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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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PostPosted: 20 May 2014, 14:15 
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I've now built 4 K12-Gs, 1 with the supplied transformers and PC board, and 2 with Edcors wired ultra linear and one with leftover transformers and point-to-point wiring. I used Auricaps for coupling, the UF diode mod and snubber cap mods for the heaters and AC from the power transformer.

When I designed the layout for the point-to-point box, I was thinking about the ultra linear wiring, it looks to me like R14, R18 and C9 are no longer needed, since pin 7 is pulled up and tied to the output transformers. Am I correct, or is there something I'm missing? It would simplify the power supply a bit if that lower voltage DC isn't needed at all.

Thanks in advance, Andy

I'm including some pics of my amp builds. The first three amps I hosted in cigar boxes by building the PC boards upside down (except the rectifier). The last build was the point-to-point housed in a Hammond chassis with the tube cage.


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PostPosted: 20 May 2014, 20:38 
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Hi You are correct. The output transformers provide the B+ to the screens. Nice looking builds.

Good listening
Bruce

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PostPosted: 20 May 2014, 22:46 
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Bruce,

Thanks for the info and the compliment. This was my first foray into tube amps and it's been 40+ years since I built a Heathkit CW transmitter.

Perhaps you can tell me what the purpose of R14 is? The B+ runs along to all the screens, and R14 pulls that bus down to C5 and on to the ground. What's the purpose of that resistor, electronically, when not running UL?

Thanks, Andy


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PostPosted: 21 May 2014, 11:16 
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That R/C combination was to provide a filtered B+ to the screens. It is not used for U/L operation. You can leave the parts out.

Good listening
Bruce

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PostPosted: 21 May 2014, 20:48 
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Bruce, as I look further at the circuit, I think those parts are necessary even in UL mode. This is B+ voltage for the screens, but also continues on through R14 to provide B1+ to the plates (pin 1). So, I think the voltage will be a bit higher without the load of the screens (pin 7), but removing R14, R18 and C9 would eliminate voltage. What do you think?


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PostPosted: 21 May 2014, 21:46 
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Hi, Yes I do believe you are correct. I don't have a schematic available at the present time and didn't check on that possibility. The screen current ought to be in the 2-4 ma range per tube. Not draining this off the B+ line will necessarily raise the voltages on later stages. Fortunately the tubes are rated considerably above any likely increase. Depending on the brand of tube, most of them are rated to handle 500-550 volts as they were often used in TV vertical deflection circuits. I would go ahead and build it without making any changes at first. Then since you already have one operating in the standard pentode mode measure the difference between the two versions. Then you can calculate a new series resistor to replace the existing one to bring the voltages in line with the other one. Without the schematic I suspect the resistor to change is R14 from your description. It is entirely likely that the extra voltage on the triode sections will not cause any problems with the sound or longevity of the tubes.

Good listening
Bruce

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