Old thread but then I'm an old guy.
I've just tried out the KT120 in my Mikael Abdellah Single-Ended (SE) KT88 Tube Amplifier:
http://diyaudioprojects.com/Schematics/ ... lifier.htm400V, GZ34, Angela Universal PT, James 6123 OPT@ 5k. 560 Rk per tube. This PT is underpowered for the KT120, Not enough filament or HT current. Oh dear......And the OPT Max idle is 100Ma where the KT120 wants about 165mA. So my amp is underpowered for the KT120.
Here are my thoughts.....
I was not happy with the sound with EL34 in triode or UL. Too thin to my old ears. Or maybe not enough power? My speakers are 99dB/W/m Goodmans Axiom 201 16 ohm full range in factory cabinets. This may be part of my harshness problem. But there is something entrancing about a full range driver that I like......
The amp sounded smoother and fuller when I doubled the current through the 6N1P.
Then I tried KT88. This was a great improvement for bass and top end smoothness. But still harsh vocals in UL. I prefer both EL34 and KT88 in triode mode in this amp. But still, a disappointing sound for my first SE tube amp
I've just tried some KT120's with no circuit changes. That's a cold bias and high OPT impedance for this tube, I know.
But the results are quite promising IMHO.
At these less than ideal operating points, triode mode sounds quite nice. It feels like there's less power than the KT88's (?) but there appears to be more headroom. I had always assumed that the CD player was overloading the 6N1P but it seems maybe it was the OP tube that was getting overdriven. With the KT120's I can wind the input pot up to full whereas with the KT88's and EL34's I could only get half way on the pot before the distortion got too objectionable. I guess this is because I am significantly under-driving the KT120's.
UL sounds a lot less objectionable with the KT120 than with the KT88 (or EL34). But it's by no means perfect. The top end harshness I have come to associate with UL is still there but not nearly so prominent. The congested midrange is still there too. But the bass improvement is (almost) worth it. I have read that this tube works well in UL and I believe it. This could be a significant advantage with this tube.
My better half likes the increased bass, bless her.
My next move will be to increase the anode current to about 100mA (max idle current for the James 6123).
This will still significantly under-drive the KT120's however.
And I plan to try the 3.5K UL option on the OPT.
I like these tubes. They sound quite promising even running under these less than ideal conditions.