I've been searching the posts for info on these topics, and found bits and pieces here and there, but I'm still lacking the whole picture. Here's my situation:
I'm building an active DI based on the Altec 1566A. I'm not including the Lo-Z input as I don't believe I'll need this feature (plus the chassis I have is limited in size). Here's a schematic:
My questions are:
-Because there's an output transformer (an Altec 15095), do I need to provide a ground lift? And if I do, is it as simple as breaking the ground connection or shunting it (as Jensen suggests <a href="http://www.jensentransformers.com/as/as066.pdf">here</a>) through a small-value resistor and capacitor?
Or, as one poster suggests elsewhere, should I just make a telescoping ground by terminating the signal cable's shield before it hits the grid of the first preamp tube?
-Speaking of preamp tubes, the Altec uses a pair of 12AX7s, with the capability to bypass the first stage via a "hi/lo" switch. What are the ramifications of substituting a completely different tube for the first gain stage (I'm thinking perhaps a 5879) and dispensing with the switch? Obviously they're very different tubes, but as long as the plate resistances and B+ supply were dealt with....
Many thanks,
Seth
I'm building an active DI based on the Altec 1566A. I'm not including the Lo-Z input as I don't believe I'll need this feature (plus the chassis I have is limited in size). Here's a schematic:

My questions are:
-Because there's an output transformer (an Altec 15095), do I need to provide a ground lift? And if I do, is it as simple as breaking the ground connection or shunting it (as Jensen suggests <a href="http://www.jensentransformers.com/as/as066.pdf">here</a>) through a small-value resistor and capacitor?
Or, as one poster suggests elsewhere, should I just make a telescoping ground by terminating the signal cable's shield before it hits the grid of the first preamp tube?
-Speaking of preamp tubes, the Altec uses a pair of 12AX7s, with the capability to bypass the first stage via a "hi/lo" switch. What are the ramifications of substituting a completely different tube for the first gain stage (I'm thinking perhaps a 5879) and dispensing with the switch? Obviously they're very different tubes, but as long as the plate resistances and B+ supply were dealt with....
Many thanks,
Seth