Making a 12AU7 Line stage in a military style chassis // Tube stereo preamp scratch built.

[@MikeFreda]

Matching Power Amp: kzbin.info/www/bejne/ZqTZo5aYjpeggqM

[@jjhack3r]

thank you for the circuit, but i must know why there aren't any cathode bypass capacitors. I don't really know their function but I am curious about why you chose to only use cathode resistors alone.

[@MikeFreda]

Hello, it's a bit complicated to formulate (and I don't profess to be an expert on the topic) but the reason for the bypass cap is to create a filter. (block a certain frequency) There is no need to ask the circuit or tube to reproduce low frequencies that you can't really hear or have a speaker reproduce. Basically wasted energy. On a power amp it is more crucial. On a preamp like this not so much. There is not a lot of power consumed with these circuits. You "can" add it to this preamp circuit but you probably won't hear much of a difference (unless you went with a high cutoff) thus wasting some money. The basic mathematical formula is: C = 1/(2 x pi x f x Rparallel). f is low frequency cutoff. 5Hz is probably pretty good.

[@pradeepgayantha7640]

Nice video

[@garydrouin143]

Fantastic I build tube amplifiers by hand

[@arend3669]

Very envious of your skills ,workshop and design! Amazing amazing work... Thank you for sharing! So cool

[@MikeFreda]

Thank you very much! I have some more videos like this planned. Cheers.

[@garydrouin143]

Fantastic I build tube amplifiers by hand so I know your skill level is over the top great job!

[@MikeFreda]

Thanks for you comment. It’s sure is a fun hobby. I use this preamp and matching power amp almost daily and I’m always amazed how good it sounds. Cheers.

[@MrCarlsonsLab]

Great Job Mike! You know, putting a 6AF6 in that bezel you have on the front would look incredible. One shadow angle for right, and the other for left. Essentially two level meters in one tube. You would need to add a compactron dual pentode (or something similar) to drive it though. Maybe in the power amp? :^) If you are interested in what the tube looks like, I replaced a 6X6 with one in a video I did last year. You can look up "1938 Receiver Restoration Deforest 7D832 Radio" in my list of videos. Like the 6X6, both shadow displays are tied together in the radio, but they can be used independent of each other.

[@MikeFreda]

Thanks for the comments. Yes, I did think about that but when I started looking into obtaining tubes like that and the sockets (plus shipping) it started to get a little pricy. I really like the idea, perhaps in the matching power amp I plan to build.

[@MikeFreda]

Done.

[@UsagiElectric]

Excellent job, that came out looking absolutely brilliant!

[@MikeFreda]

Thank you. I always wanted to do something like this but had a hard time imagining what the inside layout would be like… then after your video it the light bulb came on in my brain! The chassis was a lot of work but i think it was worth it.

[@BivacCamper]

I don't know how I landed here, but now I "really need" this thing. Good job. The military style case looks awesome. Great project!!

[@MikeFreda]

Thanks for commenting. It was a fun project and I’m glad I did it. Stay tuned for more projects like this. Cheers.

[@TheCrazzyToobinator]

Amazing machine work and the 0D3 serving both as the power indicator and regulator is just brilliant!

[@MikeFreda]

Thanks. Works best in low light. You just have to make sure you get a clear top 0D3 or else you can’t see it glow. Cheers.

[@catinatdenis5190]

Incroyable Qualité de fabrication qui me bluffe

[@MikeFreda]

Thank you! Happy New Year, cheers.

[@pmc9941]

Amazing job, Mike! Very impressed, this must be one of the most detailed videos on building a tube preamp from scratch!

[@MikeFreda]

Thank you. The video could have been longer but wasn’t sure how deep I should get into it. Not sure how many people want to watch a guy solder. The matching power amp video will focus more on the components and wiring. The chassis work will be the same as the preamp video. Don’t need to redo that portion.

[@KeritechElectronics]

Hey, it's a really neatly built preamp :). It's a nice idea to use a neon voltage regulator tube as a pilot lamp, though I'd go for a magic eye (6E5 etc.) level indicator instead, fed from a sum of output signals, possibly using an additional triode + detector diodes.

[@MikeFreda]

Thanks for commenting. Yes, a 6E5 tube would be a cool but it also requires a bit more circuitry to make that work. I do have that tube and I do plan to make something like you describe in the near future, cheers.

[@debilthomes501]

Love the military theme!

[@MikeFreda]

Thanks. It fits perfectly into my shop. I’m planning more amps in this sort of look. Cheers.

[@hardystein114]

Just found your channel Mike. Lovely attention to detail . Great job. Subscribed.

[@MikeFreda]

Thanks for commenting. I'm trying to make something a little different than the norm and to challenge myself. Thanks for the sub! Cheers.

[@mje3055]

very nice, you are an artist

[@MikeFreda]

Thank you! Cheers!

[@jimcatanzaro7808]

I love it I use all USN transformers and parts if I can I have boxes and boxes from Vietnam

[@MikeFreda]

Thanks. If you make something similar, I'd like to see it.

[@AnalogueGround]

Great build! I think using a magic eye valve like an EM34 would be more visually impressive instead of featuring the OD3 regulator. It would also show the input level which would be cool.

[@MikeFreda]

Thanks for commenting. Yes, that was my plan but the circuit got a bit involved. I will revisit it in the near future! It’s on my to-do list. Cheers.

[@bertroost1675]

Kind of has a Don Garber-ish vibe. Very cool.

[@naveenfrancis9833]

Wow.. really amazing work🤝

[@MikeFreda]

Thank you. It was a fun project.

[@josephcunningham8961]

Brilliant job. Again

[@MikeFreda]

Thanks.

[@zopilotesky3014]

Nicely done!

[@MikeFreda]

Thank you.

[@danielthomas3057]

The look of the completed unit reminds me of the HAL 9000 in 2001 a space oddessy.

[@MikeFreda]

yes, it sort of does. Would be fun to make an amp or convert a Google Home to look exactly like HAL 9000! Cheers

[@yellow_noise3]

Mike, great build, where you buy copper wire in teflon insulation?

[@MikeFreda]

Thanks. I got the solid core Teflon coated hook-up wire from Mouser. It’s great to work with. The issue is that you need to buy a large spool of it, they don’t seem to stock it in smaller quantities. Cheers.

[@itsonlyme9938]

Very nice job if I was to buy a commercial pre amp like yours I bet they would cost a fortune Would your pre amp match the impedance of a cd player ?

[@MikeFreda]

Thanks. The parts alone are under $200 for this preamp. The input impedance is around 32k ohms to 47k ohms… so yes, it’s suited for a CD player. This amp does have a lot of gain so you may need to pad the input to use the full sweep of the volume. Cheers.

[@brandoncochrone5582]

Dude that is awesome looks just like what I sent in to usagi electric

[@brandoncochrone5582]

How much for one of these

[@brandoncochrone5582]

I'd love to buy one

[@MikeFreda]

Thanks. His video inspired me to make this preamp. I don’t really make these to sell. As you can see by the video, it is a fare amount of effort in the chassis. The circuit was the easy part. Cheers.

[@vox9841]

Thanks Mike, I am loving your project build. Can you let me know what program you use for drawing your circuits? Thanks

[@MikeFreda]

Thanks. I use AutoCAD. Newer versions can be tough to obtain and A bit complicated to use and set up. I was draftsman in a past job. Cheers.

[@DerekRonin]

Hi @MikeFreda had a question on the BOM (part list), noticed you had (2x 20k -10w resistor) listed twice. I was running through the schematic and could only find 1 set of 20k 10W being used right before it goes out to the audio circuit of the two channels. Just wanted to make sure there wasn't something I overlooked on my part. Thanks in advance! BTW early Happy Thanksgiving!

[@MikeFreda]

Thanks for commenting. Good catch. That is an error on the parts list. You only need a pair of 20k, 10w. Hope the build works out for you. Cheers.

[@DerekRonin]

Hi @@MikeFreda apologize for all the questions. I noticed on the schematic you reference 1N5822 for the 6.3v. As I was referencing it over to the parts list noticed 1N5833 listed (30v 40A vs 40v 3A on schematic). Or I could just skip it and go with the AC filament route, with a pair of 100R resistors.

[@MikeFreda]

No worries. Interesting, I’ll have to check where that typo is. You want 40v, 3A diodes. You can try the AC heaters using the 100ohm virtual Center tap… but DC filaments are quieter.

[@MikeFreda]

It was a typo. All fixed up. Thanks again for noticing.

[@DerekRonin]

@@MikeFreda Thanks again DC route it is then.

[@tubeDude48]

At 13:14, that's not a gun, that's an iron. ;=)

[@MikeFreda]

Good catch. Force of habit. When I started into electronics 40 years ago I had this huge Weller "gun" my father gave me. I think it was better at wood burning than soldering! lol

[@doubledigitdreams]

hello, i noticted that both the preamps you have built share a similar power supply. The schematic for the 6sn7 shows a choke but the 12au7 has no choke. Are the power supplies interchangeable for either the 6sn7 or 12au7 circuit. Do they both use the same power transformer. Both schematics are almost identical very cool. which do you prefer?

[@MikeFreda]

Thanks for commenting. Yes, they have similar power supply. They are interchangeable but you may have slightly different voltage reading but it's not a big deal. Also, 12AU7's draw slightly less heater current than a 6SN7. A 0D3 Voltage Regulator tube needs a fair bit of compliance (higher voltage and current) to make that PSU work correctly. If you didn't want to use a OD3 tube, you can use a smaller PT. In the 6SN7 circuit, the choke does help a bit but it is an added expense. It's a bit overkill especially when using a 0D3 tube. I installed it because I had it in my parts bin. I'm not sure your ear could hear a difference in this circuit due to its topology and the low current draw. I feel the PSU using just an 0D3 tube (without the choke) works well. I hope that helps? Cheers

[@gilbertlessard4380]

Hi Mike. Really enjoyed your video. How does it sound? I have a Willsenton R8. Would it sound OK with that. I really like the sound ´as is’… but would be a nice little project is worthwhile. The only component I am not able to source is the 0D3. I am 8n Montreal. Looked online. Would have to purchases from US. Kind of risky… Do you know of a Canadian source?

[@MikeFreda]

Thanks for your comment. I find this pre-amp to have a lot of gain and not to “tubey” sounding (like 6SN7’s typically do). Very neutral to my ears. I’m not familiar with a Willsenton amp. A Canadian source is “the tube store”. That’s where I get all my tubes from. I just bought a batch of 0D3 from them. The VR150 is the same tube. You don’t really need the regulator tube but you would have to down grade the voltage of the power transformer to suit. This circuit only runs at 150VDC at the 12AU7’s. Cheers.

[@jedi-mic]

How easy would it be to have a line level output of 9 volts have a variable resistor on output as gain, it's at 2 vpp at the moment line level out? My power amps will take up to 9.7 volts input, I want to have a valve driving it, is it got a good wide sound stage how would you describe it? Why the rectifier tube the 03d not heard of that one, why is that better than a standard 5ar4 say look some nice design simple that's what you want

[@MikeFreda]

You may be able to tweak this circuit with different bias points or a higher Mu tube to obtain more output voltage. You could try 12AY7 or a 6922. Higher b+ would also work but you would have to remove the 0D3. The circuit shown uses Solid State Rectification. The 0D3 is a 150V voltage regulator tube, it's not a rectifier tube. Using a rectifier tube would require a different power transformer and will create more heat . As you can see by the enclosure I made, everything is enclosed. Tube rectification costs more money to incorporate and can induce a bit of noise in the system which would require a more elaborate PSU section. Furthermore, this circuit only draws ~10ma a 5AR4 would be a waste in this circuit. There are trade offs for sure in any circuit. I tried to keep this one simple, low cost and easy to make. Cheers.

[@aniellozimbardi7144]

I apologize for the request but I am not that experienced and I do not feel safe so if it is possible you can have a more detailed diagram of the connections as it is usually called I think layout thank you in advance

[@MikeFreda]

No worries. I do not have a layout and even if I did it would only apply to this chassis and most people would not make this chassis. If you are just doing a flat top plate design, that would be fairly easy to layout, Cheers Mike

[@suedelacy]

Beautiful! Can it be used for recording music?

[@MikeFreda]

Not really. You will need more gain and noise protection for a Mic or other low output devices. The circuit and layout would need to be adjusted for higher Mu tubes. You could probably fit the Altec 1566A Mic pre circuit into this. That would be cool!

[@benevolentessence8809]

been waiting for someone to build a preamp with the 12au7 lol..i need this in the portable audio interface version lol

[@MikeFreda]

Thanks! In its raw form, it's a fairly easy preamp to build but not very portable since these tubes do draw ~600ma of heater current. You can easily remove the 0D3 and down size the PT. If you are looking for something portable you could use a 3S4 tube. They are typically found in old school battery powered radio sets and will last a long time on some d-size batteries for the filaments and 4-5 9V for the B+. They sound pretty good! Cheers

[@benevolentessence8809]

@@MikeFreda thanks a lot for replying so fast..i'm gonna check out those tubes you mentioned..i originally thought of this when i first bought a 12au7 guitar pedal..i love the detail in the sound of it and wondered how it would sound in mic interface..like the tube version of the SSL v2 USB audio interface..how would you rate the quality of those tubes you mentioned as opposed to the 12au7? if the quality isn't as good then i guess i don't think i need it portable lol..thanks again for the info as well.

[@MikeFreda]

The 12AU7 is a slightly better sounding tube than a 3S4 but it also requires a different power supply typically from the mains. The 12AU7 is good for a HiFi / Audio preamp it's not really suited for a Mic Pre. It just doesn't have quite enough gain to take a Mic signal into a power amp without some crafty circuitry like using some really nice SUT/line matching transformers. You may want to look at the Altec 1566A for something like that. It uses a 12AX7. Cheers

[@benevolentessence8809]

@@MikeFreda i see..thanks a lot for that additional info..i know the 12ax7 has more gain, but i'm more after the clarity of sound in the 12au7, but yes, the lack of gain is a problem..what if it gets pushed with an opamp..what do you think?... i just checked out the altec 1566A tube and it says the frequency response only goes up to 10khz...been trying to find the frequency response of the 12au7 though..there's a lot of mixed info about that tube..some say 30khz.. the frequency response of at least 20hz to 20khz is what i'm after..think the 12ax7 has that frequency response, but it doesn't have the clarity of the 12au7.. do you think it will keep its clarity and quality if the 12au7 gets driven with an opamp?

[@MikeFreda]

@@benevolentessence8809 The frequency response in a circuit like that is mostly predicated on the components used. 30Khz for just a tube seems doable but it's the transformers that hinders the response. Reproducing anything after 20khz is a waste due to most humans ears don't really hear that high. Have a look here, this may having something that you are looking for: www.jensen-transformers.com/schematics/

[@moodyga40]

great video did you earth the case

[@MikeFreda]

Thanks. Yes, the green safety ground from the AC plug is grounded to the transformer stud. There is a separate ground for the signal on a stud near the tube socket and a separate ground for the DC return near the PSU.

[@moodyga40]

@@MikeFreda fabulous I am building a 300B stereo amp 6sq7 driver

[@MikeFreda]

Nice. I have all the parts for a SE Parallel Feed 300b with 5670 driver tube. Magnequest transformers. Just have to find time to make them.

[@robgrune3284]

well done! encased tubes are always my preference. no tone controls?

[@MikeFreda]

Thanks. It was a fun project! No, like my 6SN7 amp, it’s not necessary in this circuit.

[@donchonchung3622]

👍👍👍😄😄😄

[@MikeFreda]

The White 6SN7 Preamp build: kzbin.info/www/bejne/hmTJiYevi55spdU