no idea who is selling them, but in general, e-bay is a crap shoot for tubes.
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On every tube amp I've ever had the input tubes made more difference in tone than the power tubes.
If that amp is tube rectified you can also roll that for different presentation. A rectifier thats higher voltage will bias the amp towards a more agressive sound, lower voltage more relaxed.
You can run 6550C's, 6L6, or EL34 as power tubes instead of KT88's in that amp.
After you change tubes you should let them warm up, and reset the bias(140ma) as per the manual. When I swap tubes I first set the bias low with the old tubes, swap tubes, then once warm bring the bias up to spec. The eliminates the chance of being over bias on different type tubes.
For whatever power tubes you decide on: Upscale Audio's quality and customer service are hard to beat. Check out Kevin's procedures and offerings: (http://www.upscaleaudio.com/view_category.asp?cat=85) Both power and driver tubes have a major impact on your presentation. Try some NOS 6SN7GT/VT-231 (Sylvania or Ken-Rad)bottom getters from the 40's for a real treat: (http://shop.ebay.com/?_from=R40&_trksid=m38&_nkw=vt-231&_sacat=See-All-Categories) Neither are expensive right now, and both provide an expansive sound stage, excellent imaging, accurate timbre and frequency response. Here are some comments on what to expect with some of the different brands available: (http://home.att.net/~chimeraone/6sn7sound.html) I've purchased quite a few tubes on the "Bay" over the past few years, and only had one bad experience.
Tube rolling priorities -
Power tubes make the bigger difference which can be good if you are looking for major tonal changes.
Small tubes are more for fine tuning than major changes BUT, proper selection can make or break the sound of the power tube.
So there is not easy answer - as with everything else its about synergy, including synergy with your speakers and other electronics.
SED KT88's have a very different sound than a lot of KT88's in that they have a warm upper bass and extended highs. They are not a sonically linear tube. Whether you like them or not depends on your amp, which I have never heard. FWIW, I use them in two amps (a Cayin and Primaluna) and they sound fine, but in a Cary CAD50M they are far too warm/dark on the bottom and a bit peaky in the upper mids/highs. I use SED 6550's in this amp.
So I agree with Rodman 5 9's, call Keven Deal - he sells both Cary products and SED's, he will be able help you match the tubes to you sonic preference and can even help you, if you need it later, select the 6sn7's to match the power tubes AND your tastes.
Thanks. I noticed in the manual it says that the following tubes can be substituted for the KT88s: 6L6, 6CA7, KT-66, 6550C, KT-90, KT-99 or EL-34.
Just reading another thread and the poster likes the Mullard EL-34s much better than the KT88s. I notice the 34s are less expensive. What, in general, is the difference between tubes that can be substituted for each other?
The big difference between the tubes you mention is "power dissipation", or the maximum output that the tube is designed to produce at a certain plate voltage. ie: The average KT-88 will be designed to output about 35W, while the EL-34 is more between 17 and 20W. The Telefunken EL-34 was designed to output about 25W, which is high for it's type. The KT-90 and EL-34 tubes are at the opposite ends of the power output spectrum in your list, with the others falling between. There are also differences between tubes within one type, ie: the original 6L6 was a 19w tube while the 6L6GC is usually rated at 30w. A lot depends on the plate voltage the tube is manufactured to handle, and whether it's a tetrode or pentode being run as such or as a triode. Even a pair of EL-34s can output 90w with 800V on their plates. The differences in presentation/sound between all those tubes would be ridiculous to even approach, given the multitude of variables. ie: Manufacturers, amp designs, variations in bias/plate voltage, again- are they being run in triode mode, etc. There are also a plethora of personal tastes involved in the analysis of tube "sound". Again- Kevin has a vast amount of experience with various equipment and tube designs. Tell him your tastes and he can direct you to the tubes that can best satisfy them.
That's a good link(thanks)!! His observations on chinese business ethics(there's a stretch) and manufacturing, mirror mine. BTW: If you like transparency, nice freq. extension in both directions, and liquid mids- The Winged 'C' 6550C's(latest iteration/St Petersburg plant) that Kevin sells are working very nicely in my Cary monoblocks. I was a hardcore NOS tube addict, until the cost of matched quads/octets of anything decent in power pentodes became unrealistic(Dom Perignon taste/grain alcohol budget). Uncle Kev came through, in style. You could say I'm a happy customer. Does it show? (http://www.upscaleaudio.com/SED-quotWinged-Cquot-6550C_p_16.html)