V76 DI - what is the right way?

[Ilya]

What is the best way to make a DI input on the V76 preamp aside from a dedicated FET buffer?

The problem here is the fact that there's no ground at the input tx secondary. Will it be ok to just ground the lower leg and feed the signal to the tube grid as crudely shown on the pic? Or are there any more clever ways of doing it?

 

[dmp]

Use a blocking cap and a switch, then the transformer can still be elevated. I did this in my build and it worked great. It sounds way better than a fet di.

 

[Ilya]

Like this?

 

[dmp]

Yes, but you also need a resistor to maintain the grid bias.

 

[dmp]

Grid bias is set through the r57/r58 voltage divider, fed through the 100k. So connect the DI cap to the 100k/c12 with a large value resistor, ~2meg

 

[Ilya]

Thanks, I was refreshing my knowledge on tube grid biasing. Given that the arrangement here is not that trivial, this made me scratch my head a bit. I was suspecting something like that (but initially I though about 1Meg resistor to R/C junction). Thanks for confirming this.

 

[dmp]

In my first attempt I put the DI straight into the grid without thinking about it. And it worked for passive pickups I guess because they could float at the grid voltage. But blocking the dc with a cap is the way to do it.