I'm curious as to how you would go about doing such a thing? The last I seen wires have to run through the chassis to fire up the amp and see what the deal is. Unless your going to chance scrapping a chassis to investigate this issue. I really find that noise is not a problem at least the amps I have been mocking up and I mean no noise hum, hiss or otherwise. This even with the first PT being complete physical vibrating piece of garbage nothing came thru the speakers or the scope ! But get within 3 feet of the thing and you sure could hear it LOL!! Hand wound tranformer what a POS . Yes Jeff I know I know !

Craig

[mike stehr]

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On 12/7/2003 2:33:31 PM NOSValves wrote:

Mike,

I'm curious as to how you would go about doing such a thing? The last I seen wires have to run through the chassis to fire up the amp and see what the deal is. Unless your going to chance scrapping a chassis to investigate this issue.

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You would do it with the raw chassis before it is even wired. You power up the PS trannie, and move about the OPT's watching or listening for the hum, and try to cancel it out as much as one can.

You are trying to orient the transformers so the electromagnetic feilds they produce will cancel each other out, and (I think) you don't want the feilds crossing each other.

That's the best I can recall, I'll have to ask my audio friend Dan on how to actually go about it, but I know it's not that difficult.

I'm sure other's here can try to clarify what the hell I'm trying to explain.

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I really find that noise is not a problem at least the amps I have been mocking up and I mean no noise hum, hiss or otherwise.

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Right on!

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Hand wound tranformer what a POS . Yes Jeff I know I know !

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Uh oh. You bought some trannies off Ebay from the Hand Wound Transformers boy?

Yeah, ol' Dan bought OPT's from that cat. Dan measure's them of course, comes up with different primary impedances, output impedances are all the same, things like that.

He sends them back, they send another pair. Hey! Same trip!

Dan voiced his opinion on Joenet, and Hand Wound Transformer boy cut him off.

We kinda figured the guy may have tightened his quality control, since it has been a couple of years.

Guess not.

[Tom Mobley]

Thanks guys for alll your input here. I'll probably be making a PITA out of myself asking questions here in the near future. I'll be digesting all the recs for a couple days, looking up stuff and whatnot. One of the guys on the tube asylum thinks the 6SN7 I was thinking to use to drive the 2A3 is a terrible choice. What's up with that? Nice 6SN7's are easy to get if not cheap. He mentioned using EL-34 for this? What's the thinking here on this forum on this?

Tom

[mike stehr]

Your going to get a million different opinions and options, Tom.

I'd find some design you fancy and go with it.

I think the 6SN7 doesn't have enough gain to push a 2A3.

(Not too sure...)

[Erik Mandaville]

Welborne used to use 6SN7s, too. Doesn't anymore. I found the 5687s on Jeff's Horus an outstanding tube -- in my opinion easily better than any 6SN7 I've used. It's very quiet, and not prone to the often horrible microphonics of some 6SN7s.