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Old 11th August 2004, 08:56 PM   #1
percy is offline percy  United States
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Default The Next Step. (and budget tube amplifier kits/designs).

Hi,
I have always been an electronics hobbyist but never went ahead and built a *good* vacuum tube audio amp - except for a little 6C5 triode amp that I built few years ago while reading an old book(ARRL handbook?), which was more of a self initiated school project. Nothing serious. I have a fair idea about tubes and have worked with them. Have a few vacuum tube radios lying around(including an R-390A/URR) that I love working with. Yes I am a ham - KC0GVT.

So I've been around tubes but never really built a serious 'audio' amplifier for listening pleasure.
What I'd like to do now is learn more about designing audio amplifiers - feedback, SE, push-pull, power supplies, layouts,...all there is to it for building a good tube audio amplifier. Now come the questions -

So where do I go from here ? Books ? Links ? I am right now reading the Svetlana Tech notes and AudioXpress audio class articles. Especially books - which ones should I get ? I have in mind the Morgan Jones and the 'Valve & Transistor Amplifiers'(by Hood?)

And while I am taking everything in, I'd like to build a quick basic kit - like the ASL Wave 8 and the AES or S5 K-12. I saw AX84 also but I am not sure if their amps are more 'guitar' amps or typical audio amps for typical stereo applications - or if there is any difference?!

As you might have already guessed by now - I am on a budget - ideally $150. Max $200.

I'd like to build something more than an first step "learning" kit, something that will give me good performance also.

I am open for suggestions of kits that have scope for improvements(mods, upgrades) and will eventually perform much better. That could become a learning process too.

Your inputs, suggestions, will be of invaluable help to set me off in the right direction from where I am right now.
Looking forward to an exciting ride!
Thanks!
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Old 11th August 2004, 10:11 PM   #2
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At the top of this forum is a permanent thread : "On Line Tube Learning for newbies...."

On Line Tube Learning for newbies....

It has enough reading to keep your attention for quite some time, and can lead you to other links, etc...

For your budget, you might want to start with a preamp.
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Old 11th August 2004, 11:32 PM   #3
percy is offline percy  United States
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Default the next level

I did go through that thread last night. Its a very useful thread for the absolute newbie, and definitely has some information for intermediate to advanced diyers also - like some of the Svetlana Tech Bulletins and AudioXpress audio classroom series.

Someone mentioned in that thread - there should be two threads one for people who havent seen or know what a tube is, and one for people who have some knowledge. I believe that thread should progressively extend into advanced level also or atleast above beginner/newbie level. Thats the area where I am looking for information - 'the next level'.
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Old 11th August 2004, 11:55 PM   #4
Wodgy is offline Wodgy  United States
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For your budget, I'd recommend the S5 K-12. It's a great starter project; I enjoyed building mine. Even stock it sounds pretty good. There's a review of it from a sound perspective on AudioXPress' website.
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Old 12th August 2004, 04:00 PM   #5
percy is offline percy  United States
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Default Question about (Topologies used in) the S5 K-12 and ASL Wave 8.

Yes I did see the review and looks like its acousitc qualities are better than the measurements - and so does the reviewer asserts also.

I am trying to select between these two kits and I'd like to find out more before I make a decision.
I'd like to know what topology each one uses. Push-pull ? or SE ? What class ? Is feedback used ? how much ? what type ? Quality of power supply, output transformers etc...
I am guessing given the power output they produce(~8W) using those tubes they must be push-pull.
BTW, can I get just the PCB of either of these amps ? or do I have to buy it as a kit ?
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Old 13th August 2004, 01:29 AM   #6
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I built my first tube amp earlier this year and almost bought the S5 kit. Glad I didn't, as I found I could build a better amp with the same money. I chose the 6S4A amp at www.the-planet.org. 6S4's are extremely cheap but good for a couple watts of clean power in a single-ended triode amp (no feedback or tricky adjustments). This circuit worked well at first power-up. I added a second 6S4 in parallel to get 4 watts with no problems. I spent less than 150, including chassis.

The most expensive parts is transformers. Output transformers cost about 100 or more, but the Hammond 125SE series are closer to 30 apiece and sound fine to me. I used a generic 12v ct, 3A transformer for filaments, and used a 12v, 1.5A transformer to jump back up to 120v, then a voltage doubler to get over 200 volts for plate voltage.... works great.

A decent SET amp gives great dynamics and depth to acoustical music, I love it. If you prefer rock or modern processed music, you might consider a pentode push-pull design instead. The Svetlana 6AS7 SET amp design looks pretty good too, and more power than the 6S4. If you use low cost transformers, you could build that for your price range. You could always upgrade the transformers later!
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Old 13th August 2004, 01:37 AM   #7
Wodgy is offline Wodgy  United States
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As far as I know, the ASL Wave 8 is a commercial product, not a kit. It was manufactured by Antique Sound Labs a couple years ago but (if I'm not mistaken) has since been discontinued. You can still probably find them used easily enough.

The S5 K-12 uses a PP pentode output stage. You can convert it to triode mode very easily, but power drops to 2-3 watts. Converting it to SE is not very realistic; it would be better to start with something else if you plan to go that route. There is global feedback. Quality of the power supply and output transformers is poor in absolute terms (this is why distortion increases so significantly in the bass) but excellent for the price and appropriate for the kit. As far as I know the PCB is not available separately. Just in case you're not aware of it there is a related kit based on the same circuit, just in monoblock form, available from Antique Electronic Supply.

If you're looking for a raw PCB, the closest project easily accessible to beginners is DIYtube's Dynaco ST-35 clone. That whole project can come in around $300 (you'll probably end up spending as much just on transformers as the whole SE K-12 kit price, but good iron is expensive).
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Old 15th August 2004, 06:29 PM   #8
percy is offline percy  United States
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How good are the OPTs of the S5 and AES kit as compared to Hammond's 125 series (universal PP) in terms of quality/performance ? Is it night and day ? or not much ?
(Although the frequency response of the transformers is not below 100hz. The cool thing I like about the universals is that noobs like me can experiment with the impedance matching. Saw them going for ~$25 on Angela). Anyway back to the original topic -

If there is not much difference then it would not make sense to upgrade or even build this kit. I'd rather drop the idea of the kit, pick a *good* design and spend my time and money on the iron and parts there. Although I did a quick price check list for the ST-35 idea, it comes out around ~$350 or maybe even a little more. I am looking at something to complete around $200-$250 max until I can afford better.

But if there is considerable difference then maybe I can apply upgrades to the kit like better transformers, power supply etc.., learn on the way, enjoy it for some time while I plan a *real* project.

Makes sense ??
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Old 17th August 2004, 01:03 AM   #9
Wodgy is offline Wodgy  United States
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I don't have any experience with Hammond's 125 series, so I can't comment on that question.

$200-$250 is a challenging price point for a tube integrated amp. Your criteria (good design, good iron, low cost) are at least partly mutually exclusive at this level. You can definitely build something decent for this price, but you're white knuckling it. I still think you could build an ST-35 for just over $300, given that there's a guy who sells a kit version of that project for $400 (if I recall correctly), and there's got to be a profit in it for him somewhere.

Have you thought about picking up a dead Dynaco on eBay? The iron is better than virtually all iron of modern manufacture (measurements are available on DIYtube's forum), and it could knock off $150 off the total cost of the ST-35, putting it squarely in your price range, with very good iron to boot.

Another good alternative would be to build a tube preamp as your first project. You could put together a 12B4A (one tube per channel) for around $125, possibly under $100. Combine it with an older solid state power amp off eBay and you'd have a very decent system to start with until you save enough for a bigger project.
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Old 17th August 2004, 07:18 AM   #10
SHiFTY is offline SHiFTY  New Zealand
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I would second the suggestion of picking up an old stereo parts unit and restoring that. It will be a lot of fun, I have hunted down several old tube stereos at junk shops near me, ususally for very cheap. Old stereo consoles can be raided for their amps and the rest binned. There are several threads and web pages about people who have built nice chassis for console amps.

With new coupling capacitors, they can sound very good indeed.

I would recommend against the hammond 125E series, they are ok but a bit limited in the bass, and vintage iron definitely sounds better...

One thing, don't worry too much about power, 5W is quite loud for most listening!
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