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Old 30th October 2005, 08:07 AM   #1
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Question Newbie: Which amps to build?

Hello everybody, thanks for a great forum.

I am currently sporting these amps to drive my DIY three-way DALI 7A speakers:

Croft Micro multichannel preamp
The Quad II mono blocks (the legendary ones )

The amplifiers are my fathers and as I am looking forward with a somewhat boring three month break from my work as a officer in the danish army due to a knee injury, I would like to have something usefull to do while not being able to walk much.

Now, my question is: Which amps should I build? I would like them to be tube-based, stereo (1 or 2 power amps), around 2x30-50 watt of power and somewhat cool looking design, showing off the beautiful glass bottles.

I have very little experience building amplifiers (reading schematics, etc.) but I hope this forum + the internet can help compensate for that. I have experience soldering and working with electronics, though.

Best regards, Jakob from Denmark
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Old 30th October 2005, 08:11 AM   #2
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I have very little experience building amplifiers (reading schematics, etc.) but I hope this forum + the internet can help compensate for that. I have experience soldering and working with electronics, though.
In a case like yours, you'd probably do best with a kit. Dealing with VT circuits is a whole 'nother story, since this will involve much higher voltages than you work with doing solid state.
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Old 30th October 2005, 11:54 AM   #3
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Are you sure you need so much power? Bear in mind that your Quad IIs are only really good for about 10W (possibly less) and their output transformer saturates at the slightest whiff of low frequencies. I would think that a genuine 15W amplifier would be a significant upgrade. The reason I'm making this point is that valve Watts are expensive, so you really want to avoid paying for more than you need. I would suggest that a classic circuit like a Mullard 5-20 (20W from PP EL34) with a really good output transformer would be a significant upgrade. If that's not enough power, then there are plenty of classic 6550 PP circuits that will provide 30W or more. All of these circuits operate from reasonably sensible HT voltages (less than 500V). All the classic circuits can be pretty well guaranteed to work and sound good. As circuits become more sophisticated and aim for higher quality they tend to become more fragile and require better construction standards. It's probably best to start with a reasonably simple circuit, gain some experience, then improve it later. That means that the most important decision is the output valves because they determine the required primary impedance of the output transformers. Buy the best output transformers you can, and buy a mains transformer with at least 50mA spare current capacity on the HT. You can always add heater transformers, but adding an HT transformer and associated rectification/smoothing takes up significant space.

If you want significant power from SE, then I'm afraid the HT voltage will rise significantly (as will the cost) as you enter the realms of 845 and 211.

If you are safe handling mains voltages, then valve HT <500V won't cause you any problems - it's when the voltages tend towards 800V and more that the more unusual effects (corona/arcing from sharp points, deteriorating insulation, etc) start happening.
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Old 30th October 2005, 12:00 PM   #4
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Agreed, a good kit is probably the best idea, especailly if you have only 3 months or so in which to complete it. That doesn't give you much time to learn all you need to know, select a design, get a chassis made, acquire all the parts, build it and get it working. I'm not able to recommend one, because I don't have the necessary experience of kits.

Safety is a major issue, as Miles says, so it's a good idea to read some safety tips. The power levels you're planning are perfectly reasonable for tubes but, to get them, your HT will most likely need to be 400 volts or more. The smoothing capacitors can pack a lethal puch!
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