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Old 9th June 2013, 03:44 PM   #1
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Default KT88 Tube Guitar Power Amp

I have an Avid Eleven Rack that I want to use as a head. I need a power amp for it and I would like to build one with KT88 power tubes and about 50 to 100 watts. Are there any kits available anywhere or how would I go about learning to build it and find parts. I don't know much about electronics except guitar wiring.
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Old 9th June 2013, 05:41 PM   #2
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You do know how much this is going to weigh, don't you? And how much heat it's going to put out? The big problem is going to be getting the transformers; well, not really the power transformer, that's pretty standard, but different output transformers sound noticeably different, particularly when you're running into output distortion (and if you're not intending to run the output into a few percent of distortion you might as well get a transistor amp and save your muscles a bit of excess). The output transformer's going to be about your biggest single expense, and you don't want to get it wrong. Perhaps find a guitar amp you like the sound of and see if its output transformer is available as a service component; they do burn out sometimes. And even fifty watts of guitar is ear-splittingly loud (at certain concerts I've had to request a guitarist with a thirty watt amp to aim it at himself while I balanced the level in the PA, because he was overwhelming the vocals and forcing the hall volume into the intolerable).

Remember, Amps like this will be running at potentially lethal volts, that these will hang around in the reservoir capacitors for several minutes after unplugging the amp. This is not a project for the faint hearted. Still, tag board construction means easy modification, your average tube circuit is very modular, easy to see where preamp stages end and power starts, and lots of schemas are easily available; building and punching a chassis solid enough to carry the iron is going to be an interesting project…

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Old 9th June 2013, 09:46 PM   #3
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AX84 50W Push-Pull, and change the output tubes to KT88.

Board Layout for above.

Many experienced builders will be able to help you at its forum.

Jaz
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Old 11th June 2013, 07:17 PM   #4
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I don't know if a tube poweramp for someone who doesn't know all that much about electronics, let alone tubes in general, is the best idea. Lethal voltages, high currents, heat... things can seriously go wrong. I started with preamps to get the hang of it.

If you want to learn about tubes in general (triodes, pentodes, loadlines, phase splitter, feedback, etc. etc.) I can highly recommend 'Designing tube preamps for guitar and bass' by Merlin Blencowe. It's focused at preamps, but much is applicable with poweramps. Check out his site too: http://www.valvewizard.co.uk/
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Old 14th June 2013, 09:31 PM   #5
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I have just come up with a 100 watt valve design.
It uses 2 pairs of el34.
It has ecc83 front end driving a tone control then a phase splitter.
There is no feedback loop to try and keep some distortion.
I picked up a Fender output transformer on ebay for 60.
The rest of the parts come from RS Components.
I haven't built it yet so cant comment on the sound.
The circuit is quite big so isn't easy to publish on here.
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Old 17th June 2013, 09:41 AM   #6
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First find a good friend with the right tools. You need a VTVM, and at some time you're going to want an oscilloscope, dummy load, signal generator. And then some KT120's LOL.
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