Curious_George Posted February 5, 2022 Share Posted February 5, 2022 Here is a tube preamp design that uses an output tube at line level signals. The concept is to utilze the linear plate curves of a triode strapped pentode. The 12V6 is the same as a 6V6, but with a 12.6V filament and a bit cheaper to purchase. The 12V6 triode plate curve is extremely linear and makes a great preamp. One caveat though is the 12V6 can be a little microphonic, but that issue is easily handled with a silicone damper ring or a rubber grommet isolating the tube socket from the chassis. The design to use an output tube as a preamp is not new, and many types of output tubes can be used. The 12V6 is nice because without feedback, the gain is just right to take line level signals and give them just enough boost to drive modern sources with. I have included a cathode follower in this design, which means the output impedance is very low and the preamp can now drive just about any type of source/cable combination. Due to the low gain of the 12V6 in triode mode, noise is naturally low as well. Parts count is fairly low, so the price to build is not too bad. If you can find a metal enclosure around that could substitute as a chassis, the cost can be even less. The design is very forgiving, but a few areas need to be implemented properly such as grounding. Again, I am throwing this out as a DIY build. I'll try and come up with a BOM (Bill of Materials) soon to help locate parts, if needed. Some of the parts, I have and will let them go cheap because I have an abundance of them such as the LM317K regulators, mounting socket for regulator, and 8200uF/35V capacitor. An LM317 TO-220 package can be used of course, but I find it hard to mess with those small leads some times. The LM317K is a TO-3 metal case package and it is easy to work with and just. Anything TO-3 from Mouser or any parts house is sky high nowadays. I also have a lot of the resistors / caps you'll need too in small quantity. My 12V6 preamp is not anything to look at, but it sounds great. It is a test bed, so it won't get any prettier. Unity gain frequency response Full gain frequency response Quote Link to comment Share on other sites More sharing options...
Curious_George Posted February 5, 2022 Author Share Posted February 5, 2022 Unity gain distortion Full gain distortion 1 Quote Link to comment Share on other sites More sharing options...
sbcoffey42 Posted February 6, 2022 Share Posted February 6, 2022 So I have a complete beginner question. What is the software package and interface you are using? I am a lab guy but not in the realm of audio equipment. Quote Link to comment Share on other sites More sharing options...
henry4841 Posted February 6, 2022 Share Posted February 6, 2022 Any Arta charts of harmonics? I notice really nice THD+N numbers but curious on 2nd and 3rd harmonics. Nelson Pass has a couple of H2 generators to be used with SS amplifiers. One specifically a H2 SS generator and the other a true triode tube linestage using one of the miniature tubes. I have not built the H2 generator but I have built the Korg Nutube B1 preamplifier. Nice sounding with SS amplifiers but having so many SET amplifiers with enough 2nd harmonics it does not get used. I like the difference in sound of a good SS amplifier not needing any extra 2nd harmonic added. For those that have SS amplifiers that want to somewhat experience what tubes can do to the sound it can be a desirable addition to their SS amplifier for a change in sound that many enjoy. Quote Link to comment Share on other sites More sharing options...
Curious_George Posted February 6, 2022 Author Share Posted February 6, 2022 2 hours ago, sbcoffey42 said: So I have a complete beginner question. What is the software package and interface you are using? I am a lab guy but not in the realm of audio equipment. It is a stand alone unit - Audio Precision System One (circa 1995). It is obsolete by today’s audio standard in self-contained systems such as this. However, for DIY audio it is awesome. The newer systems are unbelievable but very expensive. At my previous job, I used one in the lab for 20 years. In 2003, I didn’t want to cause a conflict of interest at work, so I bought one for my home workshop. PC based stuff is out there now, so that is what people usually gravitate toward. Quote Link to comment Share on other sites More sharing options...
Curious_George Posted February 6, 2022 Author Share Posted February 6, 2022 31 minutes ago, henry4841 said: Any Arta charts of harmonics? I notice really nice THD+N numbers but curious on 2nd and 3rd harmonics. Nelson Pass has a couple of H2 generators to be used with SS amplifiers. One specifically a H2 SS generator and the other a true triode tube linestage using one of the miniature tubes. I have not built the H2 generator but I have built the Korg Nutube B1 preamplifier. Nice sounding with SS amplifiers but having so many SET amplifiers with enough 2nd harmonics it does not get used. I like the difference in sound of a good SS amplifier not needing any extra 2nd harmonic added. For those that have SS amplifiers that want to somewhat experience what tubes can do to the sound it can be a desirable addition to their SS amplifier for a change in sound that many enjoy. I can do an FFT on the 12V6 preamp. Quote Link to comment Share on other sites More sharing options...
henry4841 Posted February 6, 2022 Share Posted February 6, 2022 I like Arta better but I do not have a great FFT scope to compare with what you may have. I would like to see some 2nd harmonics and judging by the schematic it should have some. I have one class A PP tube linestage and it still has some effect using it with a SS amplifier. I do not ever use a tube pre with a tube amplifier. Too much of a good thing for me. But a touch of 2nd harmonics has a very pleasant effect when using SS amplifiers. Quote Link to comment Share on other sites More sharing options...
Curious_George Posted February 6, 2022 Author Share Posted February 6, 2022 16 minutes ago, henry4841 said: I like Arta better but I do not have a great FFT scope to compare with what you may have. I would like to see some 2nd harmonics and judging by the schematic it should have some. I have one class A PP tube linestage and it still has some effect using it with a SS amplifier. I do not ever use a tube pre with a tube amplifier. Too much of a good thing for me. But a touch of 2nd harmonics has a very pleasant effect when using SS amplifiers. I can capture the FFT like the other images I post. You can see the harmonic signature very easily. Any tube preamp is going to have predominant 2nd harmonic distortion unless it has feedback. Quote Link to comment Share on other sites More sharing options...
henry4841 Posted February 6, 2022 Share Posted February 6, 2022 Yes I know but for those that do not who may want to see and understand what us tube guys love so much it would be interesting to see. Should sound really good with a SS amplifier. Quote Link to comment Share on other sites More sharing options...
sbcoffey42 Posted February 6, 2022 Share Posted February 6, 2022 2 hours ago, henry4841 said: Any Arta charts of harmonics? I notice really nice THD+N numbers but curious on 2nd and 3rd harmonics. Nelson Pass has a couple of H2 generators to be used with SS amplifiers. One specifically a H2 SS generator and the other a true triode tube linestage using one of the miniature tubes. I have not built the H2 generator but I have built the Korg Nutube B1 preamplifier. Nice sounding with SS amplifiers but having so many SET amplifiers with enough 2nd harmonics it does not get used. I like the difference in sound of a good SS amplifier not needing any extra 2nd harmonic added. For those that have SS amplifiers that want to somewhat experience what tubes can do to the sound it can be a desirable addition to their SS amplifier for a change in sound that many enjoy. This is exactly why I grabbed a tube pre amp kit from tubes4HiFi, I wanted to use it on my main in's on my Marantz just to see what that is going to do. I still have the desire to build my own tube amp but this, for me, was a quicker way in to use tubes to influence my sound. Quote Link to comment Share on other sites More sharing options...
henry4841 Posted February 6, 2022 Share Posted February 6, 2022 19 minutes ago, sbcoffey42 said: This is exactly why I grabbed a tube pre amp kit from tubes4HiFi, I wanted to use it on my main in's on my Marantz just to see what that is going to do. I still have the desire to build my own tube amp but this, for me, was a quicker way in to use tubes to influence my sound. Careful, building audio gear is a slippery slope. You have a taste of what tubes can do. To realize the whole effect you need a SET amplifier. 1 Quote Link to comment Share on other sites More sharing options...
Marvel Posted February 10, 2022 Share Posted February 10, 2022 I have used a Juicy Music Audio Merlin line stage. It uses a single 6DJ8 or similar. I think Mark Deneen designed his preamps with no feedback. I've been using it for over 15 years, whether driving tube or ss amps. Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 12, 2022 Share Posted February 12, 2022 Great amplifier to try the 4P1L out in. I have tried 6V6 in triode mode and it is pretty good I agree but once you compare it to a 4P1L you'll never look back. There are two problems though, you'll need different sockets and since the 4P1L is a directly heated cathode you may want to rectify your 6.3Vac to DC and regulate down to 2.1v, or as I do with my 6.3v supplies is use current source instead of a voltage source and set it around 650mA to power one 4P1L per channel. That's with the filaments in parallel which is better. Heater requirements per tube is: 2.1v at 650mA 4.2v at 325mA Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 12, 2022 Share Posted February 12, 2022 I see you used a separate filament transformer for your heater supply. I imagine your power transformer has 6.3v windings? If so it should be an easy conversion. Quote Link to comment Share on other sites More sharing options...
Curious_George Posted February 13, 2022 Author Share Posted February 13, 2022 4 hours ago, captainbeefheart said: Great amplifier to try the 4P1L out in. I have tried 6V6 in triode mode and it is pretty good I agree but once you compare it to a 4P1L you'll never look back. There are two problems though, you'll need different sockets and since the 4P1L is a directly heated cathode you may want to rectify your 6.3Vac to DC and regulate down to 2.1v, or as I do with my 6.3v supplies is use current source instead of a voltage source and set it around 650mA to power one 4P1L per channel. That's with the filaments in parallel which is better. Heater requirements per tube is: 2.1v at 650mA 4.2v at 325mA I noticed the 4P is a Loktal socket. I have about 20 NOS Cinch sockets waiting for a project so I’m good there. The power transformer does have a 6.3VAC winding. I was using it in a voltage doubler configuration, but I wanted to clean up the underside and just used a separate 12.6VAC transformer up top. I think I’ll do a CCS with a 317k. Quote Link to comment Share on other sites More sharing options...
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