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Old 15th November 2007, 03:42 AM   #1
Zap is offline Zap  United States
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Default New to tube building / design - Input requested!

Hi,

I am looking at building my first tube amp. I'm an Electrical Engineering Tech student, so I have a good knowledge of transistors and electronics theory but sadly they don't teach tube theory anymore so I am going to have to learn that part on my own before I can design my own amplifier! (Any references would be appreciated!)

At any rate, I have been looking at the Darling amp for a first project, specifically the James DC Darling (DC Darling amp using James transformers). I'd love to hear if this is a good choice.

I would be grateful if I could get some input here as half of my questions/concerns are probably nonsense and the other may be valid...

First off, I was going to use the James output transformers, as well as the James 930 power transformer. I would like some opinions on using a James choke in the power supply as well, or if the Hammond would suffice? It seems to be cheaper so I'm not sure if there is a trade off. Additionally, the James choke is rated for a different current value. I presume this is simply MAX current it can run at and would have no effect on the operation vs a lower rated choke?

Also, would it be beneficial to have a manual/automatic circuit to heat the filaments prior to applying B+ voltages? I know that many amps have this, but I am not sure if this is a concern here? I'd like to build something slick, so optional improvements are still of interest.

Also, is there an issue with bias on these tubes, as far as the specific tubes being run in the amp would be concerned? Any way to accommodate this?

Finally, if there are any other suggestions or recommendations I would certainly be all ears. I want to build something great (I know it's my first try, but this stuff is expensive on my budget!)

Thanks a bunch!
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Old 15th November 2007, 04:28 AM   #2
7N7 is offline 7N7  United Kingdom
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Best of luck, Zap with your first project; be aware that this is a disease!

I cannot comment on your choice as I have no information about the Darling, but I do strongly recommend that you invest in some books on valve (tube) amplification and theory.

There is a number of interesting publications both new and reprinted, and I am sure that others on the forum will have some suggestions.
Valve Amplifiers by Morgan Jones will provide a good basic grounding in valve theory, together with a great deal of information and advice about building amplifiers. Getting the Most out of Vacuum Tubes by Robert Tomer is an interesting read and there is an excellent book that was published originally in 1947 by Philips in The Netherlands. I cannot recall the title, but the author was Deketh. Like Tomer, this is another reprint and very interesting

7N7
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Old 15th November 2007, 05:17 AM   #3
arnoldc is offline arnoldc  Philippines
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Hi,

I'm interested to know what speakers will it drive? What music do you play?

My brutally honest opinion about it-

If you're investing that much money on James iron, make a more powerful amp that is potentially better than a Darling. With that money invested, I'd sa a 2A3 (see my first question) is a better project for the same effort.

If you want to immerse yourself in this lunacy, buy the Morgan Jones Valve Amplifier + Tube Buildng (not sure) books. There are a lot of helpful people around here... Search the posts of Kuei Yang Wang (Thorsten) as he had several educational design endeavors made here, one was for me.

(oh... 7N7 just covered some books )

Just ask the questions, there are a lot of nice and patient people around

ps.

I have made a couple of 300B amps, and my latest one is posted in this forum. I also have a JE Labs 2A3 DX, a type-45, a 10Y and a 50. And of course, the Darling.

pps.

And I'm a newbie compared to the rest of the gang
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Old 15th November 2007, 05:43 AM   #4
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Lots of good information on Pete Millett's website.. Antoher on-line 'learning' site is Max Robinson's 'Fun with Tubes'. Actually, the web is full of helpful tube sites and it's woth Googling using a few key words.
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Old 15th November 2007, 07:23 AM   #5
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Default Re: New to tube building / design - Input requested!

Quote:
Originally posted by Zap
Hi,

I am looking at building my first tube amp. I'm an Electrical Engineering Tech student, so I have a good knowledge of transistors and electronics theory but sadly they don't teach tube theory anymore so I am going to have to learn that part on my own before I can design my own amplifier! (Any references would be appreciated!)
In that case, then go right to the source and download Radiotron Designers Handbook (it's a freebie). I started out as a solid state guy myself. Most of what you already know is directly transferable, with a few caviates:

*) Killer voltages. Vacuum tubes are high voltage, low current devices. VT equipment operates at hundreds of volts and tens of milliamps. Solid state operates at tens of volts and tens of amps. More caution is necessary, if you are to avoid:

*) Tubes are depleation devices, not enhancement devices.

*) No such thing as a "P Channel" vacuum tube.

*) It's an inherently low gain device, so the quick 'n' dirty formulas that serve well for transistors don't work for VT design. Use loadlines instead. You also have to give more consideration as to choice of types. Major difference between, let's say, a 12AX7 and a 12AU7. It's not like a transistor circuit where you can just throw about any "transistor anonymous" in there and expect it to work.

*) You'll be dealing with a Hi-Z device instead of a Lo-Z device. Pay attention to internal device capacitances: they do count here.

Other than that, just about every topology you've already seen still applies: differential amps, cascodes, grounded cathode/source/emitter amps, grounded grid/gate/base amps, followers. Hollow state or solid state: they still work the same.

Quote:

First off, I was going to use the James output transformers, as well as the James 930 power transformer. I would like some opinions on using a James choke in the power supply as well, or if the Hammond would suffice?


Makes no difference, go with the Hammond.

Quote:

Additionally, the James choke is rated for a different current value. I presume this is simply MAX current it can run at and would have no effect on the operation vs a lower rated choke?
If you see something like: 7H / 150mA, that current rating has nothing to do with max current capability. It's the current at which the choke has the rated inductance of 7H (give or take). At very low currents, the inductance can rise much higher, maybe in excess of 100H. At higher currents, the inductance will be lower. It can fall under a henry if core saturation occurs.

Quote:

Also, would it be beneficial to have a manual/automatic circuit to heat the filaments prior to applying B+ voltages? I know that many amps have this, but I am not sure if this is a concern here? I'd like to build something slick, so optional improvements are still of interest.
Depends. If using a solid state power supply, then either manual or automatic HV delay until the cathodes are hot is a very good idea. This is especially true if some stages are DC coupled. The HV comes up very quickly with a solid state power supply, while the cathodes are still cold, and so there's no current to establish operating voltages. Most small signal tubes don't really appreciate control grids several hundred volts positive.

If using VT rectification, makes no difference. If using Hg vapour diodes, HV delay is mandatory.
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Old 15th November 2007, 10:44 AM   #6
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Specifically this link to the book : Electronics For Physicists
http://www.angelfire.com/planet/funw..._Contents.html

It has a chapter on Tubes also. Plenty of maths. Only for those who really want a better explanation . Maybe some of the maths can be ( carefully ) skipped.
Cheers.
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Old 15th November 2007, 10:22 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by 7N7
Best of luck, Zap with your first project; be aware that this is a disease!

I cannot comment on your choice as I have no information about the Darling, but I do strongly recommend that you invest in some books on valve (tube) amplification and theory.

There is a number of interesting publications both new and reprinted, and I am sure that others on the forum will have some suggestions.
Valve Amplifiers by Morgan Jones will provide a good basic grounding in valve theory, together with a great deal of information and advice about building amplifiers. Getting the Most out of Vacuum Tubes by Robert Tomer is an interesting read and there is an excellent book that was published originally in 1947 by Philips in The Netherlands. I cannot recall the title, but the author was Deketh. Like Tomer, this is another reprint and very interesting

7N7

The Darling is a great amplifier if you have very efficient speakers - if you don't a 2A3 will give you 5 - 7X the power of the darling.. Even the 2A3 is not going to give you more than 3.5W per channel. The 300B is a little harder to drive requiring about 180Vpp for full output but will give you a healthy 7 - 8W output.

Use a tube rectifier for simplicity and that cool tube glow. 5AR4 provides a nice delay for free and current production is reliable.
Put a single '4007 in series with each plate lead and it is literally bullet proof.

Hammond 300 series power transformers are fine, very quiet and cost less than the James. They're not as pretty. Same goes for their chokes - do not over run them current wise as the inductance falls quickly and the open frame versions get HOT.

ASC PPIO motor run caps are readily available on eBay and make great power supply filter caps lasting forever and not costing much more than the comparable electrolytic. Check out midori meadows on eBay for her current selection. Do NOT use motor start types - they're not designed for continuous duty.

Jack Eliano at Electra-Print will make you very nice custom output transformers for about what the James will cost, and will prove to be a useful source of information. Plus he's just a nice guy to deal with.
(Doug will back me up on this one I'm sure.)

Sometimes there are Transcendar 3K SE transformers on eBay for around $120 a pair which are great for 2A3 and 300B amplifiers.

2A3 and 300B designs inspired by Loftin-White are worth taking a look at.

You may find some useful information on my site. (listed below)
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Old 16th November 2007, 03:19 PM   #8
Zap is offline Zap  United States
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Thanks for all of the input so far - here's the rest of the details!

Basically, I found the Darling because I had decided on something fairly inexpensive for a first project. I don't have anything fancy at all right now, just a set of Bose bookshelf speakers. While I'm aware they aren't great, they do sound better than any of the speakers I've had in the past. Sadly, I don't know if they post efficiency specs on them (maybe not good enough to?). Right now I'm in an apartment, so I don't need anything incredibly loud, but I would like to occasionally be able to get some volume. I currently have a SS cheapo receiver running the speakers so I know anything I build will sound a lot better. I wish I could figure out the wattage on it so I'd have a comparison.

I've already learned a lot from this post - the James choke isn't rated for the correct current! However, Hammond makes tons of chokes that have the value I'm looking for 5H @ 75mA. I'm not sure how to choose. The reason I picked James was after seeing a frequency response plot for a Darling with the Hammond output transformers vs Lundahl's the response was a lot better (no loss of lows/highs). I read and inquired with the designer and he said the James were a good set and should work well. The matching power transformer was done to match, I suppose. Any comments on the output transformers would be great - I want to at least build an amp that sounds better than the stuff I have, hence my concern over the output curves!

Musically, I listen to a wide range of things.. classical, jazz, vocals, organ, rock, trance/techno. Depends on the mood.

I had considered a 2A3/45 amplifier but my concerns were that the cost was going to be prohibitive to build something using all high quality parts, and that the tubes would break the bank! It seems like most of the "audiophile" tubes cost an arm and a leg just due to popularity (even the modern replicas). At the same time, I do want to build something nice...

Thanks!
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Old 16th November 2007, 03:42 PM   #9
kevinkr is offline kevinkr  United States
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I used to work for Bose so perhaps you can provide the model number? (Wood or plastic cabinet?) Most of 2 ways will be in the high 80's range so these are not good candidates at all for the darling. You can build a 2A3 amplifier for very little more (if any) money than the Darling, and even that is not going to get very loud with the Bose 201 or 301, but might be adequate in smallish room if you don't need to rock out. (I would be happy to give you a very simple and inexpensive 2A3 design to build that sounds GOOD.)

Unless you can ditch the Bose speakers for something more efficient in the relatively near term you might want to consider a small PP amplifier instead with say 10 - 15W of power, particularly if you like very loud music.

You could always build the SE amplifier with the understanding that when funds permit you will buy a suitable high efficiency speaker kit, and live with the modest spl levels you can achieve until that time.
Sound quality will still be much better provided you don't push the amplifier hard in the quest for loudness.

The Darling won't cut it with anything you could reasonably afford to buy or put together.
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Old 16th November 2007, 06:15 PM   #10
Zap is offline Zap  United States
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The speakers are the Bookshelf 301 Series V (looks like they are still in production). I certainly would be interested in considering the 2A3 amp instead of the Darling. I'm still on a student's budget, so it will probably be another year or so until I can search for something better.

I'm thinking I still want to go with a SE amp, I just wish I could audition ANYTHING tube to get a better understanding of how it would sound prior to building. I don't really know how much power I use! Do you have a rough idea of the parts cost for the 2A3? I was probably looking at spending a bit more than most of the Darling amps usually cost anyway.
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