That awesome glad you got ahold of Matt he has been very helpful to me I'm building his zebra wood spud amp as I speak also built his lacewood amp hope you enjoy the preamp I love mine keep up the good diy projects.

@SpeedyDad glad you got it sorted! And thanks for sharing the info on the different plate voltages. I wonder how Matt feels about his little preamp getting so much attention as of late? 😂

It really does sound good for what it is and how simple it is to make. And for a first time builder like myself, it went a long way to instill confidence for my next build.

@Jeff Kamen I've been out of town for a few days but plan to do a follow up soon: this thing does sound really good! No "technical" testing done yet, but it does make a lot of gain, even with a 12AU7. I probably am going to add the cathode bypass cap switch and play around with that too.

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.

Nice work! Much neater than mine! lol

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.

Hi speedy it could be that you used a 470 ohm bleeder resister instead of a 470 k resister so you are sending voltage to ground it looks like it's 470 ohms in the photo also the transformer has a 115 volt tap and a 125 tap if the 125 tap is used it will give you a lower voltage I used the 115 myself because in my house my power is 117 to 120 volts all the time hope this helps

@Jeff Kamen I love this idea of the cathode bypass cap switch! Taking it out injects a little negative feedback into the cathode, so will be interesting to see how say an unbypassed 12AX7 compares to a bypassed 12AY7 sonically, or if there is a measurable difference in distortion.

@Skunkie Well with my "unscientific" experimenting (my ear), I noticed it acts a lot like a "loudness" button. It will distort with the bypass caps engaged at higher volumes, but at lower volumes engaging the bypass caps adds some "substance". If you try it out you'll see what I mean.

@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.

I'm curious @shoreman1966, which 12A_7 tube are you using? I find the 12AU7 to have just the right amount of drive for me without overdoing it. My phono pre sounds great in front of it too!

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.

@SpeedyDad A while back I found some panel-mounted LEDs (like the one in the photo above) that runs on 120VAC. Super simple to just wire in with the power switch!

@Jeff Kamen What size chassis is that? It looks perfect!

@Skunkie I think it's the same size you used for the headphone amp. Hammond 7" x 5" x 2".

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

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

This one works:

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

Thank you!