Try this: https://toidsdiyaudio.com/forums/discussion/add-a-sub-out-to-almost-any-audio-system/

From an economical point of view and considering effort and the predictability of results you are certainly better off buying an amp. But if you look at these parameters honestly, you actually shouldn't be getting into tubes anyway. You'll get a nice SS amp for what I spent on my 6SQ7/EL34 and you wouldn't be chained to high efficiency speakers. But it sounds you don't want that much efficiency. A kit might be the best path because you can rely that they work (ELEKIT, Bottlehead, etc. are all tried and true). I have not done any mods to a Reising (etc.) because I wanted a scratch build. I have done a Bottlehead kit and liked it, but I wanted a scratch build next ... Your question is hard to answer because only you know how much time you have and what your frustration tolerance is. Mine came when a voice in my head said, "make your own circuit, use some cool driver tubes, handwind your transformers ,,," But I would build the 6SQU7 again, it was pain free and very satisfying, and I know that this kind of amp with the quality components I used would be expensive to buy. You should not have any problems given your skill level. Your DVM might be sufficient for that but a DMM is better. Except for a variac, I only used my cheapish DMM for the amp (before I bought a higher end FLUKE for the next project).

That's a nice build, congrats! A larger chassis makes a lot of sense, not only because of the size of the OT. I found it quite fiddly to make certain changes (input selector, thermistor, etc.) in the original chassis. And, those black OTs rock! Let us know how the Cornwalls sound.

Congrats on finishing the build. I like how you tucked away the cathode resistors. I also see--in comparison to mine--how less crowded the wiring looks when you use the different PT and just one choke. What speakers are you using? Enjoy your amp!

@Jeff, just to second your impression. Adding the cap actually reminds me of the loudness button, but in a good, not-artificially-adding-bass way.

Here it is finished. Looks and sounds great. And no, it won't be dangling on the CD player like this forever. I just wanted a shot with both amps next to each other. I know, Skunkie, you got a comment on these chassis looking like pulled out of the garage from a long forgotten pile of metal boxes. Much of that is personal taste, but I really like the simplicity (and the heft) of these. Thanks, everyone, for your patience and support!

Thank you, but I figured it out with the help of Matt, who designed the amp. It's different plate loads for different tubes: The appropriate plate voltages for a B+ of 250v are: 12AU7 —> 63v 12AV7 —> 79v 12AY7 —> 108v 12AT7 —> 107v 12AX7 —> 127v These voltages can vary by as much as 20% based on individual tube characteristics. I should have looked at the load lines before (actually) losing sleep over this.

And that is with the 12AU7 inserted? These numbers make sense using the 115V tap. Without the tube, I get 280ish on the cap and 275ish on the plates. But with the tube, the plate drops to 75-85V. I tried three tubes. This is not a complicated circuit, so I am probably missing something very simple. I know that the 12AU7 can work with 80V but I would love to run it a bit hotter. I should say that the amp sounds just fine, and I get more gain than I need. I just want to understand why there is so much of a drop.

Thanks, Shoreman. Not that I wouldn't be able to mix them up, but it is a 470k resistor. I get 275V B+ when I measure directly at the caps. I saw a 5V drop (around 270V) at the plates after the 100K before I wired up the socket. What voltages do you see at the plate when you run the amp? I also have around 120V in my house and therefore didn't want to use the 115V tap, it would affect the heater voltage, and, I think my B+ is fine.

Arrgh, it looked so neat and nice but looks don't count in the end. Just like in life. I had issues with the right channel. Even at zero volume it would play. When I turned the pot, the left channel came on and the volume of both channels increased evenly. There was also some hiss and a bit of hum. Very odd. Although I couldn't measure any shorts, there were some weird 0.2 Ohm connections on the PCB of the pot where there should be plain 0. So, this morning I took off the PCB and rewired the pot. This is not the final version, of course. Problem seems to be gone. Does the 100k between B+ and the plates drop about 200 Volts? I am measuring 275 B+ and then 68V on the plates. sound very low to me. And I am sure one of you will remind me of something very obvious and fundamental I missed here. @Jeff Kamen Thank you for sharing you work here! When I look at mine I wonder how I was able to clutter it up so much. And I am still not done.

Just to share my progress. I am almost done, just have to do some wiring. It's going to be tricky because, first, its' pretty tight in there (mounting the choke on the holes of the PT was a pain, trying to avoid extra holes doesn't come easy...), and second, I wanted to replicated the design and functionality of my 6SQ7, so, there will be an input selector (RCA/3.5mm) with indicator lights at the front (lights are housed in metal sockets painted matte black). And I'll have the sub-out. As to the power switch--I first wanted to go with the same angel eye I used before but found it too large, so I got a cute toggle switch and added a NOS 1960s Dialco neon indicator light. It looks just great. My proudest achievement so far is the cutout for the IEC socket. I bought a bunch of them and saw that they all have a bezel of just about 1mm. If the power supply sections looks a bit crowded it is probably due to the connectors (we call them Lüsterklemme in German) I used to connect the AC. The resistor is for the neon bulb. I should mention that for me the process of building is as important as having a functioning amp. I just enjoy straightening wires, matching up angles, etc. None of that is essential. Maybe a question: You see how the red B+ wires cross the green 12AU7 filament wires. Should I reroute the filament so they aren't near the B+ or am I entering OCD land here?

Thanks, Robert. The splinters came from drilling and not cleaning up right away. Deburring wasn't that difficult with larger drills and Dremel sanding tools. But if I keep doing this, I'll get a tool.

Cost depends on where you get your parts. I saw quite a bit of variability depending on what is available where. Overall, I think I paid between $750 and $800 I guess, including the small stuff that others might have lying around, like nuts, washers, screws, switches, etc. I do not know anyone who would build this for you if you don't want to do it yourself. Maybe you could post this as a question in the forum? The main part is actually the fabrication of the chassis, and that can be daunting if you don't do metal work often (like me). But if you know how to solder, the rest isn't that difficult. If you are not interested in doing metal work and planning all the small bits, you could consider a kit. One that is quite similar to this amp is from analogethos.com He offers a 12AT7/EL34 kit for $999 and built for $1,150. It is pretty much the same design except for the driver tube. I haven't listened to it, though.

Thank you! It's running in the back right now with Bach's Mass in B minor. Sounds incredible. I had some special order incandescent bulbs in the mail today to replace the LEDs.

I do not need to build this but started ordering parts. What is the purpose of the two 100R that ground the heater supply? Hum? I plan on using the same design that I used for the 6SQ7 amp and need some DC for the power switch and indicator LEDs. I would think that 2.5A is plenty for all this (I add maybe 100mA) but wonder if I should leave out or change the 100R resistors. Edit: That creates an artificial center tap, I see.

That would be a great idea. It probably has its limits but it might be just what your KT88 needs.

Would anything speak against using this as a preamp for an integrated amp, like the one I just built (the 6SQ7/EL34)? It is great as is, but just as the original designer, I'd love to have a bit more gain for my iPod/iPhone. I really, don't need it, But I am itching to build another amp in one of those powder coated boxes. Is the 269EX PT an updated 269E--it seems the EX is the only PT available. Thank you for sharing!

Edcor gave me a long lead time as well--December when ordering early October. I placed my order on 10/2 and had the OT on 10/16. I am just about to finish my 6SQ7/EL34 build (the original design with drop in PT and 2 chokes) and used similar sources than you have. For tubes, check out Etsy. I wanted some spare 6SA7s and EL34s and got them there.