Universal Audio 1008 Filament Voltages

[mjrippe]

Hello Group,

I might be racking a pair of these for a client, but when I looked at the schematic it has a note about the filaments:  "filaments DC & biased 60-75v positive above ground, bypassed by 150 uf min."  Why was this done, and is it really necessary when using them as stand alone preamps?  Haven't run into this on any other tube preamp yet, so can I just run the 6.3 or 12.6v DC to the appropriate filament connections?

Thanks,
Mike

 

[emrr]

AC filament biasing is pretty common.  Two resistors and a cap off of B+.  It lowers tube noise floor, and in rare cases, also moves heater more within the comfort zone for heater/cathode potential.  If you are going DC, then not an issue. 

 

[mjk]

Keep in mind that any noise on the heater wiring, including common mode noise, can be picked up by the cathode unless the heater is biased positive wrt the cathode. This occurs simply because the heater emits some electrons that are picked up by the cooler cathode IF the cathode is at a higher potential than the heater. There is also some capacitance between heater and cathode through some relatively high K alumina.

The UA1008 is a single ended circuit with relatively high cathode impedance in the first stage to accommodate the feedback and "Envelopmental EQ" circuit. This and the long heater wiring unavoidable in consoles is no doubt the reason for the special precaution. For short runs inside a rack enclosure from a well regulated clean source e.g. LT1084 and good star grounding it seems OK in my experience to ground the negative side of the heater supply. On the other hand, it's relatively easy to float the heater supply and lift it which removes a whole potential source of noise and buzz.

 

[Adam Smith]

Not as helpful as the other posts, but I'll throw in my two cents. The 1008's I use at work daily have the filaments biased. They are much quieter than about any solid state pres I have used.

 

[mjrippe]

Thanks all, I think I will try it both ways and see what works best for the client's setup.  I figured tapping off the B+ would work but it's nice to have it confirmed.

Mike