Tube suggestions - KT88/6550

bwright -- Thu, 03/18/2010 - 21:20

I just bought a new Cayin 688R on sale, a KT-88 tube integrated amp. I'm wondering if anyone has had a chance to compare and contrast the sound quality of the following power tubes (it will use two 6N6s and four 12AX7s in the preamp section):
a) GE NOS 6550
b) EAT KT-88
c) Grant Fidelity Sh. Treasure KT-88
In terms of modern tubes, I'm not much of a fan of the Winged C 6550 sound (great detail but a bit on the dry side), but don't enjoy the too-warm effect either (Golden Lions, etc.). I'd like to find something right down the middle - accurate with a reasonable degree of tube warmth and dimension.
If anyone has a suggestion I'd greatly appreciate it. This is the amp:
http://store.acousticsounds.com/index.cfm?get=detail&Title_ID=35545

Richard Warren (not verified) -- Fri, 05/07/2010 - 19:24

Hey there,
The E.A.T. KT88's are terribly expensive, but have the most robust, "big" sound to them of any KT88 that I have ever heard in any brand, model, and circuit configuration so far, and by a big margin that is anything but a subtle difference. They can make an amp sound so much more powerful and in charge that it is amazing.
KT88's in general will draw less plate current then 6550's in general. If you can get ones that draw around 40mA or so, that would be a good balance of warmth with leanness for a clear yet full bodied sound. Into the 50's, 60's, and 70's will get you a LOT of heat in your amp's chassis and sockets, risk damage, and seriously muddy up / muffle the sound. I know: I had 6550's at 65mA each in my old integrated and they did all that and then some. I also worked at Cary Audio in 2004 and did testing, QC, and so on on 17 of their models and learned a good amount about the ratings and values of plate current on sound.
Perhaps call The Tube Store or The Tube Depot or go to their websites and ask them what they can tell you from their experience. They really know their tubes and aspects of them. Personally would suggest EH KT88's of around 40mA each in a matched pair/quad, etc. if possible. I used JJ's at about 35mA for a more lean and incisive sound and for me, they were great. EH is general has a meatier sound and go for the plate current around that amount. Cary's new CAD120S KT88 amplifier uses Gold Lion KT88's of about 40mA each and Dennis LOVES "that Cary sound" of overly warm, lush, romantic, golden glow of tubes sound that I cannot stand. I like AR and Atma-Sphere/OTL.
Your amp is also manually biased, so you can adjust the bias yourself. That can give you some room to tweak it a bit within safe parameters. Call Steve Leung of Cayin Audio (the US importer) at his place in Hazlet, NJ and ask him directly. With you being a customer of his (indirectly through Acoustic Sounds), he will most likely treat you like gold. He is also a tube techy, so he knows what his amps can handle and what will not void the warranty by playing with it / too much.
You could also change out the coupling cap's to brands like Solen ("Fast Caps") or others with a more lean, clear sound. That may void the warranty though if not done by an authorized dealer or repair tech.

bwright -- Sat, 05/15/2010 - 16:56

Hey Richard,
Many thanks for the detailed and comprehensive analysis! Exactly the reason why I post here - I always get a great response.
I think I'll take the plunge on the EAT tubes and see how they work. I'll contact my audio dealer to acquire the tubes, and have them set it up properly.
Again, thanks for the sage advice,
Bill