How to make a 6DN7 tube HiFi amplifier / A low power Single End Triode tube amp that sounds amazing!

Scratch building a low power SET stereo amplifier using a low cost 6DN7 tube that you would typically find in a television. This tube has dissimilar triodes that are easy to wire into a great sound low power SET amplifier. I made to look like a old military device (this will be used for a shop stereo system)
This amp produces around 1.5 - 2 watts of sonic goodness! (I was blown away!)
These TV tubes have a wide range 25Hz - 21,000Hz (dependant on the OT used)
Deep bass, crisp/bright highs and a wide sound stage.
Specs:
Two 6DN7 vacuum tubes (1 per channel)
Two 5k Primary - 8ohm output transformer (i.e. Hammond 125BSE)
One 275-0-275 ~80ma power transformer (i.e. Hammond 270CX, 270DV, 271X)
FAQ and Mod Video: • 6DN7 tube HiFi amplifi...
If you enjoyed this video and if you downloaded the build documents, please consider buying me a coffee. I would appreciate your support:
www.buymeacoffee.com/MikeFreda
Valve Amplifiers - Morgan Jones - amzn.to/43wQjNc
Hakko Solder Station: amzn.to/3PvQaD7
Kester Solder: amzn.to/3aJKNBI
Schematic: drive.google.com/file/d/1hQbL...
Parts list: drive.google.com/file/d/1liDq...
Alternate Filament supplies: drive.google.com/file/d/1oo-j...
WARNING: Tube amplifiers contain very high and potentially lethal voltages!
Drain all filter caps and check voltages before commencing work.
Disclaimer: Some of this links go to my website and some are affiliate links where I’ll earn a small commission if you make a purchase at no additional cost to you.
0:00 Intro
1:23 Schematic
4:05 Chassis
11:22 Wiring
15:40 Firing it up
19:33 Wiring layout
20:36 Final thoughts
21:19How it sounds
Camera:
Canon M50 (Sigma 16mm 1.4f lens)
iPhone 11
Boya By-M1 Lavaliere Mic
Amp mic'd: Rode VideoMicro
Lighting:
Aputure Amaran AL-M9
Neewer 26" Soft Box
DIY Cake Pan Light: • MAKE a Cake Pan Light ...
About This Channel:
This channel has a wide variety of content. I’m interested in many things and topics. I create videos on things I like and what I feel my subscribers would like as well.
This channel is basically about two things:
1.) Show you how to MAKE things, FIX things, TIPS on how to do things and to give you IDEAS on how to make your life a bit easier! I encourage you to use your hands the to learn something new!
2.) People say I do fun and interesting things and have interesting hobbies. I like to share that with you and I hope this inspires you to do things and to get out there as well!

[@MikeFreda]

Matching Linestage/Preamp: kzbin.info/www/bejne/pojMfHilhLeKmas

[@MrCarlsonsLab]

Very nice work Mike! Thanks for sharing your project with us.

[@MikeFreda]

Thank you! Glad you liked the video and project. I'm really happy with the results. These tubes are truly a gem!

[@bentleygt3716]

OMG !!!. it doesn't only have the Military vibe. but also looks so neat and beautiful .i'm green with envy. i wish i had your knowledge, tools, and manual dexterity to make equipment like these. there's so much Pride & Joy one can take in building his own . wow. very impressive. KUDOS

[@MikeFreda]

Thank you fo ryoru kind words! I'm glad you liek the build. It's a favorite of mine and I use it every day, Cheers

[@mikevincent6332]

This is how things were built in the 60's - 70's DIY / ham radio days. Fantastic!

[@MikeFreda]

That was my plan. Glad you like it!

[@pmc9941]

Hey Mike, love that you use these television tubes! Thanks for the in-depth video!

[@MikeFreda]

You're welcome. Glad you liked it. These tubes are truly a gem! I'm blown away on how good they sound. This amp sounds better than my RH84 SEP amp.

[@PrinceWesterburg]

HOW Have I never heard of this tube?!? A triode with max 10w dissipation in an octal base thats not a Soviet model?!? I gotta download me some data sheets on this one! Thank you so much! :o)

[@MikeFreda]

Yes, it’s an oddball tube. They do have a cult following for people who are in the know. There are a couple variants of this style of tube. Cheers.

[@lego40404]

My 6DN7’s came in yesterday. Thank You again for this

[@MikeFreda]

That’s awesome. Please let me know how it turns out!

[@lego40404]

Will do, I have some Mcap 0.1’s and Auydn 0.1’s for C1.

[@MrArnprdu]

Appreciate all the effort you put into these, relaxing. You seem very patient.

[@MikeFreda]

Thanks for commenting and your kind words. Cheers.

[@lego40404]

Your killing me Mike ;-). Once again beautifully executed work. I will be building both of these for my shop and relocate my 4S pre/807SE into the living room.

[@MikeFreda]

Thanks! I hope it works out for you!

[@astroporpoise7802]

Was looking through American Resoration episodes on TV, trying to find some tube audio projects! There weren't any besides an old tube juke box. Your videos are much more what I was hoping for--and so fun and relaxing to watch with my coffee on the weekend. Thanks for the great content! Subbed!

[@MikeFreda]

Thanks for commenting. Overtime this knowledge will be lost or forgotten, hopefully people can use video’s like mine to learn and get inspired. Cheers.

[@quiverrock]

Awesome video! Thank you for sharing all of the build info - schematics and parts list is truly a gift. Smart, great work.

[@MikeFreda]

Thanks for commenting. Please let me know if you make this amp. I'd like to hear how it turned out! Cheers

[@joshuacolemanmakes7518]

My guy. Holy crap. Your process, your insights, your calm lolol. SO glad I stumbled on your channel. And you've got Mr. Carlson's seal of approval. Thanks for this so much.

[@MikeFreda]

Thank you for the comments. I'm glad you enjoy my content. Yes, Mr. Carlson is a genuine gentleman and is a wealth of knowledge when it comes to electronics. Cheers!

[@hardystein114]

I really like the look Mike. What a great unique pair of units. Just lovely builds ! 👍👍👍

[@MikeFreda]

Thanks again! They do look great in a shop! It is a conversation piece for sure! I plan to do a couple more builds like this. Cheers!

[@hardystein114]

@@MikeFreda Looking forward to that Mike. You do some fine work.

[@motjones2341]

Damn fine looking metal work and mechanical design buddy. Thanks for the tips.

[@MikeFreda]

Thanks for the comment. I use this amp daily and I still have a hard time believing this tube sounds so good. Stay tuned, I'm planning some minor modifications.

[@AndJusticeForAll...1985]

Good job Michael

[@MikeFreda]

Thank you. Cheers.

[@towerman75]

Great video. Great technique for layout and build. Very good building practices. Keep up the good work.

[@MikeFreda]

Thank you for your kind words. I love this amp and listen to it daily in my shop. It was some effort to build but it was worth it. I’m in the process of building another tube amp. Build video will be uploaded soon. Cheers.

[@martinda7446]

Hi Mike, first time visit to your channel. Subscribed. I have been working in the specialist Hi-fi industry for over 40 years. and am an electronic engineer. I have built a whole bunch of things including amplifiers.. and there are many houses esp. around London, with bespoke parts hidden behind cabinet doors. I specialised in vintage audio and was doing the whole gamut from full restorations down to biassing customers Audio Research amps and setting up to restoring turntables since the eighties. There isn't much I have not taken apart, repaired, listened to or sold..Etc.. I only mention this, not to blow my own trumpet, (well maybe a bit) but to give you an idea that I sort of know what I'm talking about when it comes to audio and amplifier design.. Anyhow, they are just beautiful. That is it!

[@MikeFreda]

Thanks for commenting and thanks for subscribing. Working with tubes and analogue equipment is becoming rare. It’s people like you and I that need to share the knowledge with the world so it doesn’t become a forgotten technology. I work with a bunch of younger people who are technical but their eyes gloss over when I explain what tubes are, what they do and the theory. They really don’t care. lol. Cheers.

[@martinda7446]

@@MikeFredaExcellent... But I've seen that 'don't care' look...

[@carlgradolph9676]

Wow, so glad I found your channel! In this video, you provide just enough technical information to allow other builders to copy your work without getting too far into the weeds. But in the future, I would really appreciate a deeper dive into the way you choose a tube and spec out the components. In my junk box, I have a variety of oddball NOS tubes, various salvaged transformers and sockets, etc. And I have some old tube manuals, but what I lack is the knowledge of how to read the operating parameters of a given tube and the translate that into a workable circuit. I have liked and subscribed; looking forward to future videos.

[@MikeFreda]

Thanks for commenting. I’m glad you like my projects. I do not profess to be a vacuum tube expert, I’m also learning things as I go. I do plan to do deeper dives into tube amp design, please stay tuned. FYI, The Morgan Jones tube amp books are a great resource to read and learn way more about tubes than I could explain in my lifetime. Cheers!

[@towerman75]

Dear Mide -- I built one simipar to it using a pair of 832's from a WWII VHF Aircraft transmitter. It also gave about 8 watss out, and sounded great. Unfortunately, it has a new owner that loves it. I sold it on E-Bay.

[@MikeFreda]

Thanks for commenting. There are a lot of different tubes out there that can make for a good audio amplifier. Just need to research and spend time developing and building them. Cheers.

[@DerekRonin]

Thanks again Mike! Just found something to keep me busy during the Thanksgiving break!

[@MikeFreda]

You’re welcome. Thanks for commenting. Stay tuned, I’m in the process of making another video about this amp. More mods! Cheers.

[@patrickdowning7674]

Absolutely amazing. Thanks for making that.

[@MikeFreda]

Thanks. Glad you liked it. Stay tuned, I have some mods planned for this amp to make it even better.

[@mikevincent6332]

Amazing metalwork!

[@MikeFreda]

Thanks for the comment.

[@akkudakkupl]

You ground shields on both sides, always. One sided shield grounding reduces the shielding effect - it makes the wire act as a highpass for electric fields, but a lowpass for magnetic shields with the shield length beinG the 1/4 wavelength of the rolloff frequency. So you might be shielding out the hum from electric fields, but might still get hum from magnetics (trafos, chokes, etc. ). Mil-Std-188-124B shows the proper shielding method.

[@greghardy2676]

Great Video Mike!

[@MikeFreda]

Thank you. The more I listen to this amp the more I like it. It’s a true sleeper. Stay tuned, I’m working on a modification video of this amp. Adding some extra pieces for even better performance.

[@franklindrebin00]

More projects like this please. 😉 Greetings from Germany.

[@MikeFreda]

Thanks for commenting. Yes, I a few more projects like this planned. Cheers.

[@dextergilford8336]

Great work!

[@MikeFreda]

Thanks! It a wonderful amp and I use is almost everyday! Cheers

[@cobar5342]

Very good explanation - appreciated

[@MikeFreda]

Thanks for commenting! Please share if you make an amp like this. Cheers.

[@MrRourk]

Beautiful build. The kits in grey from Transcendent Sound would match well.

[@MikeFreda]

Thanks. It sure is a unique look. I plan to make more amps in this style. Cheers

[@bubblehead78]

Great video. Subscribed.

[@MikeFreda]

Thanks for commenting. Glad you enjoyed the video. I have a few more coming up like this one. Cheers.

[@jimcatanzaro7808]

I made a replica using all USN parts and tubes 4 @807s 3 @ 2 6sn7 tubes and a type 83 mercury rectifier with a separate power supply I love the work you done now I’m thinking of another project based on yours the stenciling looks so factory I love it ❤

[@MikeFreda]

Thanks for commenting. Glad you like my project. Like to see what you created. Yes, there are a lot of inexpensive tubes out there that would work great in an audio circuit. Stay tuned, I have a couple more amp projects coming up! Cheers.

[@tee-jaythestereo-bargainph2120]

Great Job thanks for the video

[@MikeFreda]

Thank you for your comment. Stay tune, I’m planning a couple mods on this amp.

[@alanpecherer5705]

Fifty years ago, when I was experimenting with electronics and building all manner of simple projects like this, I struggled with any sort of metalwork beyond drilling a hole because I simply didn't have the tools. And every part other than the tubes and trannies was 25 cents. Today, I have all the metalworking tools, but every electronic part is $3, the trannies are ridiculous (if you have to buy versus salvage) them. But you couldn't get me to build such a thing today when I can buy a pair of powered computer speakers for $5-$10 at a thrift store or garage sale.

[@MikeFreda]

Thanks for commenting. Yes, you are 100% correct, building tube amps is an expensive hobby. That’s why you need to be frugal and be realistic with you build and what you actually need. Yes, you can buy inexpensive heat but it’s really not that same as a tube amp you build yourself. At least it’s cheaper than cars or horses! Lol. Cheers.

[@alanpecherer5705]

@@MikeFreda I was contemplating getting back into (guitar) amp building a few years ago; basically buying dead solid state amps for $20 and converting them to old Fenders. And after trying a few I found it's just not a feasibility...unless you're ready to do fairly meticulous work for $5 an hour. Parts are crazy expensive; you can't salvage anything from a SS amp---the pots are 1K-5K; the reverb can is the wrong impedance; the trannies are wrong voltage and impedance. I won't use plastic phone jacks at all. You can barely even solder to old parts with "modern" lead-free solder. You're 20x better off just buying a good condition used Fender whatever and selling it for $100 less (or more!) than you paid for it three years later. Add up all the parts in a Princeton or Deluxe Reverb or even an old 6G2 or 5E3 and you can't build that amp for what it costs to buy a decent used one.

[@lego40404]

Mike, I have the amp up and running and it sounds great. I’ll take a picture of the schematic with my voltage readings. A little cleaning up to do maybe on the wires.

[@MikeFreda]

Magnificent. Glad to hear it worked out. I can post a picture of your amp on my community page if you like.

[@lego40404]

Please share.

[@TitofBee]

hey ! really cool😊

[@MikeFreda]

Thank you! Cheers!

[@RobertKohut]

Nice!!

[@MikeFreda]

Thank you! Cheers!

[@jimcatanzaro7808]

My next project is using 8 807 output tubes in push pull they sound good and are about as cheap as a 6bq5

[@greghardy2676]

Since this is my 1st tube build, I am doing the chassis like a Bottlehead Chassis.....

[@MikeFreda]

That should work fine. I've built many amplifiers that way. If it's your first built I'd go with a larger plate. 10" x 12" or 12"x12" should give you enough space to place everything. Just try to keep the OT away from the PT's and leave some space for mod's like larger capacitors and a choke. I'd like to hear how you make out. Cheers!

[@greghardy2676]

@@MikeFreda Ty Mike

[@alanrogs3990]

I'd pay you for something like this setup. I really like the look of it.

[@MikeFreda]

Thanks for commenting. Yes, it’s unique style. Honestly, building the chassis was more work and more of a head scratcher that wiring the amp up. A lot of man hours in this project. I use it everyday and it’s wonderful. Cheers.

[@alanrogs3990]

@@MikeFreda The chassis makes it very cool.

[@bobbysparky4299]

Hi Mike. First of all congratulations on your amplifier project. I can only think of one word to describe it, and that is magnificent!!! I have been thinking along the same lines myself but I don't know what tubes to consider. I would like to use television tubes that are easily obtained but I've been thinking about a push pull configuration to give around 30 watts on the output. Are you aware of any tubes that I might consider for such a thing? This old man sincerely thanks you in advance. Bobcat

[@MikeFreda]

Thanks you for you comment. I'm not aware of any TV tube that is readily available that can produce that sort of power. That's a pretty hefty amp with a large PT and OT's. I do recall a while back someone made an amp like this using an equivalent tube similar to a 6L6/5881/KT66. I think the trick with these amps is using the right amount of negative feedback to not "color" the sound to much.

[@SenakaSandagomiWeerawardena]

330 thumbs up is me.

[@MikeFreda]

Thank you.

[@lego40404]

Hey Mike how’s things. Amp is coming along great, just wish I had more time. I have a question about the cathode bypass 470uf/100v cap. If there’s only 16.5v on it could you get away with a 50volt cap? I have 100v Nichicon gold foils on the way but I had put 50v Nichicons on for now. When I’m finished I’ll post picture

[@MikeFreda]

Thanks for the update. 50v should be fine. In this application it’s a good idea to oversized due to voltage spikes especially at turn on. Higher voltage is okay as well. Just make sure you choose the appropriate temp rating. Cheers.

[@tubeDude48]

For the newbie's that are out there, you should mention to them where the meter is connected.

[@MikeFreda]

I do mention it near the end of the video on how its wired. I guess I can include it on the the Schematic. It basically goes after C3 before it branches off to each channel. Thanks for the comment! Cheers!

[@Pootycat8359]

What about using 6146 or 807 RF PA tubes? They aren't too expensive, and can put out quite a bit of power.

[@MikeFreda]

Thanks for commenting. Yes, those tube are inexpensive and sound good. I do plan to make an amp using 807 tube but it does require larger transformers and different wiring scheme. This amp was more intended for a low cost way to try a single end triode amp with really inexpensive parts. Cheers.

[@alerey4363]

Do you have some measurements on the sound parameters? distortion, dc ripple, freq response and the likes? What is the power consumption on idle and what is the max power it takes with volume cranked up to 100%?

[@MikeFreda]

Thanks for the comment. I plan to do that in my next video. I don’t have some of the tools to do this but I have a friend that does. I will try to connect with him and see if he is interested in doing a video with me. Cheers.

[@MrJbhunter11]

I'm looking to do something just like this just with a little more power, maybe 20WPC. Any suggestions or plans. Love your work man.

[@MikeFreda]

Thanks for commenting. A tube 20wpc stereo is a beefy amp to build. Big iron is required. Push/pull EL84, 6V6 or even 6L6 would come close to that. It would be an expensive amp to build. The closest thing I plan on making and doing a video of is a single ended 6L6 amp. Cheers.

[@MrJbhunter11]

@@MikeFreda ahhh yes! Can’t wait to see it!

[@MrJbhunter11]

@@MikeFreda I got your next few cups of coffee on pay day next week. Great content man. Keep Up the work

[@BSutes]

Love the font. Any chance it is a free font that is available for download? GREAT PROJECT :).

[@MikeFreda]

Thanks. It was just Arial Bold that was in the Cricut App.

[@BSutes]

@@MikeFreda Wow! It looks legitimately like some sort of military equipment font. I love the aesthetics.

[@lego40404]

Mike, on your print out you have two C3’s. One after R10 in the power rail and another between R7&R8 above where it says “one channel shown and it’s not in the part list BOM. Is this the correct? value? Also for the filaments I have 15000uf on hand, this would be okay? Thanks

[@MikeFreda]

Hello, sorry, that is an error. I’ll will amend and reload. The way it’s drawn on the schematic is correct, the annotations are incorrect. 15000uf should be okay but your dropping resistor may be different. You could also go a bit lower and may not notice a difference.

[@MikeFreda]

Documents have been revised and re-uploaded. Re-download them from the video again.

[@evansabin8491]

Are there any production amplifiers ever manufactured using a single tube for stereo amplification?

[@MikeFreda]

I’m not aware of any. Most people need enough power to drive inefficient speakers to reasonable levels. Trying to mass market a lower power single tube amp would difficult since it wouldn’t cost much more to make an amp with more power which would make it more desirable. Cheers.

[@doubledigitdreams]

Are there other options when it come to the output transformers. I know edcor has a 10watt 5k 8ohm, also the AES-pt31 8watt 5k 8ohm. Could any of these be used to substitute the hammond on your build list.

[@MikeFreda]

Yes, there are. I’m creating a new video discussing the options and I swap in obtainable output transformers. (Edcor XSE10-8-5k). When it comes to OT’s. You just have to be aware if they are for HIFI or guitar amps. There is a difference in the frequency response. Cheers.

[@doubledigitdreams]

@@MikeFreda it’s good to hear that your still experimenting with this tube amplifier, can’t wait to see the results as I will be building this to your specs. I only have experience with repurposing amplifiers from consoles, and in those builds I have replaced the output trans. on a single ended el84 2 watt amp with the aes-pt31 and for the price it sounds amazing.

[@Nihil1st1347]

I wonder you use steel for the rear. Because GR Research always condemns magnetic parts next to sockets n terminals.

[@MikeFreda]

Thanks for commenting. I’ve never heard of this being a major concern. Please elaborate? There are a lot of theories regarding this but I’m not sure how else you would do it. One would then suspect that many engineers that designed guitar amps, audio amps and electronic gear since the 1900’s missed something? Cheers.

[@Dennis-mq6or]

I am curious what the RMS power output per channel is just at the edge of clipping? And is that measurement with a 4 ohm load or an 8 ohm load? If you mentioned it in your video I must have missed it. I understand you are running the plate of the output tube somewhere close to 8 watts, but have no idea what the efficiency of the output stage is,

[@MikeFreda]

Thanks for commenting. I really don't have any proper testing equipment, however, I did use a Signal Generator app on my phone and using my Fluke 115 multimeter (which is a true RMS) I measured and calculated just under 2 watts off of the 8 ohm tap. Looking at some commercial 45amps (the power section of this tube is very similar to a 45 tube) they measure very similarly. I do plan to take the amp over to a friend's place and do some proper testing for all the inquiring minds. I previously had a EL84SE amp and it was just marginally louder than tie amp. cheers

[@Dennis-mq6or]

@@MikeFreda , I think your calculations are on target. I was estimating between 1.5 and 2 watts RMS depending on the level of distortion you were willing to allow. I would be interested in knowing what the input signal was at full power output, regardless of the signal source being used (like your phone).... If your fluke is true RMS than the input level in RMS volts or millivolts would be a handy number for comparison if any future changes are being contemplated...

[@jamieostrowski4447]

How did you cut the square hole for the transformer?

[@MikeFreda]

I used a 3/16” drill bit and drilled in a line just inside the square and filed it smooth to the line.

[@jamieostrowski4447]

@@MikeFreda Yeah it looks like it turned out pretty good, but I was hoping someone had a quick way to make good square holes. I know they make square punches but they're super expensive.

[@MikeFreda]

You could use a Zip disk or a small handheld router. Use a template, a Millend and turn the speed down.

[@lego40404]

Update, all my components are in. I went with the PT-31 OTs. I haven’t gotten a chassis yet, not sure what I want to put it in. I have a few ideas though what I want to do

[@MikeFreda]

Thanks for the update, sounds like its coming slowly together! The PT-31 should work in this circuit but I believe it's more suited for guitar amps. The high end frequency may be rolled off a bit. You could see if it works then make some provisions for an OT upgrade. (ie leave some space) Hope it work out for you!

[@lego40404]

I plan on upgrading OT's for this. Hopefully by the wait time will be shorter. OD3's on the way for preamp too

[@MikeFreda]

Awesome. Can’t wait to hear and see how it all turns out. Glad my videos inspired you.

[@greghardy2676]

Only 45uf 450v capacitors I can find in stock are ones for air conditioning motors...mouser says sept...thoughts?

[@greghardy2676]

Found 1 in china cost me $80...but need 2ea...ship date is June...guess i'll keep looking

[@MikeFreda]

No. Don’t use motor run caps. Try to use what is spec’d on the schematic or parts less. Less capacitance will work but you will hear some hum. The ones I used only cost $8 each. Mouser stocks them. Radial or axial will work if you can fit it in.

[@greghardy2676]

@@MikeFreda Ty Mike

[@greghardy2676]

all at mouser are non stocked...w june delv...i can get 47uf 450v caps all day long will that work?

[@MikeFreda]

Those will work but it won’t eliminate the AC ripple as good as a 100uf. You will hear a slight hum. You can change it after the fact if it bothers you.

[@MrJbhunter11]

How come theres two resistors in the filter cap section when the schematic doesn't show them?

[@MikeFreda]

Make your you have the right version of Schematic. The first rendition of the circuit has the 1st 270ohm right after the first filter cap. It's in the 1st video and on the original schematic. The 2nd resistor is jumpered and not used. I installed it in the circuit board to help tame the some B+. I ended up not needing it. The PSU designer was a little of on actual. The 2nd rendition of the amp/schematic I replaced the 270ohm resistor with a 270ohm choke. cheers

[@MrJbhunter11]

@@MikeFreda got it and on the filter cap section am I correct to assume everything is soldered together?

[@MikeFreda]

The components are soldered underneath the board in the same manner as the schematic shows.

[@MrJbhunter11]

@@MikeFreda perfect. Thank you!

[@iceberg789]

what is a cheap, easily available, not too powerful o/p tube can be ordered from ebay ?

[@MikeFreda]

I’d probably lean towards the EL84. I’ve made a few amps with this tube. Cheers.

[@iceberg789]

@@MikeFreda i see. thanks.

[@doubledigitdreams]

For R4 249k, would a 240k or 270k resitor be ok? and for R2 and R5 are those just 220 ohms. Sorry if it seems like a dumb question, just wanted to make sure before i submit my order.

[@MikeFreda]

Hi, R4 (aka Grid Resistor) can be anything in the range. 240k would be okay. R2 and R5 (aka Grid Stoppers) can be anything between 200 ohms to 499 ohms... 1/4w is fine. High quality carbon comp or metal film preferred since they are part of the signal path. They help calm down any oscillations or RF getting in the grid. If you want to do some research on that, you can just google "Grid Stoppers" & "Miller Capacitance". Cheers, Mike

[@doubledigitdreams]

@@MikeFreda thank you so much. I’ll look that up right now. Btw I’m having a lot of fun with this project, I’m starting to lay things out and trying to figure out transformer placement to minimize interference. There’s is a lot aspects to doing this from scratch but I know it will be extremely satisfying in the end.

[@doubledigitdreams]

Hi again, just wanted to give a quick update. I’ve just completed this build with great results. It truly does sound pretty good. I’ve only played a few songs so far. There is one issue I’ve come across though ,My voltages all around are on the low side. I’m using a transformer rated for 275-0-275 100ma. I’m using the 275 winding and after the diodes It’s at 317v, The 354v for the primary of the output transformer is at 305, pin 2 346v is at 303 , pin 3 16.5v is at 14.5. and the 75v for pin 5 is at 67v. The tubes are biased at 6 watts of dissipation. So overall the voltages seem 15% on the low side. Is there a way to increase the voltage or is this acceptable. What would you recommend.

[@MikeFreda]

Glad to hear you were able to make it and fire it up. With a PT like that you should have at least ~385VDC at the first PSU CAP. You need to do some more investigating. What is your AC line voltage? How is the PT connected on the secondary side? Are you only using 2 Diodes and the CT connected to ground? What is the voltage drop across R10? Is it more than ~17VDC? Calculate the current across R10. If it's more than 65ma you are drawing too much current and that will drop the voltage. Do you have a PSU cap in backwards? Is the bleeder R9 the right size? What you can also do is disconnect the PT from the PSU circuit and just test the raw DC unloaded. If that checks out then you need to validate all you resistors sizes and connections. Check the OT primary Taps as well. That could drop the voltage a bit if there are not connected correctly.

[@doubledigitdreams]

@@MikeFreda i May have found the cause which was a blunder on my part, I switched from a bridge rectifier (4 diode) to a full wave (2 diode) with the PT center tap grounded. Originally I was using only one side of the PT,the 275 and ground windings Which did get me reasonably close voltages unloaded but obviously loaded it the PT was struggling to provide the current and voltage. Now as I type the amp has been warming up at least 30 minutes and the voltages are now stable and a lot closer to the schematic. 375 is 356. 354 is 342. 346 is 338.

[@gamiwv]

unfortunately the 6fj7 has being priced too high + her base. The output transformer is another "tragity". More than $200 for a pair (Double C nickel-based amorphous iron core 7K single-ended). May I ask for the total cost of this amplifier (+ the current sources for cathodes) ?

[@MikeFreda]

Thanks for your comment. Yes, if you source nice components you can spend a small fortune. Tube amp building is not cheap. The 6DN7 tubes are inexpensive so you will save money not having to purchase premium tubes. If you follow my parts list you should be able to build this amp for around $300.00-$350.00. I had salvaged my OT's from an old Turntable Counsel and had a lot of the parts from my parts bin. I spent around $200.00 on this amp. If you can source Hammond BSE you can save a fair bit of money. I plan to install CCS on the Plate of the input section. Those components should cost around $20.00. You could install the CCS on the power section cathodes but that a lot of power to dissipate. (~10 watts) That could be a bit risky. I feel that whatever you spend on this amp it is worth it. It sounds that good! Cheers.

[@MichaelBeeny]

I'm surprised you went to the effort of DC heaters which in the real world will make almost no difference to the sound but you did not apply any overall negative feedback, that would have lowered noise and distortion and flattened the frequency response.

[@MikeFreda]

Thanks for the comment. True, adding DC filament does not affect the overall sound of the amp but it does make it quieter which is measurable on the output, especially if you want to use high efficiency speakers or headphones. Currect, I did not use any negative feedback. I did not want to "color" the sound by doing this. Yes, as you pointed out it would do all those things if applied correctly but on SET amps I prefer not to. I find it more pure. On SEP amps, I do add some negative feedback. Pentodes seem to do better with a bit but at lower volume it find it negligible to my ears. My shop amps are rarely at full volume so I can take it or leave it. Cheers!

[@nuxboxen]

@@MikeFreda Dumb design, nice metalwork

[@Nirky]

Feedback flattens the response curve but it also reduces output level, and more importantly reduces the 2nd harmonic, which harmonic is what makes the tube amplifier sound so enjoyable.