Thus far, when playing classical music I’ve found that my Inspire KT88 stereo amp performs best with my Klipsch RF-7 speakers (sensitivity 101dB). (Speakers in my other systems: Klipsch Palladium P-37F (sensitivity 96dB), Snell Type CV (sensitivity 90dB), and JBL L880 (sensitivity 91dB)). Most of my testing has been done by connecting the Firebottle directly to the variable output of my Oppo BDP-105 universal player (i.e., no pre-amp), and listening to high-res (e.g., 24 bit/192kHz) FLAC recordings. I found that the Firebottle has ample gain for pop music, but in some cases (depending of which tubes are installed), the volume level for some classical recordings was not adequate. (In those cases a pre-amp could solve the gain problem.) My Inspire “Fire Bottle” SE Stereo Tube Amplifier HO, built in Oct 2015, was delivered as follows:
INSPIRE TUBE AMPLIFIER by DENNIS HAD FEATURING THE KT-88/6550 BEAM POWER VACUUM TUBE AMPLIFIER IN A SINGLE-ENDED CLASS A SET TRIODE DESIGN- CLASS A, 8 WATT, ZERO-FEEDBACK TRIODE
- KT-88 GENALEX OUTPUT TUBES
- 6CG7 NOS INPUT DRIVER
- 5U4 Shuguang RECTIFIER TUBE
- REGULATED DC, B+ VOLTAGE ON DRIVER TUBE
- REGULATED DC POSITIVE CATHODE BIAS ON DRIVER TUBE
- CUSTOM DESIGNED AIR-GAP 15 WATT OUTPUT TRANSFORMERS TO MATCH KT-88 TUBES IN THE TRIODE MODE
I found that substituting an Amperex 6DJ8 for the 6CG7 results in an increase in the output volume, without compromising the sound quality. However, some classical recordings still weren’t loud enough. Thus far I’ve experienced the best results with the Klipsch RF-7 using the Amperex 6DJ8, plus replacing the output tubes with Groove Tube GT6L6 GE (Gold Series, performance rating 5), and the rectifier with a JJ GZ34/5AR4. This combination provides adequate gain for all recordings when fed directly by my Oppo BDP-105, and sounds great, and has ample bass. (The organ pedal notes in the 4th movement of Saint-Saens Symphony 3 by the Kansas City Symphony (24bit/172kHz FLAC by Reference Recordings) were impressive.) FYI, Dennis wrote me as follows: “GT6L6 GE with the JJ 5AR4 along with the 6DJ8 is a great combo. If you were to go to a vintage 6L6G coke bottle shape tube you would want to use a 5Y3 rectifier tube. Other 6L6 vintage NOS best suited with a 5U4 rectifier. The 5AR4 is AOK with any of the larger tubes like the KT88, 6550, KT90, KT120 and KT150.” I found that the KT88s also sound excellent with the Klipsch RF-7 when using the 5AR4 and 6DJ8. My Snell Type CV also sounded excellent for popular music with the Inspire equipped with KT88s, 5AR4, and 6DJ8, but would not play quite loud enough for most high-res classical recordings. Other tubes that I tried but didn’t care for:
- I found that the 6N1P increased the sound level, but the sound quality lacked bass.
- EL34 output tubes provided adequate volume, but had diminished dynamic range and deep bass.
Bottom line: based on my testing thus far, my choice is currently: Amperex 6DJ8, GZ34/5AR4 (JJ sounded better than Mullard), and either Groove Tube GT6L6 GE (Gold Series) or Gold Lion KT88 output tubes. I’m waiting to hear from Dennis regarding whether or not KT66 outputs are supported. Does anyone have experience with KT66s in this amp?
