Tube Audio Amplifier

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About: I'm interested in everything that involves electronics, audio or woodworking.

I built this "tubes only" amplifier from scratch. It's a quite long project of mine and it required a lot of time and patience to make and in this summary I'll show you how i made it. If you are interested in building one of these than make sure that you take your time and get ready to face a few difficulties.

IMPORTANT! This device has lethal voltages all over inside. If you are unaware of high voltages and electronics, I do NOT recommend this project for you. If you are following along, you do so at your own risk! Definitely do not poke around electron tube devices while they are turned on!

Let's get on with it!

Step 1: The Idea.

I've found some old tubes in a drawer at my grandparents house and I was wondering what I could make out of them. After some thinking I decided to make an amplifier. I also wanted to make it special so i decided to NOT USE ANY semiconductors. I had to do some research to find out how these tube amps work and I'd like to mention here the Aiken Amps website. I've learned lots of things there about this topic.

Step 2: The Schematic & Components

This was probably the hardest part: designing a schematic. First I wrote a list of tubes that I had laying around and then I sat down to draw. What I imagined was a push-pull type stereo amplifier with tone controls, a phono and aux input, and some VU meters. The driver tubes had to be EL84 s and for the other stages i decided to use simple double triodes. I quickly ran out of tubes and had to order new ones. Which means new old stock. If you want to order tubes as well then I recommend Tubes-Store. I got mines from there and I'm very pleased. Then comes the difficult part: the output transformer. It isn't easy to find one for cheap. But after searching a bit I eventually found some on eBay. You may ask why I wrote NASS II-12 on the schematic. Well NASS stands for Not A Single Semiconductor, II means push-pull and it has 12 tubes in total. ;)

Step 3: The First Test

The rat's nest you see above is the assembly of the components in mid-air. I used two regular power transformers in series as an output transformer just to test if everything worked. Everything seemed to be all right so now was the time to find a power transformer. I had an old one laying around so I taught: Why not wind one myself? After disassembling, rewinding and testing it I quickly dropped the idea... I forgot to center tap it, which is essential for the rectifier tube. So I just took one from an old radio, thinking that this will be all right. But it wasn't. But more about that later.

Step 4: The Enclosure

For this I wanted something simple yet good looking. I thought about a brushed aluminium front, top and back plate. The sides would be made from some kind of hardwood. Sadly I had to abandon the aluminium top cover because my resources were limited. The front and back were made from a three layer material (two sheets of aluminium and a plastic one between). I don't know what it's called. For the top cover I still needed a strong and durable material, because it had to stand the heat generated by the tubes and had to hold the weight of the main transformer. So I decided to use textolite. This material has a brownish color and it's relatively strong and easy to work with. Important is to electrically shield the whole enclosure and connect it to ground at only one point to avoid ground loops. I used spray glue and thin aluminium baking sheet in this case.

I first designed the front and back panels in SolidWorks just to see how it was going to turn out. After that I used a drill press and a file to make the necessary holes for the connectors, fuses, switches, potentiometers and VU meters. For the nice surface finish I used a fine grit sandpaper and brushed it in only one direction (from left to right and vice versa) till I achieved the required look. After that I used transfer foil to print the labels and I finished it with a layer of shiny clear coat to prevent the letters from being wiped off with time.

I installed the top panel for a test fit and then I drilled out the necessary holes.

Step 5: The Wiring

After installing a sheet metal reinforcement on the top panel to help it sustain the transformers, I started the wiring. This was probably the most time consuming procedure. I first bolted on the transformers and tube sockets and then soldered the necessary components. The tone control module needed extra shielding because it really wanted to pick up noises from the environment. So I installed it into a metal box.

Step 6: The Final Assembly, Issues & Specs

So I got the whole thing assembled and after a test it turned out that the main power transformer had issues with the very high heater current, thus after around 30 minutes it got to a temperature of over 90 C (194 F). That was way over its optimal operating temperature and even after installing a small fan inside the enclosure, I couldn't keep the temps down. So I had to install another 6.3V transformer inside the enclosure. This solved the high temperature problem.

The other problem was the very high noise level. This is probably due to the ground loops that I accidentally left in the circuit. But with a rebuild this can be solved without too much effort.

In the end, despite of the small imperfections that this amp has, it sounds excellent! And by excellent I mean phenomenal. And definitely looks awesome...

This amp can output 15W RMS per channel without any noticeable distortion. It draws around 10-15W from mains when idling, and around 100W when heaters are on. You should be aware of the fact that the tubes generate a lot of heat, in winter it's great for heating up the room (not so much in summertime). ;)

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    25 Discussions

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    Hans_Daniel

    4 weeks ago

    Hello there! Just a quick update: I finally had some time to fix the amp. I've taken some measurements and modified the circuit so that now all the hum is gone and the efficiency is also a tiny bit better. Altogether now I call it finished. Here's the new schematic with measurements for those who are interested. Have a nice day ;)

    20181015_193945.jpg
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    BrownDogGadgets

    5 weeks ago

    Holy wow thats impressive. Take that solid state transistors!

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    pouet

    5 weeks ago

    Bravo magnifique travail !! j ai voté pour vous c est bien mérité !!

    Congratulations, great job !! i voted for you it is well deserved !!

    1 reply
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    Hans_Danielpouet

    Reply 5 weeks ago

    Thank you! I really appreciate it!

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    ElegantAndrogyne

    7 weeks ago

    Nice one! I like the front panel.

    I'm also concerned about EZ80 being too weak. For the record, my dual-mono (i.e. both channels have separate power supplies) EL84 PP amp that I built 5 years ago or so sticks to the "no semiconductor" rule as well. It has two EZ81 tubes, one for each channel, and one of them burned out recently, I had to replace it... I think you'd do well with a 5U4G/GZ34, or - if you want to stick to the Russian tubes - 5C3S or 5C4S.

    By the way, I've noticed that you use MLT resistors; they were very popular around the Eastern bloc, including here in Poland. I've also noticed the "Remix" capacitors, that appear to be a Hungarian factory (I've spotted some when restoring an old Orion 420A radio).

    It's interesting that you use tone controls for your amp; it's very old-school thing in a hi-fi amp, and you hardly ever find it anymore. Now it's just a phono preamp (in some amps), input selector, possibly a line stage, then volume control and power amp... I'd add a "tone defeat" switch (bypasses these controls altogether) though. I've also noticed DIN connectors, also very antique, but I like these for unbalanced connections; I tend to avoid RCA because of ground loops.

    As for the chassis, I'd go full aluminum rather than wood (the latter can be used for side panels), route these wires cleanly and do some proper grounding and insulation... 230V is no joke. You did a splendid job with the front panel though.

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    silmarillion1

    8 weeks ago

    I'm not sure how can this possibly work. As far as I can tell, you're using a single EZ80 to power 4 EL84s/6P14Ps. The maximum current supplied by this rectifier is 90mA or thereabouts, so it would be borderline even for powering just one channel of this amp. What am I missing?

    3 replies
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    Hans_Danielsilmarillion1

    Reply 8 weeks ago

    You are totally right. That tube is rated for way less current than what I'm using it for. But I haven't yet noticed any problems with it so I guess it's fine.

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    silmarillion1Hans_Daniel

    Reply 8 weeks ago

    Out of curiousity: have you measured how much current are you actually drawing from the rectifier? What's the plate voltage that it supplies? Are you actually using an EZ80 or an EZ81?

    Thanks in advance.

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    Hans_Danielsilmarillion1

    Reply 8 weeks ago

    I've been thinking about measuring the current but I haven't done that yet. At it's output, aka pin 3, I'm getting around 300V when under load. I am using an EZ80 tube. I have an EZ81 as well but that one isn't in too good shape, that's why I decided to use the lower power one. Right now I'm working on another project, after that is done I'll focus more on this one. My plan is to rebuild it to fix it's issues and take some measurements so I can give more technical details about it.

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    p_leriche

    8 weeks ago

    A nice project! I notice you don't use negative feedback - which you can get away with using triode valves and a push-pull output stage.

    @gm280 and others: You would probably find the classic Mullard book "Circuits for Audio Amplifiers" interesting. (Reproduced at https://www.scribd.com/doc/19400164/Mullard-Circui...

    This contains circuits and constructional details of various famous amplifiers including the Mullard 3-3 and 5-10 designs, and for each, interesting descriptions of the design decisions. I still have a 3-3 in the loft which I built in the 1960's together with a Heathkit bass reflex speaker, but I haven't switched it on in many years.

    4 replies
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    JohnC430p_leriche

    Reply 8 weeks ago

    So smarty... where is he missing the negative feedback? Why is it that people come up with criticism without examining the circuit closely? Is it just to make yourself sound like you know something "clever"? Yes he is using negative feedback.

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    p_lericheJohnC430

    Reply 8 weeks ago

    Did you read the "be nice" policy? Not trying to be smart - this is a good project. Perhaps you'd like to point out the negative feedback loop if you can see one and we'll all learn something today.

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    Hans_Danielp_leriche

    Reply 8 weeks ago

    If you are planning on using it I suggest you to replace the old capacitors inside the device, since these tend to get leaky through time. A leaky cap can lead to distortion in sound and might cause damage to other components as well.

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    p_lericheHans_Daniel

    Reply 8 weeks ago

    Of course. The big reservoir capacitor in the power supply, in particular, would need gently nudging back into the world of the living!

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    JohnSmith-Workshop

    8 weeks ago

    Amazing job mate. Love that old look with tubes and aluminium front panel. Big respect :D

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    billbillt

    8 weeks ago on Step 6

    WAY TOO MUCH WORK FOR MY LAZY BUTT COOL!!.....

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    Chip Zempel

    Question 8 weeks ago on Step 4

    Bravo - amazing project! Much too ambitious for a hack like me. I just use a plain ole labelmaker on my projects and never liked the way they look. Yours look so professional. Could you provide more info on how to do the foil transfer lettering (or a link to a relevant Instructable)? A search for "foil transfer lettering" here didn't turn up anything useful. Thanks - and congratulations!

    1 more answer
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    Hans_DanielChip Zempel

    Answer 8 weeks ago

    Thank you for the praise. I used some old transfer foil that my parents bought around 25+ years ago. It has a bunch of letters that you have to individually press against the surface. If these were new they would have got stuck to the surface, but considering the age I had to apply a clear coat to protect them. This is the closest one I could find that works on the same principle:
    eBay:
    https://www.ebay.co.uk/b/Number-Letters-Scrapbooki...
    Instructables:
    https://www.instructables.com/id/An-easy-way-to-cr...

    I hope these links will help you out in making great looking labels.

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    JohnC430

    8 weeks ago

    Very good looking chassis and a work of art.