Quote:
Originally Posted by robert_kc
My Inspire amp originally shipped with:
- KT-88 GENALEX OUTPUT TUBES
- 6CG7 NOS INPUT DRIVER
- 5U4 Shuguang RECTIFIER TUBE
Following is one of Dennis’ emails about tube compatibility in my amp:
GT6L6 GE with the JJ5AR4 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 think it is interesting that while my amp has the 9 pin miniature input tube socket, he is now making then with 8 pin octal socket (e.g., 6SN7). Does anyone know if there is a reason?
I just checked – he has a KT88 amp listed now … |
My KT-88HO shipped with a 6DJ8 type. I purchased it January 20, 2016. I think maybe a couple of months later he switch to an octal driver tube. I asked him about this, and his reply was: "The circuit is very similar except for the bias values on the 6SN7 driver on the front end of the KT88 HO. The 6SN7 has a bit more dynamic drive. You may give the 6CG7 a whirl in your amplifier. Similar to a 6SN7 except for the different bias on the 6SN7 driver."
This is interesting, since your amp came supplied with a 6CG7, which is a 6SN7 in a 9-pin bottle. His implication that they would be biased differently puzzles me.
I do know his KT88 amp is now $45 more expensive, and that may reflect a higher parts cost. It also appears that as his amps (and preamps) are becoming more popular he is going a little more upmarket. IIRC, Inspire started as a kitchen table retirement hobby business, originally revamping Magnavox console amps and later producing his own adaptation of the circuit. His metalwork has improved and he now uses higher end passive components. Both Cary Audio and AES exclusively used octal tubes, at least I don't recall seeing 9-pin tubes except as the output buffer in the PH-1 phono stage. It could be that he's switching based on the higher perceived quality of the octal tubes.
That being said, there appear to be a lot more options for affordable tube-rolling with 6SN7's. 6CG7's weren't made by many manufacturers and the best 6DJ8 types have gotten expensive (and frankly was not even designed as an audio tube to begin with). It seems like it is harder to find a good sounding 6DJ8 than a good sounding 6SN7. 6CG7's are electrically identical to 6SN7's but with the elements closer together in the smaller bottle there are potential compromises.
When I saw Dennis had moved to a 6SN7 right after I bought my amp I was disappointed. My options are to use what I have (and am currently happy with), preferably with a 6CG7, send it back to him to get converted to an octal socket, sell it (I have a number of amps), or buy a socket adapter to use a 6SN7. I may do the last anyway just to see how things sound.