12AU7 Pre-amp Schematic Question

[Jimthemeek]

I'm thinking about building a line level pre-amp with a couple 12AU7s I have laying around... and I've been looking at some schematics in books and online. In Bruce Rozenblit has a schematic that was in Glass Audio in 1991 that I was looking at... but I'm a little bit confused by the schematic. The 12AU7 Pin 9 (the heater center tap) is open on V1 and then one V2 pin 9 is tied to the HT power supply through a 220K resistor and then to ground through a 100K resistor. Am I missing something here? Why would I need to tie this to one tube and not the other? Other pre-amp schematics I've seen do not tie anything to pin 9. I'm not sure if I'm mis-reading the schematic or if since the pin 4s (of both tubes) are attached to the plus side of the heater power supply and the pin 5s are a tied together to ground if this one connection acts against both tubes tied together as one unit.

[mashaffer]

I would say they are doing that to raise the heater potential to 1/3 B+ for reduced hum and to prevent exceeding the cathode to heater rating of the tubes. Note that since both heaters are wired in parallel both heaters are elevated even though only one is wired to the voltage divider string.

Normally heaters are elevated by connecting the filament winding center tap on the PT secondary to the reference point but since they are doing a DC heater here that wouldn't cut it.

[Jimthemeek]

Thanks for the info on this. I wasn't thinking but now that you say that it makes sense. I was thinking about throwing this one together and seeing how it does. I was starting to lay out the negative on the bridge rectifier as tied to ground but maybe that's not a good idea. Plus I was reading in a book last night that tying one side of the power supply to ground is a great way to get maximum hum... not really what I'm wanting to acheive here... lol! Thanks again for the info!

[pchw]

This is what the original article said:

The filaments have been biased up to 95V to put the heater-to-cathode potential below the continuous rating. The DC filament supply floats on the potential across C7.

This is a mu-follower circuit and sounds pretty good as it is.

[mashaffer]

Yep, definitly do not want to ground that bridge. Bad mojo. I am using a mu follower input stage in a phono pre I am currently working on so I too hope that they are as nice as people say.