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KT88 vs. 6550 high current


 
10/8/2004 5:28 AM
MitchK
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KT88 vs. 6550 high current
Hey I just have some questions if anyone cares to answer,  
 
First I've been looking and still can't really find a difference between kt88's and 6550's, is there a technical difference between these tubes? there has to be, what is it? All I've seen so far is that the 6550's are a bit more "musical" and might not be the best for a high gain guitar amp and might be a bit more suitable for a hifi stereo amp.  
 
I'm putting in a quad of KT88's into my 5150 and I was going to go with a set of JJ's (any other comments on tube selection?) and then happened to find these http://www.thetubestore.com/bar.html at first I just liked the price but at closer inspection they look like they would be good for what I want to do. The thing is that I have no idea if I'm going to be able to run them or what modifications I'll need to do in order to. I like a challenge though. What I was wondering is if anyone knows where I can find data sheets, that type of thing on these tubes?
 
10/8/2004 2:59 PM
Brutus
KT-88's clearly have a higher plate disipation of several watts. If you need the exact number I'll dig it out for you. Also, a 6550 is rated at a max plate voltage of 600v while a KT-88 is rated at a max B+ of 800v. These stats are for the Original KT-88 made by MO Valve Company of England. I have no idea what passes for a KT-88 these days from the clone manufacturers.  
 
In general, you can use a KT-88 in place of a 6550 but you cannot always use a 6550 in place of a KT-88 because of the voltage handling difference. Under 600v they bias slightly differently but still can often be plug compatable. I use KT-88's in my McIntosh MC-60 hifi amp even though it was designed for 6550's, for example.  
 
Also, there is a difference in sound between the two tubes but not greatly. For example, I've repeatedly substituted the KT-88 for the 6550 in my Mac Amps and vice-versa and can only hear the difference on a good day. I think part of this can be explained by the heavy use of negative feedback within the Mac which would eliminate some of the perceived difference. I suspect that in a guitar amp the differences may be more pronounced.  
 
 
Brutus
 
 
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10/9/2004 5:15 AM
Greg Simon
The REAL KT88's had a plate dissipation of 42 watts, wheras the 6550 had a plate dissipation of 35 watts. There was also the max plate voltage difference as Brutus mentioned between the two, so a KT88 is better equipped to handle high plate voltages, and can bias a little hotter.  
I can't really say much about using either in a guitar amp as I prefer other tubes for guitar, but for bass, the 6550 and KT88 are very nice indeed. I just put some blue glass JJ KT88's in a Sunn 200S and it makes more power than with GE6550A's, and sounds better too with better frequency emphasis all around. The GE's did the mids nice. I've used the JJ KT88's before in the past on other amps and they are well made and reliable too, so if you decide to use them, they would be a good choice.  
 
Greg
 
10/9/2004 5:56 AM
Psycho Bass Guy
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You've just asked a pretty loaded question since 6550's and KT88's can be really different or exactly the same, depending on who makes them and what time of day you ask them. For example, the EH 6550 AND KT88 are the SAME tube, but Sovtek also makes two OTHER 6550's, The 6550WD with a plastic base and 6550WE with a metal base, AND a KT88 that are very different. Now, with the purchase of the Tungsol trademark, the former Sovtek KT88 is now called the Tungsol 6550. Fun huh?  
 
The GOOD news is that ALL the Sovtek tubes meet or exceed the GE6550A standard, which upgraded the original Tungsol 6550 to equal the ratings of the KT88: 42 watts plate, 3 watts screen, 800v B+ and 600v grid. I've run a sextet of Sovtek KT88's in my Trace Elliot VR400 ultralinear at 728 volts for over 400 CLEAN watts and they've lasted five years and are still going strong. I had another set in there for two years prior, but lost one to a short when my amp fell off its cabinet.  
 
Svetlana/SED also makes a 6550 called the 6550C which everyone else seems ga-ga over, but I don't like it as ALL. Svet/SED tubes have SERIOUS mechanical stability problems and get VERY microphonic after about six months of use. They also don't like as high operating voltages as a 6550A and need to be biased VERY conservatively to keep them from overdisappation. They'd be OK in a guitar amp, though, less vibration and MUCH lower voltage.  
 
JJ makes an EXCELLENT KT88. It's pricey compared to every other modern offering, but it's well worth it. It is odd among KT88's as having a prominent upper midrange. Most modern KT88's have a "scooped" sound. 6550's tend to emphasize the upper midrange.  
 
China has been making great KT88's and shitty beyond belief 6550's for years and now makes a 6550A on the same machines that GE made them on. They're essententially identical to a real GE 6550A. Ruby sells them. China also produces a KT100, basically a selected KT88, no matter how they advertise it. I have four Chinese KT100's and they're identical to all my other Chinese KT88's, but do bias hotter. Believe it or not, they're all great tubes.  
 
ALL of the above tubes are BEAM tetrodes and have a beam former instead of another control grid for true pentode operation like an EL34, however, both EI and Sovtek make a tube called the KT90 (you'll also sometimes see them called KT99's) which was derived from the 6KG6 TV sweep tube and is a true pentode. They take HIDEOUS amounts of voltage and can put out more power than a KT88. They're not as bass heavy as a KT88, but have more low-end than EL34's.
 
10/9/2004 9:26 PM
MitchK
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Hmm well I mean what I'm going for is the mid-scooped sound, I want a somewhat sterile sound. I'd have no doubt that I'd be happy with the JJ kt-88's although those chinese kt100s sound interesting. Its just that these high currrent 6550C's by SED are about half the price which means I could afford to get some new preamp tubes aswell. I mean all in all I have a peavy 5150 which is biased really cold and when I change that around I'll probably like the sound of the 6l6gc's a lot more, but basically I'm doing the mod for the experience of doing it, but also I would like a bit of a different sound, I'd like to stand out a little (I realize that seems like a contridiction compared to what I said about wanting a sterile sound). I just basically want to know if there is going to be a huge difference between the JJ KT88's and the SED 6550C high current tubes, and where I can find datasheets to compare the regular SED 6550C and the high current one and even ones for the RUBY 6l6gc's (what I have now) and the JJ KT88's. I need to determine if I can even actually run these things and then decide how much I'm willing to spend on buying better tubes.
 
10/9/2004 11:25 PM
Greg Simon
Psycho gave some great info about the tubes available. I'm not sure where you can find datasheets, although I think there was some available for the JJ's. You can't go wrong with the JJ KT88's, as they are very well made and will last a long time and sound great. I don't mind the Svet/SED 6550C's, and have six of them in my Ampeg SVT. I liked the sound of GE 6550A's in there better, but they SED's are MUCH cheaper and sound fine. They hold up well too. I haven't had any issues with them at all and have had them in the amp since 1999 with no problems. It gets regular use. I would say if price isn't an object, then get the JJ KT88's. If price is a sticking factor, then go for the SED 6550C's or one of the other offerings.  
Greg
 
10/13/2004 4:43 PM
CZ V-KNIGHT
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Beware Mitch: Let me tell U my story:  
I retubed my SLO clone with Tung Sol 6550 (sovtek 5881WXT previously). My amp has 480v B+. With the 5881 the bias was about 50mA. When I plugged the 6550, it jumped to 120mA!!!!. Somehow, these tubes drains more bias current. So you may have to check this out with other brand/type.
 
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