Making a 6SN7 Tube Linestage // DIY Tube Stereo Preamp // New all Tube Shop Stereo System build! 

Thank you!

Thanks for commenting. Hopefully this will inspire you. I’m not sure VU meters are that important on a preamp. Would be more ideal on a power amp. Cheers.

@@MikeFreda Really? That's interesting. I thought VU meters would be useful to make sure the preamp is delivering the optimal signal to the power amp with different sources (CD player, computer, phone etc) having different output gain. Is that not the case?

Digital sources are pretty well balanced. A preamp doesn’t have a lot of gain. The millivolt difference between the channels would be negligible (unless you built it with components that are way out of tolerance L/R channels and the measured voltages are way off) Even still, unless you have a balance control in the circuit it would only really be for aesthetics. In a power amp, there is more gain. You would want the VU on the output to help balance. Again, if built correctly and all the components and voltage reading match between both channels you shouldn’t really hear an imbalance…. But it is possible especially if your room is not symmetrical or you are off to one side. Yes, you can put VU meters on both but not sure if it is worth it especially on a preamp.

@@MikeFreda I hear you but I am still unsure. Even on my digital sources there is great variation between the volume of different albums. Some albums are mastered super-loud, whilst others are really quiet. Having VU meters on the pre-amp would allow you to tweak the volume on the pre-amp so that the output was consistent, wouldn't it?

Yes, that is method for looking and changing the overall volume level to be consistent throughout your playback and listening session. You will be adjusting the volume anyways to suit. You could also just get a decibel meter. I agree, big VU meters look cool but I’m not sure it’s really necessary in the grand scheme of things.

You're welcome. Cheers

Thanks for commenting. To my ears I find the 6SN7 more midrange forward. Some may say “tubey”. When I A/B them in the same circuit they are close… with the 12AU7 being a bit more neutral. Cheers.

Did you consider using a 6GC7? Also don't you find the gas filled VR adds noise? Lastly why monitor the current on a preamp; surely its class A?

Thank you.

Thanks for watching, glad you enjoys it!

Thank you.

Thanks! Yes, it's a bit of an unorthodox build. I typically don't use tube rectification in my HiFi systems. It's an added cost. Tubes tend to have sag and voltage drop across it. Using SS rectifier allows me to use higher value PSU caps to help control the AC ripple better. Makes for a quieter amp with less money spent. cheers

Thank you. A matching 6L6 amp is being made as well. Doing a video of that as well.

Thanks. I’m still pretty happy with it. Looks and sounds good in my shop. Having a good stereo in a shop is very under valued. This pre-amp has more that enough gain to power any amplifier.

@@MikeFreda I imagine having a good stereo anywhere you spend a significant amount of time can only be a good thing, but until and unless most of us can find a way to make that an affordable option, it will simply be an expensive and unaffordable luxury. Hence, one reason why so many take an interest in building their own gear. :)

Thank you. Fun project. I’m really happy the way it turned out. I use it almost everyday.

Thanks, glad you like it!

Thanks for commenting. I did not bend and metal in the chassis. The sides are channel aluminum and the top and bottom plates are bolted on with small machine screws. The point of this built that you didn’t need any special tool to make this chassis. Cheers.

Thanks, glad you liked it.

Thank you, cheers!

Yes, and oldie but a goodie. I’m actively looking to upgrade my test equipment… a lot to choose from! Cheers.

Thanks for your comment. I already made a Phono preamp (have a look at my playlist for a video I made several years ago). A phono pre is typically a stand alone unit. Just add an extra set of input RCA’s. A phono pre can get rather complex due to the RIAA circuit and the gain that is required. They need to be wired pretty exact and methodically or else it will introduce noise into the system. There are a lot of “tricks” to employ to make it work effectively. RCA Receiving Guide Manual has a phono pre in it. You can Google JE Labs Phono stage. That will also get you close. Bottlehead makes a pretty good phono kit as well.

A 500ohm choke is okay. The Triad C-3x is a good choice. The circuit is flexible and you can omit it if you like. Due to the 0D3 clamping the voltage at 150V, you can use various PT's. This circuit does not draw a lot of current. Any 300-0-300 between 25-60ma will work. You just need to make sure you have enough filament current available. ~1.5 to 2 Amps. The Hammond 269JX, 270BX, 270AX will work. The 270X will also work but it has a 5v winding. I'd caution about having a bluetooth module in the amp and/or running off the amps power supply. This may induce some noise or grounding issues. If it was me, I'd use a seperate bluetooth unit with its own power supply and just RCA into this preamp. I wouldn't worry too much about the time delay circuit. The 6SN7's are pretty hearty. You could just use a tube rectifier and modify the PSU a bit by lowering the first filter cap to ~40mfd and using a 272BX or 272X (a 5Y3 will drop the B+ a bit) I get most of my parts from Mouser. Cheers.

The capacitor would be too high for tube rectification. I modeled this in PSU designer (Duncan amps) and it does not show that using solid state rectification. You can go lower but you may hear some hum/ripple coming through. Cheers.

Thanks for commenting. I did this video for more high level “this is what I did” sort of thing. I didn’t realize that people would actually want a deeper dive. I may do that with theory and a good layout if people want (like I did with my 6DN7 amp). Cheers.

@@MikeFreda I would love a step by step tutorial (on video or in writing) along with the full component list 🙏🙏

@@clemleg123 Please have a look at my 12AU7 Preamp. It's very similar and I also include a parts list. Doing a full build document is a lot of effort, however, a layout is something I can put together fairly easily if you and other would like that?

@@MikeFreda Hi Mike. I am planning to build this preamp, I like that it has sub-out which would fit perfectly with my DIY speakers. I would appreciate if you can add the layout you mentioned here. thanks.

I am working on the layout but it occurred to me that if you don’t build the same chassis my layout won’t work for you. I think you could pause the video of my layout and use the schematic to draw something out that would work for you. Cheers.

Both these tubes sound somewhat similar in this circuit. Some people say that 6SN7 is more "tubey" and has more mid range presence. There is a lot of reviews online on the tubes people like. For me, tube rolling is part of the fun. I have a whack of 12AU7's and 6SN7's because of that. It's always fun to find a NOS treasure and see how it compares to a newly manucfatured tube. At the end of the day, there are more things to worry about in a circuit than just the tube itself (i.e. Resistors, Capacitors, grounding... etc..) Cheers

Hello, Yes, this is a common topic for discussion. Its hard to explain over comments. I really should do a seperate video on this. Have a look at this link on the Hammond website: Design Guide For Rectifier Use: www.hammfg.com/electronics/transformers/rectifier PT is just power transformer. You can omit that form the stock number. It's the same thing. The Hammond 269GX is a 225-0-225. >>> 450VCT. The DC output voltage will vary depending on the what sort of rectifier and PSU input circuit you use. Set up as a Full Wave Capacitor input, it will give you B+ of 319VDC >>> V(PEAK) DC = 0.71 x sec VAC [319 = 0.71 x 450] of course this is unloaded. In this circuit you have a range of PT voltage that will work since the 0D3 regulates the voltage at 150V. You need at least 75V-100V of compliance so, any B+ from 250VDC - 350VDC will work in this circuit. I hope that helps? Please feel free to PM me if you want more info.

@@MikeFreda Thanks for the quick reply. In searching for an economical PT, I found a Hammond 270DAX, which is 520V CT (260-0-260, if I understood you correctly). The spec sheet says 520V CT @ 104 ms, with a filament #1 of 6.3 V @ 3.5 amps. Do you believe this would be a good choice? Would the fact that it is a little larger than the 269GX be a problem or a benefit? Thanks, Ron

The 270DAX will work but overkill. Wasting some money here. The preamp circuit doesn't even draw over 20mA. Your B+ will be way higher than listed since its rated for 104mA. The 270DAX would be suitable for a EL84 SE amp. The 270AX is more suited for this preamp. If you are looking for budget, try the 261C6 or D6. Use a full wave bridge rectifier and leave the CT floating. (Do not connect the CT.) I've used this PT several time and it works okay for what it is. It's a good practice to have the PT current rating slightly above the amps current draw. You can go larger but you are spending extra money on a larger transformer that you don't need and you may need to tame the B+ by adjusting the PSU dropping resistors. Cheers

Thanks for commenting. The transformer I'm using in this preamp is actually used in a Fender Champ amp and its overkill for this circuit. This amp does not draw a lot of current, around 8-10ma. The tubes are bias very lightly to help minimize gain and distortion. The PT you quoted is rated to give 300-0-300 at 50ma of current draw. If you have small amount of current draw you will typically see the B+ voltage being a bit higher. Will it work in the circuit? Yes, it will. The 0D3 will cap the operating voltage at 150VDC but you may have to adjust the large dropping resistors that are placed right in front of the 0D3. Simple ohms law equation will get you in the ballpark. You "may" have to go slightly larger. Its hard to tell until you build and test it. I hope that helps?

@@MikeFredayes, that does help! I’ll do some more research in to a better suited transformer, but I’m finding some champ transformers that should work either way. I’ve built a few paint by numbers, high gain guitar amplifiers… but this will be my first hifi build! This looks like a great build! Thank you… I will post a video when I finish!

Thanks for commenting. Yes, you could use a 12SN7 tube and you are correct, you would need to figure out how to do a 12V filament supply. Another option is using a 12AU7. It's a similar sounding tube and they are a bit cheaper. Cheers

The link is in the description section of the video. Cheers.

Thanks. If you goto my 12AU7 video there is a parts list. It’s a similar circuit. That being said, you can easily calculate the ratings using ohms law (V=IxR) once you know the current in amps, then use the power equation. P=IxV. You will typically get a low number like 0.001amp or 0.1 watt. A good rule of thumb is to double the power number to get the resistors rating for a bit of a safety margin. 1/4watt resistor is 0.25 watts. This circuit does not draw a lot of power. 1w on most of the resistors if more than enough. If you have the space and don’t mind paying a little extra, there is nothing wrong with spending a bit more for a larger resistor. Better larger than too small. I hope that helps?

Thanks! I really appreciate

Thanks for your comment. It’s not a power amp, it’s a line stage. Output is around 10v RMS, gain is around 10dB, input sensitivity is 32k to 47k ohms.

Yes, I am using a 0D3 in the power supply section. 0D3's can be noisy if they are not applied correctly. I have it installed after the PSU filter section. I'm using a lot of capacitance to remove most of the AC ripple and it seem to work great. I have not physically measured it but the "ear test" tells me it is working fine. I think it creates more of a black background. You can really notice it between breaks in the music. Honestly, I hear more "noise / hum / buzz" from AC filaments and bad grounding schemes. cheers

@@MikeFreda Oh, so you can hear something then? As it is, I have seen this regulator tube used in amp designs before, particularly Grommes amplifiers. I know at least two that use it. But I don't see it in pre-amp use. As for filament supply hum, yeah, I would go DC on those to get the hum down. I like the 6SN7 design you used in that it only has one capacitor in the signal and I am thinking of building this very circuit myself, with the omission of the OD3, of course.

@@jukingeo I do not hear anything unpleasant with the 0D3 in this circuit. If you plan to omit it from my circuit, you will need to use a Power Transformer with lower voltages or install some larger dropping resistors. You may want to model that using Duncan Amps PSU designer. Cheers

@@MikeFreda I could possibly try it both ways, but like you said, if I don't find I like it, I could always add another RC stage to the filter portion of the power supply. Otherwise, it looks like a really nice simple design that has only one capacitor in the signal path. I also have loads of 6SN7 tubes, so the choice of that tube is a no brainer.

Thanks for commenting. This preamp is for 2 channel audio playback. You can input a CD or tuner directly into this. For record (aka Phono) playback, you would need a separate turntable (Phono) preamp with a RIAA equalization circuit ahead of this preamp. There is not enough gain to record with this preamp. You could not directly connect a microphone in this... well, I guess you could but the signal would be low and there is no equalization adjustment. All this preamp does is allow for a bit more gain and a volume control for normal audio playback. I do plan to make a one channel mic pre in the the near future. I hope that answers your question? cheers.

@@MikeFreda I think you mean you plan on doing one for recording soon ? That would be awesome Do you tell how much it cost to build them in your videos?

@@MikeFreda hey Mike, if you’re going to make a recording preamp can I suggest a feature can you make a stereo unit that can be used for recording but also gonna be set up for monitoring your daughter as I understand it the PMs need to be set in a different part of the circuit to do this correctly for mastering purposes

Thanks for commenting. It is not possible to have a headphone Jack with this preamp without some major topology changes. This preamp doesn’t have enough current and there is a huge impedance mismatch. To drive 8ohm headphones you will probably need small output transformer with a 8ohm tap and figure out a way to get more current out of these tubes. Realistically this is not a headphone amp, it’s a preamp the drive power amps. Yes, it could be reworked to drive headphone but there are better schemes to to that with these tubes. Cheers.

Thanks for commenting, You can and I do. I've never measured the output impedance of the prepamp when doing so but I can't hear a difference. You won't notice it on the sub at all and its questionable if you can on the power amp. The output impedance on this preamp is around 500ohms and adding the resistors to the subs outputs helps with isolation. Try it, see if if works for you! Cheers

Thanks for commenting. Are you asking in Tube amps in general or just for this pre-amp? When it come to sizing transformers you need to know what B+ and current you are trying to achieve/meet and what the filament voltage and current you need. The filament current is easy. Figure out what tubes you using, refer to the tube data sheet to see what their currents are and add them up... then add a bit of a safety margin. It is advisable to not run the filament current at 100%. Selecting the right B+ voltage is a bit more difficult since you need to know what sort of Bridge rectifier you are going to use and what is your desired operating voltage and what current will be. The Hammond website has a Rectifier useage and calculation guide that is helpful. Duncan amps also has a PSU designer that is helpful as well. I use both all the time. Cheers

@@MikeFreda thanks for the quick response! First, I was specifically asking about this design. What model? I wasn’t familiar with the “300-0-300” description. But I believe I understand 300vac out rating with a 0v center tap? This is my first attempt at a tube amp so I’m trying to keep it simple. The 150vdc after the OD3 set by adjusting the voltage divider seems straight forward. I think I found a suitable transformer on the Hammond website but was looking for validation of what a 300-0-300 transformer. Thanks again and love your videos.

You're welcome. This preamp does not draw a lot of current. 25ma is all you really need. The 0D3 caps the voltage at 150v so as long as you ~75-100v more you are good. That's the reason you need a higher voltage PT, you need some compliance for the 0D3 to operate. The Hammond PT269GX may be a good option. It should give you ~320VDC at the first cap. If you want to keep your budget low, you could omit the 0D3 regulator tube and use a 250VCT transformer. This is a good topic to learn. The Hammond Design Guide For Rectifier Use and Duncan amps PSU simulator is time well spent.

Thank you. No, what you hear is how it was recorded. Also, adding pots in that part of the circuit would actually induce a bit of noise in the system. One could add some switches to add or change values but it not really necessary. Cheers.

@@MikeFreda uh, respectfully, not exactly. what you hear is how your speakers and amp play back, not how it was recorded. true, pots could create some noise.

Yes, true. That’s what I meant to say. Play back what was recorded. Now that I think of it, I’ve never made a hifi amp or preamp with tone controls, only guitar amps.

You can but this is what I had on hand and is easily available for people to purchase. This power supply has great ripple rejection and it seems to be quiet enough for my speakers and power amp.

The current meter is mostly for aesthetics... to make it look "old school" but it can help determine if there is some funky going on with the circuit or if one of the tube is acting off. This has happened to me once when the OD3 was oscillating. You don't really need it in this circuit. Cheers.

@@MikeFreda i'm really like the way you put the meter. Thanks for sharing your experience to us.

55VDC and on the output? If that is the case, I suspect you have a faulty Capacitor. Retrace all your wiring and re-flow the solder joints. Try swapping the capacitor to see if the voltage issues follows. The Capacitor blocks DC and creates a filter on the outputs.

@@MikeFreda OK. I checked wiring and found bad solder joint on a capacitor. DC is gone now but I see 24-25v ac on output. Is that normal? Thank you for the assistance!

Well, you should see a bit of AC on the output however it depends how you are measuring it and what sort of input you are testing it with. Under proper testing procedures you should see 9-11v RMS ( 29v pk-pk) before clipping. This preamp has a lot of gain. Plug it into an amp and keep the volume low to start.

@@MikeFreda Hi. I just connected a solid state amp and got a huge hum. Still seeing around 22-24v ac on the outputs. I'm stumped. Could the regulator tube be bad? The voltage there is 149.9v. I'm stumped.

The VR tube is working correctly. The three main sources of hum are the PSU (filter caps not working or values are off) , faulty or poor grounding scheme, heater (filament) supply circuit. Another obscure issue is the output impedance of the preamp does not match the input impedance of the power amp. It’s been my experience in building amps that the grounding scheme and PSU section has the most impact on hum.

Hello, thanks for commenting. I used a standard 4 position, stereo (2 pole?) switch. I made the extender shaft out of 1/4" steel rod and a 1/4 coupler with set screws. I have no clue if something like that is purchasable via an online store. PM me if you can't find one. I may have some metal left over from previous projects. Cheers.

@@MikeFreda Hello Mike, thanks for the quick response. I think I will be able to put something together. This looks like a great circuit due to it's simplicity. I was thinking about replacing the cathode follower with an SRPP stage. Any thoughts?

The circuit I used incorporates a Cathode Follower. It transforms a high-impedance signal into a low-impedance signal. You basically want to match the output impedance to the input impedance of the power amp (or close to it). This circuit creates more gain than you really need for most amps. Not knowing your SRPP topology, it’s hard for me to comment if it will work or not. It “could” but you could also have a huge impedance miss-match. Cheers.

No, there is not a complete parts list for this preamp, just the schematic. However, my 12AU7 preamp is very similar and there is a complete parts list for that. You can use that as a reference. Cheers.

Yes, this preamp inverts the signal.

This preamp is meant for a stereo system (HiFi). Its topology is not really suited for Guitar or Keys. I guess the circuit "could" be tweaked to accommodate that for studio or line-in applications. Cheers.

I used a old Fender champ PT. You could substitute a Hammond. 270 or a 272 series will work. This circuit does not draw a lot of current. Most of the current draw is some by the OD3. If you omitted that from the circuit you could use a very small and inexpensive PT.

Sorry, I don’t sell amps or tunes anymore. This is a simple circuit. You could build it or get a kit. Bottlehead.com makes great kits.

Thanks for commenting, unfortunately I don’t really make tube amps for sale. By the time I buy all the parts retail, and add up my labour you can buy one from a proper manufacturer for the same cost. You could try building one from a kit. Cheers n

No. Two completely different tubes. A 0D3 is a voltage regulator tube (150v) and a 5Y3 is a rectifier tube (converts AC to DC). They are not interchangeable.