David McGown wrote:
Grover,
One thing I wanted to mention. The ECC99 is fairly equivalent in gain to the 6SN7, it has a mu of 22 vs 20 for the 6SN7. That is pretty close and should not be a problem. The transconductance is considerably higher, and the current sink in the tail might help achieve the full gain out of the differential amp, so there may be an increase overall. Perhaps that is what you meant. I am picking the ECC99 for drive capability into the KT88 grid. I debated going with 5687, but that would limit tube swapping should I choose to due to the basing.
David
That wasn't what I meant, I was just thinking about the slight increase in gain. But now that you mention it, if the current sink allows for full gain of the ECC99, that really is a big jump--from 12.7 for the 6SN7 drivers to 22. That's going to affect your feedback arrangement significantly, no maybe about it, I think.
I understand that you want to improve the driver capability but I don't think it's necessary and I'm concerned that you may not hear what it is the original front end imparts--an ease and naturalness to the sound that makes it so lovely. Since you already have one channel built, consider trying the original circuit with a 6CG7 in place of the ECC99 just to see what it sounds like.
ETA: Also, if the overall gain is increased that much, your input sensitivity is going to be so high that I'm not sure it would be practical.
Forgive me, I don't know that much about current sources and current sinks so I'm only guessing here.