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slor 12th April 2008 03:20 AM

Tube DI Help, Please: Substitutions
I posted much of this ramble on the "Musical Instruments" forum, but I'm looking for some tube-specific help here, hope that's not breaking too many protocols....

I'm building a tube-powered DI box, mainly for recording bass. It's based on the Altec 1566A, though 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 through a small-value resistor and capacitor?

-Speaking of preamp tubes, I'm thinking that I won't need as much gain as the Altec provides. The original 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 pentode) 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,

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