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Old 8th November 2007, 12:31 PM   #1
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Default I have $250 to spend..what amp should I build?

Hi, I am new to building tube amps. I can read schematics and am looking for a good push-pull AB amp. I want 15 watts or more of power. I am interested in best sound quality (I know its subjective, but that is what I am aiming for). Anyone have a particular design they think sounds great? I am open for any suggestions, thanks.
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Old 8th November 2007, 12:59 PM   #2
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I think you'll find it tough to build a STEREO amp for $250 - transformers alone could exceed that. But if you're willing to scrounge some used parts, it could be done...

EL34 or 6L6 can give you 20W or more in class A. 6BQ5s or similar can be done for less, if 10-12W is enough (Some rated them at up to 18W, but they were exceeding the tube's voltage ratings...)

If you've never built anything like this, rebuilding an old amp (even one from a stereo console) is a good first step.
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Old 8th November 2007, 01:00 PM   #3
SY is offline SY  United States
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How good are you at scrounging parts from surplus? Aiming for "best sound quality" "15 watts" and "tubes" is a challenge at $250 if you have to source all new parts.
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Old 8th November 2007, 01:26 PM   #4
jduffy is offline jduffy  United States
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Default I remember reading about a cheap amp somewhere

There's something called "El Cheapo" based on a schematic and some guys who can help you. I forget what the specs were or the total amount of money you would need to spend in order to build it.

Do a quick search in Google and something should pop up.
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Old 8th November 2007, 02:05 PM   #5
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Thanks guys. I will do some hunting around online. I have a friend who has a bunch of old output transformers and some tubes and old enclosures so a lot of the more expensive parts will be pretty cheap to me.
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Old 8th November 2007, 02:27 PM   #6
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The "el cheapo" circuit can deliver a few good watts, but not 15, regardless of output tube - since it uses them triode-wired. With pentode or ultraliner wiring, it would need some negative feedback to lower the output impedance - and I don't think there's enough extra gain for that. Add another stage, and it would be fine - you'd have the EICO circuit.

Find some good-sized output transformers, 3K to 8K center-tapped, and work from there. Better to buy the power iron - it's cheaper - than to try to match outputs to what you have.
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Old 8th November 2007, 03:05 PM   #7
rdf is offline rdf  Canada
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If you have time every once in a while stereo jukebox amps show up on Ebay. Mine is a Rowe R-4359A that went for $50 plus shipping, decent iron.
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Old 8th November 2007, 10:34 PM   #8
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The K-12 / K-502 Tube Amplifier Kits will fall within your budget once you add in shipping and an enclosure. They are good little amps, but fall short of your power request (15W). They are rated at 8W, but beyond 4-5W, I hear too much distortion.

Be warned, once you build one, you will be looking for more and bigger budgets!
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Old 8th November 2007, 10:57 PM   #9
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I'd check out the tubelab simpleSE. "could" be done for $250.
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Old 8th November 2007, 11:50 PM   #10
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The "El Cheapo" schematic is here.

You can get approx. 15 WPC from that topology in 2 ways: ultralinear mode "12" W. O/P tubes and triode mode 7591s. 7591s will require about 425 V. of B+. The 12AT7 splitter/driver has enough gain. Please observe that 9% of the speaker drive voltage is returned as loop NFB.

$250 is unrealistic for all new parts, including "iron". Copper and other commodity prices have been on the rise. If you can get your hands on a pair of recycled O/P trafos (with UL taps) from either a 6BQ5 or 6V6 amp at little or no expense, that $250 goal can be achieved. Jim McShane has bundled the necessary "small' parts into kits at fair prices. Look here. A highly satisfactory power "iron" set can be purchased at modest cost from Allied Electronics. I enumerated the necessary items and their costs, here.
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