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Thread: sawing a log and reassembling it WITHOUT warping?

  1. #16
    Join Date
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    Cameron Wood has listed the tricks I know of in post #3. Cool Idea for an amp. It is very unique.

  2. #17
    Join Date
    Apr 2017
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    You have a lot of stuff in there. Here's a suggestion to ease the crowding.

    Carve the face then hollow out the log to a shell with no top or bottom. Make a top from plywood about an inch bigger all around. Paint it or cover it with leather or a grass skirt, something to look nice. Give it 3 ribs to hold it up 1/2" from the log and with stubs to keep it in place. 3 ribs to accommodate movement in the log, 1/2" for ventilation. From the lid hang a board to the bottom and as wide as the opening. Mount all your electronics on this. Just lift out for great access. Put a base on the components board so it will stand up when out. Allow wire slack.

  3. #18
    I'm a little confused at how the ribs and stubs work. I'm picturing three strips of wood, 1/2" deep and say 3/4" wide, running parallel to each other and evenly spread out underneath and running to the edges of the top. Do they fit into rabbets around the inner shell of the log? Which would make them taller than 1/2" to give proper ventilation clearance. I also don't see how the stubs fit into the picture - are they dowels that mount into the rabbets? Wouldn't such a fixed stub warp or crack as the shell moved?

  4. #19
    One problem you're going to run into is those tubes are gonna get hot. And that's going to quickly want to warp the front of the housing more than the rear. Your best bet is to carve the face, let it dry for a couple of years, then split it and hollow it out. I understand that's not really an option for you. So the next best bet would be to build a frame and mount the face to the frame to allow it to expand and contract. Then use the frame as the connection between the two halves so it can close properly. Or maybe ditch the back half altogether.

    Is this a stereo, class A amp with an integrated preamplifier? That's what it looks like to me. Or is that preamp tube a phase inverter on a monoblock power amp or a dual cathodyne PI on a stereo power amp? I ask because if it has an integrated preamp, then with the wood housing and low voltage/high impedance signals in the preamp, you run the risk of picking up radio interference. It might not be an issue where you live, so it might not show up in tests, but if you relocate or sell it, it could become a huge problem, rendering the amp virtual useless. That's why most amps are made with grounded metal housings to create a faraday cage. If it's just a power amp, then grip stoppers and shielded (and grounded) wire are often enough to avoid the issue. So I would want to build that frame I previously suggested out of sheet metal. Then maybe thin out the wood a lot. If the wood is thin enough and attached to a metal support, the metal will likely keep it from warping too much on you. Of course, it'll crack easier, and being as heavy as it is (those OT's are huge!), it might be difficult to move without damage.

  5. #20
    It's a class AB push pull with integrated preamp. The bigger eye tube is a GZ34 rectifier and the top two nose tubes are 6SL7 dual triodes, similar to a 12AX7, but with larger glass bottles. The circuit is essentially https://diyaudioprojects.com/Schemat...-Schematic.htm with a preamp and volume control. So far radio interference hasn't been a big issue; all the signal wiring is shielded.

    I'm going to try the hollow cylinder route for the next one. That should not only address the warping issue, but also allow me to install a metal cage on the interior of the log for additional RFI shielding. I'll start a thread in the Projects forum to chronicle the build.

  6. #21
    Ah. Yes, I didn't notice that the top two "big" tubes were slightly smaller. I thought you had 6 identical power tubes in the nose, and a rectifier and probably a 12AX7 or similar preamp tube in the eyes.

    You can get away with just using shielded wire for everything instead of using a shielded enclosure. I've done that before on a true PTP amp. I had some noise issues to work out, but it was really simple circuit, so it wasn't impossible to track down. But it's quite a bit more difficult to pull that off on a turret or eyelet board, as you typically have more wires running everywhere and longer component leads to pick up RFI (and often a more complex design). On a PTP, you can keep your leads short and run shielded wire everywhere. It'll look like a bird's nest, but it can still sound great if you don't get too complex with the design. Since you're using the same shielded wire everywhere for everything, it can be difficult to track down problems versus color coded wire, as you have to manually trace each wire to know where it's actually connected. So minimalism wins here. But, I have a personal philosophy that amplifiers just sound better the less components you use, so I don't shy away from minimalism.

    But one of the things I've learned with audio circuits is there can be a lot of philosophy and science behind them. But ultimately, it's your ears and experience that need to do the thinking. Sometimes problems will pop up that make no sense and shouldn't exist, but there they are. And sometimes all logic says you should be experiencing a problem, but for whatever reason, it's working anyway. There's a bit of voodoo involved with audio circuits that defies understanding. More than once I've run into an issue where every tube amp expert and EE I've talked to, said my problem was impossible to have, but there it was. And the fix made about as much sense as the problem, but it worked. So I try not to put too much stock into conventional wisdom and mathematics with this stuff, and focus on what my ears and scope are telling me.

    I think you've got a really clever idea going on here, and your electronics look very nicely organized as well. I think it'll be worth it to run through a few "prototypes" to get the bugs worked out and really perfect this idea. And the metal frame and thinner wood should not only add stability and RFI protection, but also lighten the amp a bit. So I think you're on the right path. This will be fun to follow.

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