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Old 7th July 2002, 04:05 AM   #1
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Default DIY Hi-fi Stereo Tube Amplifier

Wow! Just discovered this site, what an amazing resource! You guys are like, family, man!

My question:
I would like to build a tube-powered, Hi-Fi Stereo Amp. I am going to use this to drive a set of bookshelf speakers, and possibly a subwoofer in the future. I need at least 50 watts per channel into an 8-ohm load. I will construct the speakers in the future. This setup will be run off the soundcard of my computer. I was hoping to get a schematic or circuit from something Harman Kardon, I really admire the old designs, but am not familiar with the model numbers, and do not wish to risk an E-bay purchase.

Any advice?
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Old 7th July 2002, 04:38 AM   #2
PedroPO is offline PedroPO  Portugal
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ASSASSIN!!!

It is ILLEGAL to use tube amps with PC soundcards!

Ask anyone!

You Should go for a kit, but If I were you I'll stick with a dosen of watts. 50 of them is much for valve amps (altough not impossible).

Buy a cheap CD player (but decent) (yes don't use MP3) if your low on . Or a turntable, if you have one.

My valve amp is a Audio Note Kit one and it is a kit as you can imagine.

The thing in valve amps are the parts.
If you don't buy a kit you may find a little dificult(yet possible) to find the transformers, valves, chassis.

If you want I can send you some pics or schematics os this amp or goto www.audionote.co.uk and browse the kits.

This amp has 8W per channel, class A operation Single ended design, no feedback, Directly heated 300B Triode.
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Old 7th July 2002, 05:30 PM   #3
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I beg to differ - I have a tube amplifier hooked to my computer (only five watts per channel though) and it sounds MUCH better than my previous sound card, which had an IC ampifier.

Fifty watts is a lot for a tube amp - 25-40 is about the limit for a single pair of output tubes (of course you can double that by using parallel pairs, but it gets expensive fast). If you use a separate subwoofer amp, you can probably get by on quite a bit less power.

The Harmon Kardon Citation II is one of the best amps ever, according to many, but it would be difficult to duplicate today. The key is the transformers - hard to find anything comparable now.

Hammond and Magnequest, to name a few, make good iron today. Select an output transformers, output tubes, driver circuit, power/filament/bias transformer(s), and get out your soldering iron... or look for a kit, if you haven't done this before. I tend toward the complete DIY approach, but I haven't built much in recent years - I am getting back into it though. Check my (temporary) website - http://www.audiophool.2y.net
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Old 8th July 2002, 12:53 AM   #4
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thanks tom!
pedro, i guess i should have mentioned this is not my main listening station, merely an upgrade to the crappy stock soundcard/speaker setup. no need for alarm...i would be the first to hang my head in shame if this were otherwise....
i did look at your reccommended amp, and yes, it is sweet, but i fear, out of my budget, and, not high-powered enough. this is not to say that someday i will not want one, i have heard about the 300Bs...

tom-
checked out the hk cit2 as advised. sweet! the 4x KT88s in the output stage would give me plenty of juice. observe: us guitarists crank out about 100 watts of power from that particular setup, so ~ 40 to 50 watts of clean power into an 8 ohm load with 90+ dB efficiency JBLs should present no significant difficulties, right?

still-
i have an allied radio/knight stereo integrated hifi amp rated at ~20 watts max output, 4x 6l6s in the output stage. it was not loud enough! when i first got it, it needed a tube rectifier, it then ran BEAUTIFUL (albeit quietly) for about a month. then,
"she jes' up an' died!" this was a year ago, so bear with my memory. no filaments, no hum, no glow, no nothing. it is in my basement back home, i am in school, now. any suggestions? fix? relace? i have not been able to locate a schematic.
thanks again.
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Old 8th July 2002, 01:36 AM   #5
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Find the Knight model number, and we'll see what we can do. No filaments - that sounds like the line fuse or (perish the thought) power transformer. Even then, maybe $75 for a new one - not THAT bad - assuming you can find one.

A Citation 2 goes for about $1000 in original shape - and they're not that easy to find. But most modern amps with KT88s or 6550s will be as much or more.
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Old 8th July 2002, 02:05 AM   #6
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thanks, dude!
i go home in about 4 wks, so i can get the model number then. you know, i may end up replacing the transformers with some higher-rated Hammonds anyways, as i will then be able to tweak some values and get more output, correct?
40 or 50 watts is still my main goal. this should be no problem from 4 6l6s.
i also have an rca tube manual that has schematics in it. one for a 50 watt stereo int. amp, with very impressive specs. it even has a parts list!
as far as the citation goes, that is way too much money for me to spend, maybe i could build a citation clone with the knight?
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Old 8th July 2002, 02:51 AM   #7
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Yeah, a pair of 6L6s can do 50 watts, but not 50 CLEAN watts... I wouldn't hope for more than stock performance from the Knight- the driver probably isn't up to it even with new transformers and a bigger power supply. For 50 watts, I'd want KT-88s or 6550s.
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Old 8th July 2002, 03:28 AM   #8
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allrighty then,
what about 5881s? or are they the same as the 6l6s?
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Old 8th July 2002, 04:40 AM   #9
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No, 5881 is a shorter version of 6L6 - it's not rated as high as the 6L6GC, either.
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Old 8th July 2002, 08:56 AM   #10
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Quote:
Originally posted by el capitan83
i may end up replacing the transformers with some higher-rated Hammonds anyways, as i will then be able to tweak some values and get more output, correct?
40 or 50 watts is still my main goal. this should be no problem from 4 6l6s.
The Hammonds are good iron for the money, and picking up a pair of these in the right size and impedance will put you in the best position to get your 50W

I agree with Tom below that a pair of 6L6's might not be the best for trying a 50W power level. KT 88's or 6550's are a much better bet, especially for long term reliability. The 6L6's would be running a bit hot for my taste.

Quote:
i also have an rca tube manual that has schematics in it. one for a 50 watt stereo int. amp, with very impressive specs. it even has a parts list!
Might be OK, but it would depend on the circuit design. Resaerch around on the net, and see how the design is regarded, to get some idea as to how good it will sound. Low feedback designs, or using exactly the OPT specified are the best ones for you too be looking into.
Quote:
as far as the citation goes, that is way too much money for me to spend, maybe i could build a citation clone with the knight?
I reckon a well restored or mint Citation is probably excellent value for money, especially compared to some of the high-end dross that's being sold for big money today. There are reasons why designs like this and the Marantz 8 are still highly sought after and fetch good money. They were very well thought out, designed and optimised for the purpose by people who <i>really</i> knew what they were doing.

As for making a Citation 'clone', sorry, you won't. The Citation uses nested feedback loops and is dependant on the exact OPT as specified or it probably won't be dynamically stable. Stu Hegeman (original designer) might make it work with a Hammond, but you or I won't.

So the big question: do you really <b>need</b> 50W or is it a number you have in mind? If you can deal with 35W, then the Dyna Stereo 70 is a possibility, and there are tons of them out there, lots of mods and spares and knowledge. It's a great design that uses EL34's in ultralinear. There are lots of nice sounding, cheap new EL34s, and Magenquest even make an exact replica of the A470 OPT with better materials, if you can't find a pair cheap on ebay. BTW the difference between 35W and 50W is only 1.5dB, hardly a huge amount. If your speakers are at least 85dB, 35 W is plenty. Remember tube amps don't clip hard like SS amps do, and seem to give lots more level than their rated power would suggest. Take a look at this <a href="http://www.stereophile.com/fullarchives.cgi?357">article</a> on the subject.

I disagree with Pedro at the top of the thread about parts availability for things like tube, transformers and chassis. They are easy to get online at lots of sources in the US, Europe and Asia. Let me know what you want and I probably have a URL or two for it.

In the meantime, look at
http://www.worldtubeaudio.com/directory/directory.htm
www.tnt-audio.com/ampli/dynaco1e.html
www.tnt-audio.com/ampli/dynaco2e.html
http://www.audioasylum.com/forums/tubediy/bbs.html

HTH some
Cheers
Brett


np: Alison Krauss and Union Station: so long so wrong
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