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PostPosted: 20 Jun 2018, 22:23 
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Joined: 20 Jun 2018, 21:11
Posts: 2
Location: Brazil
Hi folks,

It's my first post in this forum, nonetheless I have read a lot of the information here lately.

I always wanted to know what "tube sound" is, so I did that the hard way: I decided to build my amplifier by myself, from drawing the chassis to determining the power transformer characteristics. As I am an electronics engineer and work mainly with digital and RF circuits, I thought that it would be funny to play with a technology that is considered obsolete ***in my field***.

I finished "Der Koloss" one week ago. It's based on Mikael Abdellah's Single-Ended KT88 Tube Amplifier Schematic, with some modifications. For example, I used two CCSs with LM317 to bias the output tubes. The power supply uses a solid-state rectifier, and I added an 8H choke for each +B channel, to reach the ripple level of -90dB and to guarantee the channel separation. The filaments are fed with DC to avoid hum. I also included a switch per channel for choosing ultra-linear or triode mode. There is also a switch to turn on a negative feedback loop, but used it only once for testing purposes.
I ordered the output transformers in a specialized factory, which made the transformers according to the impedance, power and frequency response I wanted for my project.

I even bought a pair of auricap capacitors for coupling the pre- and output tubes, as some people say that they are the best for this purpose. I didn't want to threaten the sound quality of such huge (expensive) project with cheap capacitors, so I used them.
After finishing the circuit, connecting the pair of Electro-Harmonix KT88s, turning it on and measuring all the key points to ensure that everything was working as expected, I connected it to my turntable. What I can say is that I had never heard music like that, with many details that I had never heard before! I connected this amplifier to my turntable, iPod, CD Player, DVD player, bluetooth adapter, and everything seems to gain a new life!

You can see some photos below. I'm also a photographer, so I shot some long-exposure photos that show the blue glow of the tubes.
Here I have a question: if you look carefully at the photo #6748, it's possible to see that the shape of the blue glow is different between the tubes. I found it strange, as the tubes are the same model, bought as a matched pair and physically look the same. Has someone already seen this difference between tubes?

I noticed that it is impossible to conclude what the best brand/model of tube is, based on the opinions found in the Internet. So I ordered a pair of Psvane KT88 tubes and a pair of Tung-Sol 6550 tubes to hear the differences by myself, just for fun.
About the eternal question, "which one is the best: transistor or valve?"... I built this vacuum tube amplifier, and my colleague is projecting and building a transistor amplifier. We will listen to them carefully and figure out the differences, and MAYBE reach a conclusion.

P.S.: "Der Koloss" passed the most difficult and aggressive test: my wife loved it! :)


Best regards,
Roben de Nulagio.


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PostPosted: 21 Jun 2018, 16:24 
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Joined: 04 Jun 2008, 20:59
Posts: 3814
Location: Arizona, USA
Hi, The blue glow is gas in the tube. It will glow if the B+ is high enough. Typically over 450 volts. The actual pattern is of no consequence. You will notice it will dance around in time with the audio. Nice looking build, however the transformers are closer together than I would do. I like at least 5 cm spacing. Less likely to pick up hum that way. I would also put the driver tube further away from the power transformer for the same reason...but then I am known to be fanatical about hum and noise levels.

Good listening
Bruce

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PostPosted: 21 Jun 2018, 19:00 
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Joined: 20 Jun 2018, 21:11
Posts: 2
Location: Brazil
gofar99 wrote:
Hi, The blue glow is gas in the tube. It will glow if the B+ is high enough. Typically over 450 volts. The actual pattern is of no consequence. You will notice it will dance around in time with the audio. Nice looking build, however the transformers are closer together than I would do. I like at least 5 cm spacing. Less likely to pick up hum that way. I would also put the driver tube further away from the power transformer for the same reason...but then I am known to be fanatical about hum and noise levels.

Good listening
Bruce


Hi Bruce,
Thank you very much for your comments! In fact I did have some problems with noise: there was a huge noise when I adjusted the volume control to less than approx. 85%, even with "star" grounding system. This behavior was eliminated when I used a shielded microphone cable to connect the potentiometer to the 6N1P tube grids, but I can still notice some hum when I'm close enough to the speakers. I will take your tips on account if I build another one someday :)


Best wishes,
Roben de Nulagio.


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PostPosted: 22 Jun 2018, 06:42 
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Joined: 17 Mar 2016, 07:26
Posts: 28
Location: PrĂȘles, Switzerland
Hello Roben,

Congrats for your first build ! Since I have lots of tubes at hand, I have considered building something in that direction as well when my engineer amp casing is finished. Have you measured the output power ?

Thx,
Charles


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