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PostPosted: 26 May 2010, 22:08 
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Here are some photos and build notes of an Elekit TU-879S (SE 6L6GC) Tube Amplifier Kit that I recently put together.

Quick Links:
* - Note these schematics are for the previous TU-879R version. Elekit has indicated that they do not want to share the TU-879S schematics with those who have not purchased the kit.

Photo of the printed circuit board (PCB). Good layout and there is ventilation underneath the diode bridge and the larger resistors.
Attachment:
EleKit-TU-879S-Tube-Amp-Kit-PCB.jpg


The Elekit TU-879S Tube Amplifier Kit Transformers supplied with the kit. The power transformer is a Kitamura Kiden R-core transformer.
Attachment:
EleKit-TU-879S-Tube-Amp-Kit-Transformers.jpg


Comparison of the EleKit TU-879S audio output transformer versus a 15W Edcor GXSE15-8-5k audio output transformer.
Attachment:
EleKit-TU-879S-Tube-Amp-Kit-Transformers-vs-Edcor-GXSE15-8-5k.jpg

As can be seen, the Edcor GXSE15-8-5k output transformer is a little larger. I would estimate that the EleKit TU-879S audio output transformer are about 10W.

A photo of the PCB coming together. The electrical tape I put over the tips of the solder posts as they just cleared the pcb and I did not want the AC line rubbing against sharp metal ends which are connected to the OPT. :hot: Note the reverse mounting of the FET. Later it mounts to the chassis which acts a heat sink.
Attachment:
EleKit-TU-879S-Tube-Amp-Kit-PCB-2.jpg


Here the tube amp kit is coming together. The enclosure is compact, very attractive and well ventilated.
Attachment:
EleKit-TU-879S-Tube-Amp-Kit-Chassis.jpg


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PostPosted: 26 May 2010, 22:16 
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Bringing up the finished tube amp kit slowly on a Variac. Note that the tube amplifier is attached to a speaker dummy load as you should not run the amp with a load on the output. Shown in the photo are the kit supplied EH 6L6GC and a nondescript made in China 12AX7B.
Attachment:
EleKit-TU-879S-Tube-Amp-Kit-on-Variac.jpg


Front side of the kit shown here with new production Sovtek 5751 and Tung-Sol KT66 tubes.
Attachment:
EleKit-TU-879S-Tube-Amp-Kit-with-Sovtek-5751-Tung-Sol-KT66.jpg


More to follow shortly. ;)


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PostPosted: 29 May 2010, 11:54 
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So far I seem to prefer the Sovtek 5751 / Tung-Sol KT66 combo the best. This weekend I will be trying 6550 and KT88.

With regards to modifications, I removed the input caps and have tried orange drop, solen and auricap capacitors in the coupling role. All three are good upgrade choices with the slight edge to the auricap.

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PostPosted: 29 May 2010, 13:21 
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Hi Gio,

looks very nice. Good job.

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My DIY Audio Projects:
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PostPosted: 02 Jun 2010, 20:13 
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I've gone through all the tubes now and here are my picks of the bunch.

Favorite power tube of the bunch: Tung-Sol KT66
Favorite Driver tubes (in order): Sovtek 5751, JAN-Philips 5751, RCA Black Plate 12AX7.

I've also cycled the caps, Auricaps (~$8) were best here, but improvements over Orange Drop 715p (<$2) a Solen ($<2) were subtle. The stock (kit supplied) capacitor is on the far left.
Attachment:
Stock-Orange-Drop-715-Solen-Auricap.jpg


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PostPosted: 03 Jun 2010, 11:55 
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Gio,

Thanks for the thread. I built the TU-879S about six months ago (my first tube project). I don't have the knowledge or skill set to be making upgrades so I appreciate your detailed analysis (made a few anyway, guess I'm hooked). Please continue to post photos and observations of your upgrades.

One question- How would you characterize the difference in sound between the 5751 and 12AX7? I understand the gain is different. Why do you favor the 5751?

Keep up the good work!


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PostPosted: 03 Jun 2010, 20:10 
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I'm not fond of any12AX7. ;) I find the 5751 to have better focus and sound more natural. I generally find that 12AX7 tubes sound soft and fuzzy.

Attached is a real simple (and free) change you can try on the Elekit TU-879S. Try remove the input capacitors - you can do this provided that your source that you are feeding into the tube amp is free of DC.
Attachment:
remove-input-capacitor.png

Cheers


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PostPosted: 04 Jun 2010, 11:28 
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Thanks for the input, I'll be trying a 5751 soon. So far I've only tried a couple of different 12AX7's. I need to build up my tube collection so that I can do some listening comparisons.

Also thanks for the clarity on the input capacitors. Exactly what I need. I'll plan on making that change soon as well.


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PostPosted: 04 Jun 2010, 12:32 
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surface tension wrote:
Also thanks for the clarity on the input capacitors. Exactly what I need. I'll plan on making that change soon as well.

This seems strange to me. I'm currently doing research on caps and the properties of dielectric materials. To quote from the Auricap website:

"one would expect polypropylene to have a lower dielectric absorption than polystyrene, which is not the observed result. This can be reasoned by re-examining what is being polarized by the field in each. In the case of the polystyrene, the pi-electrons in the aromatic rings (which have been modeled, in the past, as a "free electron gas") can orient electronically, with less mechanical change in the polymer structure. Hence, it can relax faster. In contrast, the polarization of polypropylene involves more mechanical change of the structure, and hence a slower relaxation rate."

This seems to suggest that polystyrene (Auricaps are polypropylene) would be a superior choice as a dielectric for audio caps. Why does the manufacturer of Auricaps indicate on their website that the the dielectric they use has a higher dielectric absorption and a slower relaxation rate than polystyrene? Why don't they use polystyrene if it's "better"?

Sorry to go OT, but this is confusing. :confused: That's what I get for asking too many questions!

Nice amp BTW.

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PostPosted: 04 Jun 2010, 12:44 
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IMO, polystyrene is the best non-oil filled cap for sound. However, finding them in sizes greater than 0.01 uF is a challenge. Polystyrene capacitors are really well suited for use in a RIAA equalization circuit.

"The Sound of Capacitors" by Steve Bench is a worthwhile read on the subject of capacitors and audio.

Cheers

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