Forum Posts

Dec 05, 2021
In Show and Tell
Here is my finished 6SQ7 EL34 build. It is based on Skunkie's original two choke drop-in PT design with a few custom elements. It turned out great. The amp is quiet, no hum, no buzz, nothing, and I cannot hear any distortion even when turned up to almost 100%. I am running it with a set of Klipsch RP 600ms--only a coincidence that Skunkie has them as well. I consider myself a confident solderer and was for a short time on a track to becoming an electrical engineer before I changed subjects. I have average experience with metal work, and did most of the fabrication with the Dremel cutting wheel and a step drill bit. The Dremel is great for the PT and IEC cutouts. Work slowly and you get great holes. My worst hole was the one for the power switch. I overshot (overstepped, ha ha) and made it too large. Next thing I see myself doing is turning a thick washer into a bezel and painting it with hammered paint. It looks fine, and I can sell it as a design feature. One thing--these steel burrs and small pieces of metal that are left after drilling are nasty and will bug you for a long time. I had multiple splinters. Use gloves and put a newspaper on the bench. As you can see, to make things easier and avoid klutzi scratches, I taped the whole chassis with masking tape and drew my markings on it. Easy enough. Overall, I am happy with how the chassis turned out. I am perfectionist, and really had to come to terms with what tolerances I could possible keep. Next came the part I was really excited about, the soldering and putting it all together. Here is the end result: If you watched the series, you know the design and what is happening. Here are a few things I made differently or added: The large cathode resistor that Skunkie put on top of the chassis are inside. They get air from all sides and are away from the caps. Given that they are run below their wattage rating I wanted to give it a try. They get warm but not so much that it worries me. I got one of these Alps pcbs (eBay). They make soldering the pot super easy. If you look closely, you'll see that only the small section between pot and the 6SQ7s is shielded at the ground point there. I added another star ground at the inputs, and that's were I grounded the longer wire from the inputs to the potentiometer. In the lower left of the chassis you see (barely noticeable) a thermistor (CL70) that I put on the hot side of the switch. That was a pain to get in because I wanted the terminal strip connected to the screw that holds the IEC (that middle connector is not connected to anything). I also wanted to have at least two inputs--RCA and maybe another RCA or 3.5mm headphone jack to connect my iPhone or another device. I considered a few options, maybe using relays that switch ground and signal (sold on eBay) but saw how tight the space is and decided to work with a simple switch and just switch the signal with a common ground. The lower pins were used for a two LED indicator at the front. I spent considerable time fussing over the color of the LEDs and how to run the wires elegantly. I am currently using blue (don't judge) and an amber colored LED (it's my third orange'ish try). I am still not happy and try little grain size bulbs next. The switching works well, just as planned and it is great to be able to move between inputs. To get AC for the LEDs (including the power switch) I put a rectifier under the 6.3V connector and then soldered a little board with a 240uF on top. I am getting around 8V from it. Since I used different colored LEDs, I had to adjust resistors for each LED. This is as far as my mods went. The rest was straight forward. I was quite nervous when I turned the dial on the variac for the first time but all my voltages checked out. My house has 120V, my B+ is 418V, 415V after the additional filter for the 6SQ7s. I am seeing 28V at the EL34 cathodes and 1.82V at the others. And that is with a 150R. The original BOM listed a 100R, not 160R, and 150R was what I had. I am seeing 251V/248V at the plates of the pre-amp tubes and 412V at the EL34s. This amp is great to build because of the very detailed video series. I would not say, however, that this is a beginner's project. There are so many things you can still do wrong, and I personally would not have started it without a very solid understanding of how the circuit works and experience soldering. So, to close, this is a fabulously sounding amplifier that looks crisp. Thanks, Stephe, for putting so much work into making the amp the series!
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