It's crazy, I know...I'm finally biting the bullet and building a single ended amp. All the fuss you single ended advocates have been making over the years has piqued my interest and I'm going to try it! I'd appreciate any feedback or thoughts anyone may have on this design. I have only ~90 dB sensitive speakers, so I need something with a little power behind it. A flea watt type won't work at all for me. Minimum power I can live with is maybe 10 watts or so in my small-ish listening room. Here's the schematic I worked up. First thing you will notice is it's not cathode biased. I specified fixed bias only because I didn't want the heat dissipation of a big ol' cathode resistor under the chassis, plus it will give maybe a watt or so more output power (not that I will be able to notice...). I've heard very good things about these Edcor transformers. The CX line is the biggest baddest SE OPTs they make. So I'll try them... I'm not a fan of directly coupling the driver to the KT88 because that means either 1) pushing up the cathode voltage of the KT88 so high that you need a big electrolytic to bypass it, and I just can't imagine that a big electrolytic will sound sonically better than a small film coupling cap, or 2) shifting down the driver plate and cathode voltages with a negative supply rail so you can direct couple via a fixed bias approach, but that then requires capacitor coupling the input. So if I need cap coupling anyway, might as well place it in between driver and output because that is the simpler solution, and requires only a very small current negative bias supply. Ideally I only wanted one stage in front of the power tube, but driving a KT88 with only one stage in front of it is a bit of a challenge. The only tube I know of that can drive a KT88 with the gain needed, and that is readily available in new production is a 12AT7. But the other thought I had here was to use a 12AX7 and direct couple it to a buffer (cathode follower), which then drives the KT88 (cap coupled). I know that would work well, and ultimately I may end up going that way, but it adds another stage. The buffer could be made from a 6FQ7 or any tube of that ilk. One reason I didn't go that way for now is I wanted it as simple as possible. I could use a mosfet driver, but that's again getting too complicated, plus I'm not sure I want sand in the signal path. (one day I will probably try it though). I think a bit of global feedback is good (I know I am blaspheming against the true SE crowd). With 7.4 dB feedback, the math says input sensitivity is 2V rms. That's a good place to be. But I would like to add a little more feedback (say 10 to 12 dB), but the gain of the 12AT7 doesn't support it without raising the input sensitivity. I'd like to keep sensitivity at 2V rms to work with a wider variety of preamps, but ultimately the higher level of feedback to get a better damping factor may win out over a single driver stage design. I am aware of a type of feedback taken from the OPT primary (either from the plate or from the UL tap) and feeding it back to the driver. I will probably eventually experiment with those configurations. But for now, I just need to get a simple base line amp built so I can get some experience here. The KT88 idles at 80% of max plate dissipation. The math says with both channels driven, voltage sags a bit to 430V, and power output is then about 12 - 13 watts at the speaker terminals (that's assuming an ~80% efficient transformer). That should work in my listening room. The power supply is straight forward, but you'll notice I'm using silicon rectification. I will snub the secondary (not yet shown on the schematic) to reduce noise a bit more. Initially I wanted to use a 5U4 or 5R4, but the voltage sags too much at full power and drops the output power to something like 7 or 8 watts (both channels driven), so that won't work. Getting a larger power transformer to overcome that voltage sag puts the tube's operating point a bit out of class A. So 450V to 470V high voltage rail is about right for a 5K primary, and a 200 mA 350-0-350 transformer is a very common off the shelf item, so it shouldn't be difficult to obtain. Also, using silicon rectification I can make the first cap larger and get the ripple down so the amp should be dead quiet (but we will see...). The bias supply is conventional. Nothing extraordinary here. Just need some adjustment range to dial in the tubes to the requisite bias level. So that's what I've got. This is my first attempt at a single ended design, with no bench prototyping done yet. I welcome any constructive feedback anyone may have on my design.