|Size comparison of Tung-Sol KT66s vs a Svetlana 6L6|
I wanted to try some different families of power tubes but hifi amps tend to be less forgiving than guitar amplifiers when it comes to voltage and power-draw changes, so my options were quite limited. Some guitarists replace 6L6s with KT66s, but KT66s pull 1.3 amps of current, which is roughly half-again as much current as a 6L6. However, while researching the KT66 varieties, I found this article which mentioned that Tung-Sol's KT66 happens to pull 0.9A, just like a 6L6!
According to several places online, the Tung-Sol KT66 is so similar to a 6L6 in performance and characteristics as to be "not a real KT66" or "just a 6L6 in a different bottle." However, I found it to sound significantly different from the Svetlana 6L6s I had in my amp before. The KT66s are much brighter, with crisper highs and punchier lows, at the cost of reduced midrange response.
Once I used my preamp's EQ to crank up the mids to be more balanced, I liked the sound signature of the KT66s better than the 6L6s. Comparatively, I would say the difference is akin to toggling an amplifier's "loudness" button on/off. That is, the frequencies that are accentuated by the KT66 sound more lively and the crisp high-end really made my ears perk up. On the other hand, the sub-bass suffered to the extent that I needed to add my long-dormant subwoofer back into the mix to get audible/tactile sub-bass at reasonable volumes.
So, pros and cons, overall. I'm currently digging the change and having fun listening to my favorite albums with the KT66's different coloration but I'm definitely keeping my 6L6s close by for if/when I want to switch back to their warm, balanced tone.
Some other things to keep in mind if you want to try the Tung-Sol KT66 in the place of 6L6s:
1.) Pin 1 of these tubes is tied to the metal base, so be sure to remove any tube clamps that would come in contact with the base to avoid ruining your tubes and/or amp.
2.) These bottles are big af, and so is the base. The glass is about 2" in diameter, so make sure you have enough space between tube sockets to hold them, and if your sockets are recessed, make sure you either have enough clearance for the base or pick up some 8-pin "socket savers" (available for about $10 for 4x from Chinese sellers on eBay), which is what I did.
By the way, you may notice in my pic above that I also picked up some "tube stabilizers" (aka silicone o-rings), which, for the record, made absolutely zero difference in sound quality. They may do something at very high volumes (i.e., when the tubes are getting physically affected by vibrations), but at normal listening volumes, the rings do nothing. Don't bother.