Nice video! I can explain why the output of many tube-preamps is cathode follower. It reduces the output impedance which needs to be at least 10 times smaller than the input impedance of the power amplifier. For example: input impedance of power amp = 100k so the highest output impedance of the preamp is 10k. If it is higher than 10k for this case, the signal will be distorted. It is just like plugging 4 ohms speaker in 16 ohms taps. In this circuit, the output is a pot which varies the output impedance as you turn the volume up or down. Let's say this is a linear pot and you set it at a half position, then the output impedance will be 250k/4 = 62.5k ohms. Hence, your power amp's smallest input impedance has to be 625k ohms. If it is lower than 625k, the signal will be significantly distorted . That also explains why the pot should be placed before the gain stage to avoid the problem I have just said. About the distortion, somebody may say they don't care about the distortion as long as it is musical. However, we want the pure the distortion from the tubes and not be covered by the distortion from the pot. Hence, I don't think placing the pot between the preamp and the power amp is a good idea. Anyway, just go for it! I am waiting to see how it goes.
@@SkunkieDesignsElectronics Yup! Let' s see how it sounds. For this circuit, I recommend using the low gain tube such as 12au7 or 6922. The lower the gain, the less noise it will produce. Besides, you don't have to turn down the pot too much which will degrade the sound.
Dear Steph, as a believer in the "less is more" philosophy, I think this build is amazing, the possibility of testing all the 12A family in a single preamp is very appealing. Great idea, I will wait for your measurements before going into the build, anyway sounds like a winner. The octal version would also rock, it would also be great to have the version for the 6922, 6N1P, 6DJ78 family with 6 volt heaters.
Yay! After building this one and sharing it on your forum, I’m excited to see your version and what you think of it. I love the one I built and use it almost daily!
@@SkunkieDesignsElectronics I think lots of people are confused by the variable output impedance. It's really not an issue if matched to an appropriate amp.
Great !! A mix of tubes in a preamp shoot out. I'm new to vacuum tubes and l have learned so much from your channel. I got interested in tubes with a toy preamp the FX TUBE-03, rolling tubes, opamps and Caps before then all I known is SS. I like the tube sound added inline with a SS amp. I'm now in the process of fabrication of the Boggie Factor preamp found on Audio karma. Love your channel, you are so easy to follow. Thanks again and Happy Building 😁
The PS is very well designed ( somebody has experience and the correct knowledge ).......the values of the filter capacitor ' first cap ) and the reservoir capacitor have the right ratio !!!!!! ( to prevent ringing/oscillation when responding to dynamic power demands) ..............ratio 1/3.............!!
I have known of this for a several years and have wanted to try it but wondered if it was good. So this is something to im very interested in and the 6SN7 version is a great idea. You always do great projects.
I built this a number of years ago to drive my KT88 SE. It really is a nice clean and simple design. The one thing I wish I would have done was to add a 3rd channel out for subwoofer connection.
I've built one of these and am using it in my main rig. I've got Russian teflon caps on the output and I had to put a 100k pot on the front end as I couldn't source a 250k one. Apart from some hiss it's very quiet, I love it. I cant wait to hear what you think of it either way. Very exciting!!!🤗🤗🤗
It looks like a very good runnig amp, the mechanical base is also solid.... I prefer always the 6SN7GT version.....in Germany it called "Weitverkehrsröhren"...and means long distance tubes...thats always a Quality factor...its much more bariumoxide on the cathode, than other tubes have.You can also say its a longlife version. And By the way.....the potentiometer..... I use 50kOhm as a volume pot....because the modern amps have so low impedances, you dont need such high values like 100k anymore! and of you try it,,,,you will hear it...less noisier. That looks interesting! And Thx for the Vid.
Hey, Skunkie! Yes, I am enjoying the channel. If you do the octal version, here's something you might try. There is a dual power triode called the 6AS7G/6080WA. Very low mu, very low plate impedance. They're great for SE power amps. They also share the same pinout as the 6SN7 and 6SL7. If you don't need much gain, but want to drive those long cables without a cathode follower, this is your tube. The plate and cathode resistors will have to be considerably different (the book says not to use this tube with cathode bias, but I've done it for years), but it would still be possible to add a switch to choose between it and normal SS dual triodes. One caveat: You MUST use an indirectly-heated rectifier with this tube. If the plate voltage arrives before the cathode is up to temperature, the tube will be ruined and you'll get a light show for your trouble. Later!
Since you mention this, what would be a reason to have long cables between a pre-amp and the amplifier? I'm trying to understand why someone would ever do that.
Looking forward to the next video! Maybe a fun extension to this could be modifying the design to work as a phono preamp. I take it it will need more gain & RIAA equalisation.
What a strange thing lol I was setting here getting ready to push the buy button on the Chicom 6sn7 preamp. I like the idea of using the 6sn7 tube, on paper it’s a very nice tube. I’m definitely in on building this preamp, but I know not to get ahead of you an let you work out the bugs. No sh###t I was in the process of ordering the amp when you’re video notification popped up. Thanks as always Ms. Skunkie love your stuff.
Haha! Yeah, I'm real interested in how this first one works out and if it sounds good, I plan to do a single 6SN7 preamp too. Then you only have to buy one tube!
Thank you so much for doing this preamp. Building one is so much nicer than buying one. I know you saved me money and gave me something to look forward to. I’m also thinking of building the Darling Amp. 3/4 of a watt lol. The reason (I think, could be wrong) people are so interested in preamps is being able to hook up there iPhone to there tube amp. Like when you’re piddling around in the shop. I know my iPhone doesn’t have enough ump to run my tube amps to full volume. LPs an turn tables don’t go well with impact wrench’s, grease, an sawdust. Hope you’re doing well
i have watched all videos of this serie. I am wondering , the 12AX7 is suitable for a phono stage? Or maybe two stages of it . Thank you for the knowlege you share with us
The phono stage I use, and have a video series on, uses 3 12AX7 type tubes. It's the EAR834 clone. I do use the 7025 variant because they are lower noise in a high gain application.
Love your videos. I've probably watched them all , sometimes several times. I have built many different topologies of Set2A3 and SET45. I built a 6h6p preamp and it has way too much gain. I have many left over parts and you vids have inspired me to build something like a 4s but dedicated to 12au7 only. I could really use some of your advice on finding a schematic. I'm torn with where to put the pot on the front or rear end. I have 150hz Tractrix front horns with 102db 8" Lowther drivers. for the top end and 4 -18" push pull open baffles self powered. I really look forward to chatting with you. Thanks , Les Hudson
i am going to buy a transformer, which i like to use for both preeamps (the phono and this one simple pree). How many mA will i need . In schedule you write 1,8 ma. is this correct?
I am building this pre-amp right now, what a coincidence. I have everything in except for the IEC plug that should be here tuesday. The other thing is the pot I couldn't find a J taper so I'm using a linear potentiometer. I don't know if I need a J taper because of the way the pot is on the tube output but we will find out. Are you installing the ground lift switch?
