Vintage Asylum

Classic gear from yesteryear; vintage audio standing the test of time.

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Mark

Posted by 1973shovel on May 15, 2017 at 10:43:21:
I've now watched the Blueglow MK-III rebuild videos. He does nice work, and I generally agree with most everything he said. Thanks again for alerting us to them.
A few things I noticed. If you go with the SDS power supply boards (which I recommend over the multi-section can cap), be aware that Triode has stenciled them incorrectly. Or, at least the two I bought were. Caps one and two are meant to be in series, but that's not the way the boards are numbered. It won't matter at all if you're using all the same value caps, but I used a slightly lower value as the first cap(s), so my smaller value caps actually went in positions one and five, rather than my board's indicated positions of one and two. I bring this up because, although Blueglow stuffed his own boards, he made no mention of them being mis-marked. Perhaps Triode's corrected this on the newer boards.

The second change I would make would be to do the "Diode mod" (shown below) and add a couple of UF4007s to the socket of the 5AR4. Eli Duttman has frequently recommended this as cheap insurance against today's "less robust" 5AR4s. Total cost, about 40 cents or less.

The last thing I took note of, and the biggest thing I take exception to, is his AC wiring. I really like his idea of installing a thermistor, and would encourage you to follow his suggestion. But he then wires up the power cord the same way Dynaco did it fifty+ years ago, which is the hot through the fuse, and the neutral through the switch (or vice versa).
There was a long thread on DIY Tube Asylum some time back about which should come first, the fuse or the switch. Although there was no universal consensus (although the majority said "fuse first"), everyone agreed that by today's safety standards, the hot leg should be wired to the fuse and switch, and neural directly to the power transformer.
That's not the way Dynaco did it, and BlueGlow simply followed their antiquated method. Take note that at the 43:18 point of video #7, he wires the hot (black) to the thermistor and then the switch, while the neutral (white) is wired to the fuse holder. It's simple enough to do it correctly, and I've changed any of the Dynaco pieces I've worked on to the proper method
I also noticed that he added a chassis ground to each amp, via the green wire of a three pronged power cord. Be aware that multiple grounds may cause you ground loop hum problems. I suppose you'll just have to see how that goes once you get yours built.
As to your two other questions, there's plenty of "joining two wires together" tutorials on youtube. Watch a few, and that should show you the best method for extending transformer wires.
Your other question about stripping Teflon wire, I have no good answer for. Blueglow's heated strippers looked tempting to me too. But I only do this for a hobby, so the investment wouldn't be worth it for me. I strip my Teflon wires by hand, using a razor knife. It's a pain in the ass, but works OK once you get the hang of it.
Good luck with your project, and please keep us posted.
Edit: One more thing I forgot to mention is that Blueglow had to reuse the original Dynaco 11.2 ohm bias resistors. I get that these amps were not his, but he was working on them for a customer. Perhaps the customer might be confused by the new bias setting. Dynaco's 1.56 battery test method was fine for testing a VOM fifty years ago, but today's digital meters are generally very accurate. I'd trust my meter more than a fresh battery checking exactly 1.56V. That said, I'd install a 10 ohm, 1W resistor in place of the "unobtanium" 11.2 It makes calculating the bias setting a whole lot easier.