My impression of output tubes. First I have say to with any of the tubes different manufacturers of the same tubes all sound different. An example a SED KT88 sounds different than a Gold Lion re-issue and EAT KT88 is even more different. Just comparing KT88's the SED sound very warm Gold Lion picks up more detail and the EAT has even more detail and a lower noise floor. The only tubes I have from the same manufacturer are the SED's. KT88 very warm with a little more bass than the 6550C's which are very linear. The EL34 has a mid range magic but lack bass and power. My KT90's are Ei KT90 type 2 and is more forward sounding in the mids with good bass but a steely sounding high end and the most power of them all. I haven't heard or tried the EH KT90's so they most likely very different. As far as 6550's I have 3 different quads. SED's and Tung Sol original solid grey and black plates. Even with the Tung Sol's the black and grey plates sound different. My favorite of the 6550's are the black plates with the grey's being very close. The SED's in my opinion are not even close to the Tung Sol's. None are bad mind you.
That's the long version and the short version is you really just need to try them.
Short version. Output KT88s are more detailed and more powerful. EL34s are warmer and more euphoric especially the midrange. My amp is not bass shy with EL34s due to massive iron, I believe. I do not doubt as the previous poster stated different tube manufacturer's output tubes will sound different among the variants. My experience is limited to Ruby KT88s and Sveltalna, JJs and EH EL 34s. Personally I like the EL34s better in general in my amp which will take either and I listen to more jazz than anything else. If I listened to mostly classical or rock I might opt more for the KT88s. On the driver tube front, I believe the manufacturer absolutely has more to do with sound differences than do tube type. Of course, YMMV.
I f have found that the design of the amp has more to do with it than the particular tube. I have two great sounding amps, one has 6sn7/kt-88's and the other has 12at7/6ca7's. Both are equally musically and emotionally involving. This is true only for push pull amps Single ended is another story
I tend to enjoy most of the circuits I've heard that were designed for (and use) EL34's with 6922 or 12AU7 driver boards. However, it really comes down to your tonal preferences, the efficiency of your speakers, and which designs will work best in combination.
I'd suggest you try a preamp that uses 6SN7 tubes. It probably will be a more full sound than with a 12AX7 type -generally-, many exceptions. There are still many available and affordable NOS tubes to have fun "rolling". The 6SN7 are just a great tube "family".
Your speaker's efficiency will dictate your power requirements but KT88's/6550's are powerful enough not to limit you too much, allow for tube switching fun and decent bass. (Quicksilver amps are a good value choice here.) I find EL-34's to tend to sound thin in inexpensive amps. Do try EL-84 based amps if you have more efficient speakers. They'll surprise you!
I prefer the EL-34 over the KT-88. In regard to the 6922 as opposed to the 12AU7, although they are two entirely different animals; I like them both. My impression of the 6922 is of a very neutral, high definition tube that can create a 3D soundstage; while a 12AU7 is warm and beautiful, it sounds best with female jazz vocalists.
EL 34 s are the best and most beautiful sounding. The 12ax7 is a very good sounding tube and lasts longer than any of the others and is the easiest to get either on the NOS market or brand new. My humble opinion after 35 years of listening. Shay
In my push-pull amp, I prefer the EL-34's to the KT-88's. I do not find the EL-34's to be bass shy, either, but I think this depends greatly on the amp and the speakers. They are warmer sounding, and do a better job with both vocal and instrumental timbres. I listen to about 90% classical and 10% jazz.
I,ve owned two tube amps recently from the same manufacturer, Rogue. The Stereo 90 uses KT88, and I loved the authority in the bottom end and the headroom of 90 wpc. I have the Atlas now, and the el34 gives a fantastic 3d sound. A beautiful midrange through my Nola Boxers,. The bass is a bit lacking, but I'm using a stand mounted 2 way speaker.
If you have an amp that will use a variety of these tubes, it won't be a true reflection of each various one, from another number family.
IOW, an amp that can use a KT88/6550 type, and an EL34/6CA7 type, can't reflect the true sonics of each type.
Switching brands of each type can make your amp sound totally different.
The overall design of the amp will have a great influence on the sound also.
In general, the KT88/6550 do have a more detailed and dynamic sound, and can almost sound like a solid state in the right amp.
The EL34 is more euphoric, and seems to focus mostly on the midrange. It can still give deep bass, and extended treble.
With all things being equal, the 6SN7/6SL7 is considered the best by most. The 12AX7/12AU7 a close second best.
These two are similar, but the 12AX7/12AU7 was made smaller, to fit the miniature tube bottle, and socket. The 6SN7/6SL7 is the older, larger octal type.
The 6922/6DJ8, the third choice by a lot of audiophiles who have had a large variety of tube amps over the years.
By the way, the 6DJ8 was originally designed for TV tuners and can work in circuits to produce a ridiculously broad frequency response. An amp that uses the 6DJ8 in conjunction with the KT88 can fool you into thinking you're listening to solid state. I find that the 6DJ8 with the EL34's keeps the warm mids with increased bass and highs when you compare it to the 12AU7.
I think el84s may be the best of the bunch, if you can live with 15-30 watts,
I've had a Rogue Cronus Magnum with the EH KT90 to my ears a transparent tube with good definition top to bottom. For me the best new issue KT88 is the Shuguang Black Treasure - dynamic with punch and excellent across the frequency range. Likewise, if you have an amp that takes 6sn7s and the amp allows for swapping in cv181s might want to try the Black Treasures as well. Like the sound of 12au7 , especially the NOS CIFTE Mazda. Have found them more dynamic that NOS Tungsols & Sylvanias that I have. Only tried El34s once, found them lacking beyond the mid-range, prefer the KT88s.
Facten - I agree about the non-mid range response of the EL34; however, I have an original circuit design ST-70 (upgraded every component/increased capacitance, but didn't alter the circuit) that outperforms all of my 200W/ch SS gear in the bass and highs. In general though your thoughts on the EL34 are correct.
Also, the Shuguang Treasure are great.
IMHO...it all depends on the speakers you're running, amp design, etc. In my particular case, when comparing EL34-based monoblocks to 6550-based stereo amp by the same manufacturer, using very similar design...I preferred the 6550-based amp.
It didn't come down so much to performance at the frequency extremes - it was more that the EL34's midrange occasionally sounded grainy compared to the 6550, and that dynamics seemed compressed with the EL34's. This is all partly due to the fact that both amps use 4 power tubes per channel, so the 6550 just has more power (and my speakers are able to do something with the extra power) - but it also comes down to resolution IMHO. The 6550's just seem to be higher resolving to me.