KT-77 vs. KT-66

As I've had my QS Mono 60s, I've tried a variety of tubes. I have sold off my KT 150s, and use KT-88s from time to time, but what I really love are KT77's.

I'm thinking of getting some KT66s, thinking that lower powered tubes are better (to my ears) matches for my system. I just don't know what the KT-66's sound like. 

Question: If you have compared KT77 and KT66 tubes, what would you say the sonic, audible characters of each are? Is there a difference you can hear? What does the KT66 sound like -- to you?


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!