as a generality, kt66/6l6 tubes are basically a weaker (less powerful) el34 -- they will usually sound weaker/slower in the bass, and sometimes a touch more ’beautiful’ in a classic tubey manner in the lower treble, mids and upper bass
but obviously, tube makes within a type, can have sonic differences that may overwhelm that across tube types (a gec kt77 vs a shuguang el34 may would sound more different than a modern russian sovtek kt66 for instance)
I'm not familiar with your amp. I think your question tends to be amp specific. In my Decware ZMA, I preferred the KT66 over the KT77. however, steve Deckert designed the amp around the KT66. I did a long journey of tube rolling and ended up right back where I started. That said, my second favorite in this class was the inexpensive russian 6P3S. Won't cost much to try it.
And of course tube manufacturer plays a big role.
KT66 and KT77 are both great tubes as @jjss49 states above the 66 will sound more classically tubey and a bit warmer but they sound great to my ears. Worth exploring since it's an option for you @hilde45
Outstanding question since I have that question too. Im using the QS MidMono, so should be 20 watts lower than the 60. The 88' are slightly fatiguing....the 77' slightly less bass punchy, so I suppose the 66's would be even less punchy.
Suppose I'll stick with the 77's
They’ll sound different in different circuits. I’m running GL KT66 in my Dyna/VTA 70s, and prefer them over any of the 6CA7s and EL34s I’ve tried. Midrange and lower treble are clear and lush. Bass had more extension with the Nesstone 6CA7, but the KT66 only feed the midbass and tweeter from 80hz+, where they do extremely well.
All things being equal, KT66=6L6 w kinkless filament. KT77=EL34 kinkless filament. That being said, I prefer the Gold Lion KT77 over my Tesla, Svetlana, RFT, VEB and Psvane (the worst) EL34s. KT77 has more of everything of the EL34 sonic signature.
i have Sylvania 6L6 GBs and GCs, and Tung Sol 5881s. Different sonic signature than EL34, less bass but more airy twinkling treble. I don’t have KT66 (yet!) to compare, but going off KT77, if you like the 6L6 sound in your amp, you’ll love the KT66. I’ve had excellent luck with Gold Lion new production from EH.
My tube rolling amp is a Dennis Had Inspire KT88, best $2,000 SEP on the planet, IMO. Very revealing of tube changes.
Thanks for the replies. I realize how much variance can come from amp, manufacturer, etc. and so trying it is the only way.
One thing which patterns these responses that helps me is that the KT66 is definitely going to differ a lot from KT-88's. That's the kind of nudge which helps me move ahead to try it.
@trivema My amp will *not* play a 6L6 so I don't know what it sounds like.
KT = Kinkless Tetrode. So designed to remove the "kink" in the curve of the anode characteristic in older designs.
KT77=EL34 kinkless filament.
@noromance I think Bizarro World has a character named Kinkless Friedman to balance out our own.
I’ve been playing both these tubes this week. I have primalunas, both tubes are from gold lion, new production. In my system the differences are very easy to hear. The 66s have more top end emphasis, and perhaps a bit more transparent, though that could just be the added detail on top. The 77s are warmer, more midrange emphasis. The bass is about the same on both. I could see some people saying the 77s were more natural sounding and the 66s more “HiFi”. Overall I prefer the 66s. I was told by someone at Upscale that the primalunas mate better with the 66s based on the amps’ transformers, but I don’t know any more about that. Having owned the stock El34s, other El34s, and the 77s, I can say that the 66s were definitely a change in sound overall and I was glad I bought them. What do the KT88s sound like? I need to try those!
KT66 will definitely sound different than KT88. KT88 is Europe’s answer to 6550. My Inspire will run both. I love the St Petersburg Svetlana 6550 in my Inspire, also have GL KT88 which is slightly different than the 6550, hard to put my finger (or ear) on difference. Also have Shuggy Nature series black bottle which is a very nice sounding tube, closer to GL, further from 6550. I really like all three.
What’s really fun is swapping the driver tube between 6SL7 (higher gain, more top end) and 6SN7 (lower gain, less top end detail, smoother and touch darker). The Inspire runs both. Great combo is the 6550 or Nature bb w a 1945 JAN RCA grey bottle 6SL7, get warm dark bass and mids, with the 6SL7 providing more detail and sparkle on top.
obsessed and addicted
What do the KT88s sound like?
@jimmy2615 The GL KT-88’s in my system have "slam" and bass response; lower mids have punch. Mids and highs are a bit harsh; this can be tamed a bit with changes in my preamp 6SN7 tube.
Curious how others might answer your question.
Thanks. I thought I’d also add, after noting some of the comments about 6L6s, I have some NOS RCA blackplate 6L6GCs. These sound like JJSS was describing above. The KT66s are a lot more dynamic and not as warm in my system.
@jimmy2615 I have a Primaluna Dialogue Premium integrated and have rolled tubes a bit, including KT88’s and 6550’s. I, always, come back to KT77’s. To me, I find KT88’s (Tung Sol & GL) and 6550’s (EH) have more bass, but loose significant detail. KT77’s have it all. I was not impressed with Siemens el34, either. I would like to try some Amperex el34’s double-D getters, but need another matched pair. Also, I would like to hear how vintage 6L6’s sound in a Primaluna. A friend swears by them in his guitar amp. Though, not the same equipment.
I've been using my Dennis Had SEP tube amp again lately (I switch between it and a Pass XA-25) which can utilize a really wide variety of "KT" type tubes and I really like KT77s (Gold Lion) in the thing. A fave for sure. I discovered a forgotten pair of GL KT66s I bought just before the Russian tube world imploded and recently stuck 'em in the amp...it seems they need a little while to break-in...they're great sounding! Really hard to say if they're better than 77s or to describe the difference, but hey...try 'em! The Gold Lions are great looking tubes by the way.
Concur with wolf_garcia the GL KT77 is a fantastic output tube in a Had Inspire, detail, warmth, linear up and down the frequency range with excellent harmonics (my way of saying sounds great!).
toddnig comments are helpful identifying KT88/6550 differences. More bass, lower mids, loose some sparkle and detail on the top. Still a great sound depending on the music and mood. But the GL KT77 is the overall winner, has it all top to bottom good tight bass, rich warm mids, sparkly highs with detail 20Hz to 20kHz. Like I said earlier, everything the EL34s have but more of it and better.
Now I’m curious what a GL KT66 would sound like…oh jeez, here we go again!
@wolf_garcia Thanks for weighing in. I love the KT77's so much I have two pair. I'm very curious if you hear any real difference with the KT66's because they've gotten kind of expensive and hard to find. The GL's are very very hard to find. JJ is more gettable.
I think the Gold Lion 66s have a little more low end maybe. Although unless I listen to them and quickly take them out and put the 77s in its really hard to tell critical differences, and I generally don’t do that. Suffice to say, after break-in (a day or 2) the 66s really came into their stride and sound great. I can’t say much about the JJs except that a pair of JJ KT77s I bought were just not good at all compared to the Lions. I also have at least 4 or 5 (one died) GL 77s so I’m set with those for years, as well as Tung Sol 7581As, KT120s, etc. I was perusing Viva tubes recently and thought I had to try the GL 66s, and looked in my tube drawer and found a pair I must have bought just before I started using the tubeless Pass XA-25...nice surprise although even at today’s rough pricing for Gold Lions I would have bought ’em. Note my single ended Had amp uses only 2 power tubes (four tubes total including the input 6SN7GTB and a rectifier tube) so tube expense isn’t much of an issue.
@wolf_garcia Thanks for the additional info. I need 4 tubes in my QS monos. I also have the Pass XA25, which is a fine fallback if I have a tube go out.
So you need 8 power tubes...at today's prices that gets an "ouch." Yeah I love the XA-25's utter transparency, but my Had amp just looks sad sitting there and it's easy to swap the wires over to it so there's that, and I'm always amazed and entertained by the differences between these amps. Fun...it's just fun...
Just for some new audio kicks NOS 6L6GC subs can still be found for reasonable cost. You're probably familiar with Brent Jesse?
"Also, I would like to hear how vintage 6L6’s sound in a Primaluna"
toddnlg- if you're referring to the LOW VOLTAGE 6l6G(not GC), been using RCA 6L6G in a PL HP as the preferred compromise for EL34 mids/ KT88 bass.
Talk about going full circle. One of the best sounding amplifiers I have ever owned was the Audio Research D-40 with 6L6s. I recently paired it with an Audio Research REF 6 and it added another dynamic to my Sonus Stradivari. Much more detail. Just could not live with the low power so I have gone back to a REF 160S with the KT150s. I am using a Rogue Atlas Magnum as a test bed since it can accommodate the KT77, KT88, KT90 & KT120 family of tubes for comparative purposes. Something to be said about the older tubes. I am leaning toward the KT88 with good bandwidth and power. Glad I kept my classic collection of tubes from my Dynaco 70 days.
Thanks. Mike Sanders at Quicksilver has told me directly that 6L6 and EL34 cannot be used on my Mono 60 amps.
@hilde45 If you can't use 6L6 or EL34 tubes it's likely you can't use KT66 or 77s either as they are each very similar to a 6L6 and EL34 tube respectively.
@jond I'll double check my email. My memory is likely at fault! Thanks for pushing back on my claim -- if I am wrong it will open up some options.
The M60s were actually designed around the EL34 tube per Mike and will happily use these as well as 6L6s. I ran both in my M60s with grrat success.
Those are different amps!
M60 Mono Amplifier
Found my old emails.
Mike Sanders said about my (later) Mono 60's
"They should be able to use KT66 or 6L6 tubes.
The Sixty watt amps have a plate voltage of 500 volts."
He also said,
"EL34s and 6CA7s should not be used in the Sixty watt amp."
So, that's the word from the maker regarding my amps.
@hilde45 I am still a bit baffled by this he says not to use EL34s but you can use KT66 and 6L6 but also KT88s and KT150s?
Jond, FWIW, it's probably the the plate difference between the two amps which determine which tubes you can use. I don't know, but I'd risk a bet that the amp which can use EL34's has a plate voltage of 450 volts. The 500 volt unit would probably stress the EL 34's and cause them to fail. FWIW KT66's, 6L6gc' s and other 6l6 variants are not all equal, electrically speaking, so I wouldn't recommend that you use them without making sure that your amp would be appropriate for a specific tube.
I think it's a plate voltage thing. I'm not an engineer and just follow instructions!
just remember, flexibility in use in a piece of gear (especially tube gear) usually trades off peak performance optimization
even with true auto bias now offered in some modern tech tube amps, if a designer/engineer truly optimizes his unit for a certain sound and certain performance parameters, you will hear what is intended in an amp that is specific to a particular, specifically chosen power tube (think audio research amps over the years)
power tube type selection, matching to its driver stage, and output transformers is key as these elements together provide a chosen performance envelope into whatever is anticipated as a range of speaker loads
the optimization ’point’ can be loosened into an ’envelope’, which buys you something (the option of running various types of power tubes) but it gives up something too
think of it in a photography analogy -- given a certain aperture and light condition, there is a specific focus setting that gives the sharpest image at a certain object distance... crude auto focus or fixed focus cameras still let you shoot the picture, but what is captured becomes necessarily murky compared to what is optimized
@jjss49: excellent point. The designer/builder of our 75 lb. 100w monoblocks which run 2 x 6BL7 + 4 x KT77 told me (paraphasing) ’everything is designed around optimizing sound from KT77s. The only other output tube that can be correctly biased (they use an individual tube fixed bias system, ie. trim pots on each output tube) is EL34, but it won’t sound as good as with KT77s. No other output tube can be correctly biased.’
Note that the output transformers are rated to 200 watts, so thought I’d try some higher wattage output tubes, but he was right; couldn’t get correct bias with KT88s...also tried EL34s and he was right again; while the tubes biased up just fine, the amps didn’t sound as good. Funny, I would call what was lost "focus".
Super focus with KT77s though!
Thanks to all for weighing in. Good points, and I learned a lot!