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 Post subject: 12J8 Headphone Amplifier
PostPosted: 09 Jul 2018, 22:45 
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Joined: 28 Dec 2010, 22:07
Posts: 349
Hi all,

I was playing with a 12J8 tube before and found that it did okay. I was using those little 1K:8 audio transformers that Radio Shack used to sell, which are capable of a few hundred mW into a speaker. Not ground-breaking, but decent for headphones. It's altogether possible that a larger transformer could provide better fidelity and soforth.

https://frank.pocnet.net/sheets/127/1/12J8.pdf

Anywho, I am proposing a headphone amplifier that will use these as output tubes. I would like to implement some kind of gain stage prior to the tetrode so I can effect some negative feedback. That might make the tiny transformers more willing to perform well. I do remember that the sound was kind of rich in the highs and I could mellow it out by placing a 100nF capacitor from the plate to ground. I might look for a 12U7 (which I'm convinced is a 12AU7 proven for low-voltage) or I might use one of my clear-top RCAs.

Either way, I will provide more information when there is more information. I'd like to make this one exceptionally hum and noise free, something I haven't achieved with other tube amps I've made. The last headphone amplifier I built using tubes was a 12AU7/IRF510 model. IIRC, the gain of a 12AU7 is something like 5 at 12 volts if the next stage is a high-impedance load like a MOSFET or even an LM386.

I'd consider getting better transformers, but I also have no interest in getting Edcor transformers that will cost at least $50-60 for two models (when shipping is factored in) and will take months. Maybe some feedback from the experts here could be helpful.

Ed

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PostPosted: 10 Jul 2018, 06:55 
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Joined: 19 May 2011, 05:38
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Those transformers bottomed out at about 300cps. they wee made for transistor radios. you might try anything else. Stay away from communication transformers (telephone duty)


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PostPosted: 10 Jul 2018, 09:15 
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Joined: 28 Dec 2010, 22:07
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When I get back to my work bench, it might be good to run a test and plot out a frequency response curve for the amplifier. What you're saying makes sense, because the sound was very bright and airy when I wired it up. It would be okay for a radio receiver (especially with some audio filtering) but for a "hi-fi headphone amplifier" I'd want to use something else. It's also possible that I could drive a FET or a transistor with 12J8, as that is what they were made to do in the first place.

I also have one or two pieces of 70v, 10w PA speaker transformers. I could wire one of those to see what the frequency response curve would be. I have used one of those in the past with 12AU7 and 24 volts and it worked well. What I don't want to do is corner myself into buying $25-30 worth of transformers for an amplifier that will put out 20 mW. I still like the idea of using no semiconductor parts in the amplifier. If I decide to use the extra diodes in the 12J8 to make a level indicator, I may not be able to get around that though.

Ed

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PostPosted: 12 Feb 2019, 02:26 
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Joined: 28 Dec 2010, 22:07
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I got the tube sockets and volume control for this amplifier today. I'm going to wire up the circuit on a copper clad board soon and start working up a chassis.

Although I'll still try those tiny transformers for testing, I'm going to get some better irons. Either I'll find some 70v speaker transformers or I'll order those Edcor line matching irons someone else used for a headphone amp here.

I'm going to continue looking for transformer options. I don't want to spend a ton but my Edcor order might not be so bad if I order some other transformers too.

Ed

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PostPosted: 12 Feb 2019, 21:27 
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Location: Arizona, USA
Hi, The "matchers" workfine for audio, but be sure to keep the dc current within low levels. They are not rated for much ...particularly if it is single ended. The core will saturate and the sound will suffer.

Good listening
Bruce

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PostPosted: 13 Feb 2019, 01:50 
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Joined: 28 Dec 2010, 22:07
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Yes. DC can mess with a transformer's ability to pass AC power. The max power of a 12J8 tube is 25 mW. In this case I am not worried.

Ed

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PostPosted: 18 Feb 2019, 04:27 
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Joined: 28 Dec 2010, 22:07
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I ended up making an aluminum case for the amplifier tonight. I bought some 0.019 inch sheet from the local home store and folded it into a chassis. Although it's thin, bending the metal gives it some strength.

The case is 8 x 6 x 3 inches and I'll be able to post pictures of it when I can shrink the photos a bit. I may drill some vent holes or add some accent pieces of wood or something.

The PCB for the amplifier is going to be 3x4 inches, which means the transformers should have lots of room. I thought about painting the interior black so radiated heat from the tubes would have somewhere to go.

Ed

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PostPosted: 18 Feb 2019, 17:03 
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BowToEd wrote:
The PCB for the amplifier is going to be 3x4 inches, which means the transformers should have lots of room. I thought about painting the interior black so radiated heat from the tubes would have somewhere to go.

Ed


Colour shouldn't do anything to cooling, I suggest is probally better to cut more neet holes in the case so air can flow inside.
Is mandatory to have some sort of holes for cooling, a easy way is to drill multiple m2 or m3 holes


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PostPosted: 18 Feb 2019, 17:16 
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Joined: 28 Dec 2010, 22:07
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Airflow would be effective for the simple task of keeping the heat inside the box from getting too high. It would do nothing for the dissipation of the tube plates but would prevent other components (mainly capacitors) from overheating and failing.

Yes, color would have something to do with cooling. If I painted the box black inside and out, the infrared radiated from the tube plates would not bounce off the inside of the aluminum box and find its way back to the tube. It would be absorbed because black objects absorb more light (which is what IR is). Then the aluminum box would warm slightly. It would be able to conduct the heat away but would also transfer the heat to the air around and also radiate it a bit.

So yes, color will affect the temperature. I suggest looking at the stand-up 6EM7 amplifier on the project page. The designer (whose name I forget) painted all the tube enclosures black. There's a reason for that.

Ed

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PostPosted: 18 Feb 2019, 17:32 
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Here's a link to pictures of the chassis.

[url]12J8 Headphone Amp Chassis https://imgur.com/gallery/UyqR6on[/url]

Ed

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