If the amp has KT88s in it, you can directly substitute the 6550s but be careful about using the EL34s. You didn't say what model amp you have but the EL34 tube usually requires a little circuit modification if you want to use it. Follow the manufacturer's instructions very closely on this point.
The KT88 tube has a nice liquid euphonious sound. The 6550 tube may have a bit more solid bass and be a bit less "3D" in my experience, but it is also supposed to be a more rugged tube.
The EL34 is known for a wonderful midrange but tends to be lower power and without the bass and treble extension of it's bigger brothers.
Of the tube amps I've had in the past I think the KT88s were my overall favorites, but we're talking pretty subtle differences.
I would not be able to make that description. However, seeing as how you just bought the amp, why not listen to it as is for a few months?
EL34s are closest to solid state sound of the three tubes...clear and articulate, not as much bloom or romance as the KT88, truer timbre and more accurate tonal balance, a tad more clinical, but without the transistor signature or chip sterility of solid state. I do not agree that the EL34 has diminished bass vs. a KT88. The bass of the EL34 is tighter and more accurate.
Of these two tubes, the KT88 offers the largest soundstage and is perhaps the most colored tube...technicolor...compared to the EL34. The KT88 is more euphonic, more tube like, and a little less accurate.
By the way, I own an Audion Silver Knight PX25 amplifier. I have come to really appreciate the PX25 tube...
After having owned two EL34 push-pull amps, I would hazard a guess that the bass from these tubes has as much to do with other design elements as the tubes themselves. I would suspect the output transformers first of all.
Facten, while a lot of folks will speak out on sonic characteristics, you will find a big variance in sound between the same tubes of different manufacturer ... so much so that you cannot really ascribe a sonic character to any tube. This, coupled with the fact that the design and component quality of an amp has more influence on the amp sound than the tube itself, makes defining the possible characteristic is somewhat academic to say the least.
Listen to the PX25 tube...there is no doubt that this tube has more grunt...it is faster handling transients than a 300b, and not as warm, nor is the bass response as rolled off. If you listen to tubes within a line, you will find that every line imparts some bias...Cary and Audio Note are darker overall than Audion. But there are characteristics of each tube that transcend the lines, and may be magnified in some product lines, and diminished n others, but those characteristics define the sound of the tube. You need to decide what is important to your ear. If you like hard rock music, perhaps the EL34 or PX 25 would be a better choice than a 300B? Vocals and string instrumentals, acoustic guitar, the female voice, perhaps the 300B is the tube for you. Big soundstage, technicolor...maybe KT88s. Just make sure you listen to the tubes in the equipment that you intend to purchase, including the speakers and the front end. And you owe it to yourself to listen to Single Ended amplification before you decide.
Thanks for everyones' comments.
Mcpody - the manufacturer states that you can roll the KT88s EL34s and 6550s; can you explain how the PX25 and 300B fit into that equation - txs for your help
Facten ; are you sure that it says EL34 and not EL84 ? The 84's are usually transferable with 6550's and KT 88's .
What model is the amplifier in question ?
+++ The 84's are usually transferable with 6550's and KT 88's +++
Not sure how a KT88 or 6550 will fit into an EL84's socket or vice versa. The EL84 is a much smaller tube. They [KT88 & 6550] use identical tube sockets to EL34s, 6L6s etc and these tubes are interchangeable in many amps.
KT88, 6559, EL34 are NOT interchangeable with 300Bs
Sorry, but you cannot use teh PX25 or 300Bs in your system. I was just using the 300B and PX25 as an example of tubes that have generally acknowleged sonic signatures, because the previous comments stated that the sonic signatures were totally equipment dependent, and that is not true. With the tubes that you have mentioned, the EL34s will give you the best mids, the KT88s the biggest soundstage and a technicolor presentation, and I am not familiar enough with the 6550s to give you a fair assessment.
Thanks again guys for your feedback and clarifications. Saki70 the amp is Sound Quest SQ88, my first foray into tubes. I have only listened to it for about 10 hours so far and already enjoy it. Though I am keeping my Plinius 8200 and Portal Panche, the more I listen to the SQ it may have me hooked on tube amps going forward. The amp comes with Shuguang (Chinese Military tubes), seem good across the range; but I will be interested in hearing other tubes after the amp break-in awhile.
My bust , sorry for the incorrect info .
Facten ; welcome to the tube side !
I'm late to this thread, but wanted to ask what brand of KT88 people like the most. I'm sure each brand has a different sound.