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Old 8th April 2011, 03:14 AM   #21
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The switch toggles need clearance when installing the main circuit board.
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Old 8th April 2011, 03:23 AM   #22
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The output transformer wires are quite a bit longer than needed, so I trimmed them to fit, consistent with good wiring practice. Beginners might want to skip this part, since if you trim too short you have to solder a piece of wire back on.

The transformer wires are also very stiff and need a firm hand when twisting.
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Old 8th April 2011, 03:36 AM   #23
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The amp running Tung Sol 6L6G tubes. These have 6L6GC innards in a big ST bottle, which is what the original 6L6G came in.

I know Victor likes NOS RCA 6L6GC's for this amp, but I thought NOS prices were ridiculous 10 years ago, so you can imagine what I think now. Vendors test NOS tubes these days, but it can still be a crapshoot. I like to fool around with the best of current production instead. More on tube rolling at a later date.
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Old 8th April 2011, 04:54 AM   #24
vkung is offline vkung  Canada
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This is another sleeper tube. A high-voltage beam tetrode with an ultra-cool plate cap. Can be used in circuits designed for 6L6GC?

This photo was from one of my customers. He use RCA 807 to replace 6L6Gc.
Octal(8-pin) to 5 pin adaptor is used.

I bought a pair of RCA 807($35.00) and an adaptor from Ebay. I will let you know the result soon.

Last edited by vkung; 14th April 2011 at 01:23 PM.
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Old 8th April 2011, 07:21 PM   #25
w5jag is offline w5jag  United States
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The screen on the 807 is rated for 300 volts, and the TU-879S has more than that on the screens in its stock configuration. Most 807's will probably be OK with the overvoltage, but there may be some at the wrong end of the bell curve that are not.

I didn't hear much effect from tube rolling in my amp. I subbed in a real Mullard 12AX7 for the stock Chinese input tube and really couldn't hear any difference. There is a NFB loop, so I really didn't expect to hear much difference. I could tell the difference between 6L6's and EL34's, but they are very different tubes, construction wise.

I wound up converting my TU-879S to use smaller tubes like the 6V6 and 6F6 - the B+ voltage is modest in this amp, so it is an easy conversion.

To convert the amp to the little tubes, I replaced the cathode resistor with a 750 ohm 5 watt resistor, and removed the jumpers feeding the screen grids on the output tubes and replaced each with a 27K 1 watt resistor. I use the 4 ohm tap on the OPT's to reflect a 6K load back to the tubes with my 8 ohm speakers. After these changes, and with 5992 tubes in the amp, I measured the B+ voltage at 337, the screen voltage at 308, and the cathode voltage at 21 volts. This equals 9 watts power input, probably 4 out. This is plenty with high sensitivity speakers. I haven't made any changes to the NFB loop, yet.

I like the TU-879S and the Elekits in general. My TU-879S is at my second house so I don't get to listen to it much, unfortunately. I liked the TU-879S enough that I bought the Elekit preamp, but I haven't had a chance to build it yet, and I am thinking hard about getting the Elekit headphone amp, and maybe another TU-879S for a new office I am building. It's a classy looking amp.

Thanks to Victor for making these nice kits available here in North America.

Win W5JAG

Last edited by w5jag; 8th April 2011 at 07:44 PM. Reason: spelling
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Old 8th April 2011, 07:38 PM   #26
w5jag is offline w5jag  United States
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Quote:
Originally Posted by halitun View Post
It's my first SET;
Wired as per the schematic, it is actually a single ended pentode amplifier.

It is possible to wire it as an SET. For the type of listening I do, I have grown to prefer single ended pentode.

Win W5JAG
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Old 10th April 2011, 04:09 AM   #27
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Default Listening Impressions

Listening Impressions
The amplifier I was using most of the time before the TU-879s was a Class D unit with Tripath chips, from hifimediy. This pulse-width-modulation amplifier sounds very detailed and smooth. What I noticed right away when first firing up the 879 was that the sense of musical flow was completely different (some folks use the unfortunate acronym “PRAT”). I much prefer the flow of the tube amp. Art Dudley of Stereophile also talks about the presence of sound in the room and the ability to emphasize solo instruments as unique characteristics of single-ended amplifiers. The 879 certainly has both of those characteristics.

My vinyl days, and collection, are long gone. I listen to CDs and SACDs, mostly classical, mostly relatively new, all well recorded (quite a few from Harmonia Mundi). For jazz I seem to have gravitated to the ECM sound. Popular music is all over the place - I have been playing folk and rock bass guitar since 1968 and like lots of artists.

I am using this amp with a NAD T535 multi-disc player that has a very nice Cirrus Logic 4382 DAC with OPA 2134 chips in the output filters. This was a bit of an accidental find, bought on closeout for DVD use, but it immediately put my modded Music Hall 5.2 CD player on the shelf. Speakers are Focal 806s, at 90dB/2.83v, but only moderately “tube friendly” with some big impedance variations. I’m using the 8 ohm taps on the TU-879s.

12AX7 tubes
: JJ ECC803s gold pin, Tung Sol ECC803s gold pin, Mullard 12AX7/ECC83, all new production. Used Philips 12AX7/ECC83 of uncertain date, with long plates. The JJ has short plates with a shield between and over them; the Mullard plates are long, the same length as the Philips plates; and the Tung Sol is somewhere between in plate length.

Power tubes
: SED KT88, Tung Sol 6L6G, Tung Sol 6550, all new production. Pre-2000 SED EL34.

Why these tubes? The last time I played with tubes was about 10 years ago, and I had high hopes for Svetlana since they were into audio and not primarily guitar amps, but alas ‘twas not to be. JJ I found to be reliable if not especially exciting. Now it seems to me that New Sensor is the one to root for, and Tung Sol seems to have their most audio-oriented variations, plus maybe Mullard.

JJECC803s: This tube has a nice even frequency response, but the sound field is not out in the room at all. The details of initial attack on percussive sounds and the ambiance surrounding sustained musical flow that are found with some other tubes are just not there. Said another way, there is little tube nuance, or bloom, or whatever it might be called. I am glad that JJ is surviving in the business, and not surprised that folks like this tube as V1 in guitar amps, but it’s not my pick for this audio amp.

Tung Sol ECC803S: I keep coming back to this 12AX7, it has the best mix of features in my system. It’s a little laid back in the treble, and slightly lacking in punch compared to the long-plate tubes. The nuance and bloom aspect is definitely the best of this bunch. Focal tweeters can be quite forward, and the combination of this tube with the Focal speakers works very well.

Mullard 12AX7: This sounds remarkably like the Philips long-plate right out of the box. The sound field is nicely out into the room, but the overall sound is more businesslike and straightforward than the Tung Sol. It’s also somewhat clearer sounding, and a bit punchier in the bass. I really want to like this tube better than I do, if that makes any sense, but in my system it keeps having shrill moments. As an aside, I wonder what the difference is between this tube and the long-plate Sovtek 12AX7LPS.

Philips 12AX7: Sounds just like the Mullard. I kept trying it “one more time”, but then taking it out after a while as being a bit fatiguing to listen to.

SED KT88: This is an expensive and well-regarded tube, and was very clear-sounding, as well as punchy and quick in the bass. In this amp it was rather workmanlike, not offering anything special. Looking at the specs I think it would be happier in an amp with a considerably higher plate voltage and a stronger driver tube than a 12AX7.

Tung Sol 6L6G: These are fun tubes, with the innards way up high in the ST glass envelope. They were the most out-in-the room of this bunch of power tubes, with the most nuance, bloom, and all that. They also could be a bit wooly in the bass – the bass was deep enough, but sometimes blooming too much into a sort of ongoing drone. These would be fine in this amp, but personally I liked the 6550 better.

Tung Sol 6550: KT88s and 6550s are pretty similar, but these sounded closer to the Tung Sol 6L6Gs than the SED KT88s in this amp. They combined the best aspects of both tubes: the overall sound was along the lines of the 6L6Gs, but not as far out there in the “romantic” direction, while the bass was almost as punchy as the KT88s.

SED EL34: I love EL34s and have been playing with them in push-pull amps for many years. This set, from the Svetlana glory days in the late 1990s, sounded fine in this amp, with a well-balanced presentation. If they were the only option, it would be a good option. It just happens that I found the Tung Sol 6L6Gs and 6550s to sound even better.

Conclusion
: Everything is still burning in at this point, with maybe 50 hours of total listening, but the differences among the tubes are pretty consistent. I made comparisons several times before coming to the conclusion that among these tubes, the Tung Sol ECC803s and 6550 combination is the most pleasing to me.
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Old 11th April 2011, 08:33 PM   #28
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Default Elekit 879's

I ordered a 879's kit from Victor, at first I am ambivalent to order the 879's because I don't even know how to solder and I am the kind of plug and play guy. But Victor sent me all the necessary documents for the build and it helped me. I went to you tube to learn how to solder. April 7 the kit arrived, It took me 8 hours to build it, check then double check, the moment of truth, I turn it on. No smoke, or burning smell, success!. It sounds good.... and I could not believe that I build this amp... Thanks Victor.
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Old 12th April 2011, 03:48 AM   #29
vkung is offline vkung  Canada
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For $75.00. It is worth a trial to replace 6L6GC. It is quite good for my ears.
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Old 24th April 2011, 10:01 PM   #30
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Default Minor mod to help cooling

I am still listening and tube rolling. I think everything is still breaking in - the sound keeps getting better, especially the small details - although the pace of change has slowed down.

I have re-tried almost all the combinations mentioned in my post above. I still prefer the Tung Sol new-manufacture gold-pin 12AX7 and 6550. The long-plate 12AX7s still are a little bright; the 6L6G is still somewhat unpredictable in the bass; and the KT88 would be my second choice to the 6550.

With the 6550s in the amp, and even more with the KT88s, the outside surfaces near the output tubes get very hot - too hot to keep my hand on for very long. I noticed that the 120 volt transformer just barely fits into the case, since it seems to be a little larger than the original 100 volt one. It blocks off about half of the vent on the bottom.

I am sure the surface heating is from radiant heat from the tubes, but just to let a little more air flow through the inside of the amp, I found some small washers and put them between the amp case and bottom cover. This gives a narrow slot, maybe 2 or 3 mm wide, all around the bottom to let some more air in so convection can help keep the components cool.
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