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Midwest Audio Fest

It’s that time audio enthusiasts!

Registration for the 2019 Speaker Design Competition is now open! Visit midwestaudiofest.com for details and to list your speaker project.
We are excited to see all returning participants, and look forward to meeting some new designers this year, as well!

Be sure your plans include a visit to the Parts Express Tent Sale for the lowest prices of the year, and the Audio Swap Meet where you can buy and trade with other audio fans.

We hope to see you this summer!

Vivian and Jill
2 of 2 < >

Midwest Audio Fest

It’s that time audio enthusiasts! Registration for the 2019 Speaker Design Competition is now open! Visit midwestaudiofest.com for details and to list your speaker project. We are excited to see all returning participants, and look forward to meeting some new designers this year, as well! Be sure your plans include a visit to the Parts Express Tent Sale for the lowest prices of the year, and the Audio Swap Meet where you can buy and trade with other audio fans. We hope to see you this summer! Vivian and Jill
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SET Amplifiers

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  • #16
    I came across this in the TT project gallery:

    http://projectgallery.parts-express....ock-amplifier/

    Other than concern for the high voltage, it seems like an interesting possible project.

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    • #17
      Those huge tubes do look awesome and I'm sure they sound great... but talk about space heaters ;) 32.5 watts per tube just for the filaments! And 950 Vdc B+ is way beyond what I want to work with. Most normal hookup wire is rated for 600 V so I keep my project under 450 V. Power supply caps prices are reasonable at that voltage.

      I'm perfectly happy with the ~ 10 wpc from my KT88 SET amp. Most of my listening is at pretty moderate levels due to having a wife and two young children. When they aren't home and I feel the need to listen at 90+ dB I just hookup my 500 WPC ClassDaudio amp I built for half the cost and abuse my woofers and ears
      Craig

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      • #18
        My space heater. Built in mirror image monobloc pair. Seven pieces of iron for each amp, all I can do to lift them. Very comfortably drive my 3-way Scan Speak filler-driver transient-perfect design speakers. I am very surprised that an 6SN7 can swing enough voltage to drive the GM70 to 25 watts.

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        • #19
          sure don't know how I did that. Kept telling me to enter password, which I did.

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          • #20
            Good points, and I am thinking about ways to avoid a premature death given the voltage. I can ask some of the "up-the-hill" engineers who teach high voltage systems where I teach for assistance.

            From the little I've read and seen though, this mono amplifier design may be an elegant way to try an SET amplifier. The sound may be worth hearing and comparing to more conventional amplifiers. It may be worth the trouble of extra caution and insulation. It's certainly not a project I intend to start without careful consideration.

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            • #21
              I've built both the JE Labs 2A3 and 300b SE amps. I enjoyed sourcing parts, constructing and listening to both. Neither was cheap to build (it's been over 15-years, but as I recall, the 2A3 cost about $600, and the 300b was about $1200 at the time)

              The 2A3 did have some hum that I couldn't adjust out with his heater circuit, although the 300b was reasonably quiet. I'd encourage you to build a stereo amp and do some real listening if you go through the effort. Both of these designs share a single power supply, so the cost of stereo is only maybe 40% more.

              I won't get all "audiophile weird", but the SET's were very pleasing to listen to. Whether or not it's "tube magic", or just in my own head, I thought they sounded exceptional playing vinyl, acoustic music and vocals. Loud rock and overly produced music isn't their forte though.

              The components for my 2A3 are now sitting disassembled in a box. I was going to re-build it into two mono amps, but never got back around to it. The 300b amp is still on my shelf, awaiting some new power tubes (once I get around to ordering them.)

              Oh yah - stay out of the really hardcore tube forums. Those guys are insane ;)

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              • #22
                Good advice, Chad. TN: let me tell you a few basic safety points to implement, not just think about. If you can be fastidious about these, you can safely build a high voltage SET.

                1. Never work barefooted on a concrete basement floor.
                2. Wear running shoes and sit down for everything you do.
                3. Buy a 5 amp Variac so that you can work on power supply, without tubes, at low voltages, and scale up for input voltage from (say) 40 volts AC to 117 volts.
                4. When you have to do measurements in a powered amp, first block up amp upside down with books, connect first probe (alligator clip) with power off, then with power on and with one hand under your butt, take measurement with probe in other hand.
                5. Do not even think about doing 4. above unless you have installed a shunt 270k-400k 1/2 watt resistor across the terminals of EVERY capacitor, whether the schematic shows it or not. With these in place, the caps discharge every time you turn the power off.
                6. do not think about doing 4 and 5 above until you cover all but 1/16" of your DVM probes with heatshrink tubing.
                7. Before you start, pause to reflect that the electrical path from your left hand to your right hand passes through your heart and if you do not keep one hand under your butt, your wife and kids may have a number of questions for your life insurance agent. You do have a life insurance agent, don't you?
                8. do not wire any high voltage circuitry on the very convenient printed circuit boards. It is fine for low voltage circuits but definitely nothing higher than a few hundred volts.

                As you read, you will probably find other safety tips. If they are reasonable and logical, add them to my list. Finally, don't do any work on HV if you are not feeling 100% mentally.

                Roger

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                • #23
                  Regarding above, EVERY capacitor does not include interstage coupling capacitors, only those big enough to shock you.

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                  • #24
                    Here's an 805 SET 35W design I have on hand. They sound very nice. But I think I'll try modifying the filament voltage to DC.

                    Click image for larger version

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                    R = h/(2*pi*m*c) and don't you forget it! || Periodic Table as redrawn by Marshall Freerks and Ignatius Schumacher || King Crimson Radio

                    Byzantium Project & Build Thread || MiniByzy Build Thread || 3 x Peerless 850439 HDS 3-way || 8" 2-way - RS28A/B&C8BG51


                    95% of Climate Models Agree: The Observations Must be Wrong
                    "Gravitational systems are the ashes of prior electrical systems.". - Hannes Alfven, Nobel Laureate, Plasma physicist.

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                    • #25
                      Roger,

                      Thank you for the cautions. I have a healthy respect and appreciation for electricity. I built a Tesla coil at about 14 years of age using a 25KV ignition coil from a coal fired power station for the primary coil voltage. My father warned me that it could kill me, evidently I listened. It worked a few milliseconds until the old bubble laden glass in the capacitor I built blew up. Old glass was what I could find, didn't think about the bubbles as flaws. I'll never forget that unplanned explosion.

                      I have access to labs and people accustomed to working with ship and power plant systems. I've lived 70 years without life insurance, despite the Tesla Coil, seems late to change.

                      Chad,

                      Your comments seem particularly useful. I'm wondering if the expense and effort are worthwhile. Building an SET amp would be an interesting project, but perhaps ultimately not as worthwhile as a few other projects.

                      Pete,

                      Am I correct in observing the power transformer in the schematic requires 220V?

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                      • #26
                        Originally posted by Pete Schumacher View Post
                        Here's an 805 SET 35W design I have on hand. They sound very nice. But I think I'll try modifying the filament voltage to DC.

                        Click image for larger version

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                        Pete, why a cascode input? For low input capacitance I guess?
                        Francis

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                        • #27
                          Originally posted by TN Allen View Post
                          ...Chad,

                          Your comments seem particularly useful. I'm wondering if the expense and effort are worthwhile. Building an SET amp would be an interesting project, but perhaps ultimately not as worthwhile as a few other projects...
                          You're here in a DIY forum, so you probably enjoy the process of a build as much as the end result. Maybe look at some simpler circuits to keep the intimidation factor low. Pete's above looks more complicated and costly than some. If you build a modest amp and hate it, it's less of a loss. If you love it, then your onto bigger and better. Maybe do a "virtual" build." Find a schematic you're interested in, spec and price the components and lay it out on paper or cad (component placement, wiring, etc.) I think if you got that far, you'd want to go the rest of the way.

                          Heck - my 2A3 amp ended up on my desk at work as part of my PC audio for a number of years. The part where I explained to co-workers what the heck it was, and that I scratch built it myself made it worthwhile to me (the cleaning lady thought it was some sort of bomb.)

                          It doesn't necessarily have to be your primary amp for your main system. Make it part of another setup and have fun with it.


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                          • #28
                            Hi TN:

                            Given that you have some HV experience, I would encourage you to do whatever you want. The GM70 amp looks straightforward, I am concerned the power supply B+ caps don't have quite enough margin of safety between the cap rating (for two caps in series) and the B+. I would use three caps in series. I was close to a motor run capacitor that let out its magic blue smoke just due to age/defect. It sure gets your attention. Second, I would be concerned that only one choke is used in the power supply. I have two chokes in series and that is barely enough to keep hum acceptable. Otherwise, it looks straightforward. You might find that an intermediate approach such as Chads 12-15 watts SE KT88 is optimum. The transformers and chokes are certainly more available and cheaper. A second system with 90 db speakers and 12-15 watts would be a very enjoyable undertaking.

                            When I first started asking questions about 845 amplifiers over on the Tube Asylum one particular guru asked me what I had ever done with high voltages, I replied that I had been a tube hobbyist as a kid, had built four tube amps recently, had been around truly high voltages in graduate school (3 Megavolts) and had more respect for a kiloVolt than I have for an agitated cobra. He thought that was adequate...;-). I think you too have the background to take it on so go for it. It is a lot of fun. When I did it 20 years ago, I could still hear high frequencies. At my present age (77) I don't hear them well enough to really justify trying to get the ultimate sound. I would do it anyway just for the entertainment, hell, it keeps me out of the bars! And I have some mods planned to try as soon as I finish a couple of other projects now underway.

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