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Home » Audio » Thermionic Emissions » Opinions please-New EL34, 6CA7, KT88 and 6550
| Opinions please-New EL34, 6CA7, KT88 and 6550 [message #59313] |
Sat, 30 May 2009 11:34  |
hurdy_gurdyman
Messages: 416
Registered: May 2009
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Illuminati (1st Degree) |
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I'd be interested in hearing opinions from everyone who has used new production EL34, 6CA7, KT88 and 6550 power tubes. What differences there are in sonics, what to avoid and what sounds good. I have listened, with my TAD-60 amp, to EL34's by Sovtek, Electro-Harmonix, JJ and Shuguang. I have been happiest so far with the Shuguang tubes, but am thinking of trying something new. I am concerned with longevity as well as sonics. It never hurts to have an opinion database to study from those who have more experience when it's time to upgrade.
Dave 
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| Re: Opinions please-New EL34, 6CA7, KT88 and 6550 [message #59321 is a reply to message #59313] |
Sun, 31 May 2009 12:24  |
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Wayne Parham
Messages: 18787
Registered: January 2001
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Illuminati (33rd Degree) |
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I have just about all of the 6550 / KT88 tubes available. I also have a handful of EL34's.
For some reason, I find myself drawn to the Светлана (flying C) tubes (both KT88 and 6550), although the original issue Tung Sol 6550 tubes are even better. Those are my top choices. I kind of like the Electro-Harmonics, they sound good and I'm not afraid of running them (i.e. easily replaceable). That's a second choice for me. Same with new issue Tung Sol 6550 tubes. Those would probably be my third choice tubes, and I like having a bunch of them and the EH 6550's on hand for daily use. Sound good, no worries.
I have a pair of RCA 6550 tubes that are absolutely wonderful, but you know, it may be that I know they're supposed to be wonderful. I tend to save them anyway, like precious jewelry. I've been told they're rebranded original issue Tung Sol tubes, purchased by RCA and relabeled.
The last ones on my list are the Svetlana (no flying C) and the JJ KT88's. I'm not as thrilled with those. The Svetlanas are probably OK, but they seem a little grungy to me and I've popped one, not 100 hours out of the box. The JJ's are definitely a grunge tube, novel 'cause the glass is tinted blue but not nearly as good as any of the others.
My selection of EL34's is a little more limited, just blue JJ's, Electro-Harmonics and Светлана. I haven't done as much tube rolling on my amp with those tubes in it, so I can't say which I like better. I will say that each of those tubes are still available, so I'm not as concerned about audio jewelry (irreplaceable tubes). If I find one I like better than the others, I can always buy more for my personal stash.
I gotta put in a plug or two, while I'm here. I've done business with a lot of the internet tube companies and BOIAudioworks is my hands-down favorite. They're a sponsor, but it isn't just that. We solicited them because we liked them, not the other way around.
Aki (at BOI) is always friendly, and honest to a fault. Many (if not most) of the tube vendors out there are not. You may already know this, or maybe not. Ebayers are notorious for claiming any garage find as being NOS, but I've found this is also true of some of the biggest most popular internet tube sellers.
One, in particular, sold me a "NOS" tube that was obviously used hard for many years. You know, I expect a tattered box, or a plain white box even. But a NOS tube should not have scratches on its base, insertion wear on the pins and weak conductivity. That doesn't even pass as a good used tube.
Who was that? I'll give you a hint: They advertise aggressively on various forums, and they sell at prices that are way above the norm.
So deal with Aki, if you can. If the tube is still current production, definitely get it from BOIAudioworks.
If you need an old radio tube, something used in a television or old tube radio, I suggest Antique Audio Supply. They sell used tubes, make no pretensions and have reasonable prices.
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| KT88 and 6550 - Operational shifts [message #60429 is a reply to message #60376] |
Sun, 19 July 2009 14:32 |
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Wayne Parham
Messages: 18787
Registered: January 2001
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Illuminati (33rd Degree) |
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I run KT88's or 6550's in an Audio Note amplifier about 4 to 6 hours a day, on average. It sees a lot of service. In general, the Audio Note Kit 2 is a reliable amplifier but if an output tube goes bad, it will probably smoke the cathode resistors and may kill the output transformer or power transformer as well.
What I've found after many years of service is the most common failure mode is an gradual increase in quiescent current, rising until it becomes excessive. It is like the control grid and screen grid become shorted, although that's not what's really happening (I've measured). I think something shifts internally to make the screen grid become more influential on cathode-to-anode current flow, especially after a few minutes of warm-up. Whatever the cause, the symptoms are an increase in quiescent current, just like a bias shift.
I spoke with Albert Porter (writer for Positive-Feedback Online) at LSAF 2009 and he said his amplifier uses a bank of sixteen 6550 tubes. With that many in service, it occurred to me that he has even greater visibility of statistical failure modes than I do. Sure enough, he said that was a fairly common failure mode for him too. When the grids "short", he has plenty of stock to swap from.
I keep a large stock of tubes too, but the problem is the tube still works, and the amplifier sounds good so it hasn't shifted so far on its load line to become non-linear. You can't tell the tube is approaching this condition unless you open it up and put a meter on it. Usually my first indication is a burning smell (no kidding) from the cathode resistors getting hot.
To me, this screams for a simple solution. I think 6550 and KT88 tubes are best used with a fuse in series with the cathode or the plate. More in the post below:
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| Re: Opinions please-New EL34, 6CA7, KT88 and 6550 [message #63117 is a reply to message #59338] |
Wed, 09 June 2010 21:25 |
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gofar99
Messages: 1949
Registered: May 2010
Location: Southern Arizona
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Illuminati (5th Degree) |
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Hi Most interesting, The more I work with tubes the more I find that they are not predictable in the usual sense. In the amps I have designed (Oddwatt series, info mostly on another forum) I have found that unlike you the JJ tubes work well particularly the Blue Glass EL34 and KT88s. I have tried several brands and sizes as the amps can be switched from low power (EL34, KT77, 6L6GC etc) and higher power (KT88, 6550, KT90 and just this week EL156). There seem to be essentially two types of output based on the type and brand, either really great bass and very crisp mids and highs or smooth and well rounded, what I usually call a polite sound. The JJs fall into the latter area, many others are in the first group. It may well be the design of the amps that makes this so. Class A push-pull U/L with a CCS forcing the amp to stay in class A mode. Just a thought. It may also relate to my main speakers which are Martin Logan Vistas (ESLs). They have characteristics that will influence the sound of the amps as well. One tube that did not sound good at all was the New Sovtek 6L6WXT+. I would describe the sound as raw, like scratching on a chalk board.
Good Listening
Bruce
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