[poseidonsvoice]

Fellas, when you talk about the 75V bias, do you mean raising the filament heater voltage by 75V? If so, that's super easy. Its exactly what I do with my preamp I have now.

Anand.

Yep, that is what they mean.  It is just one of the many things that should be included in a tube preamp as good practice, like tightly twisting the heater wires.  It takes stress off the cathode to heater insulation as well. 

Actually I take that back.  You don't always want to bias up the heaters.  In this case you do because you are stacking the tubes on top of eachother and you want to take the stress off the cathode/heater insulation of the top triode.  In a regular grounded cathode amp, I think it is more case by case. Best thing is to look at the datasheet to see what the max values are for the heater to cathode voltage and design around that.  However, biasing up/down the heaters so that it is relatively near the cathode is usually a good thing for noise.

Josh,

Well...I don't think you need to take it back (at least with the applications discussed)! I always observe the parameters so I don't violate the max heater to cathode voltage rating, however, as you have said, biasing the heaters up is a great thing to reduce noise. I know Allen Wright supports this approach. You just have to bias enough to not violate the heater to cathode voltage rating. Not a question of "if" as far as I'm concerned, just a question of "how much." At least in my preamp designs. If there is a design where you wouldn't bias the heater voltage up, share it with us. Enquiring minds would love to know  aa.

Best,
Anand.

[JoshK]

Anand,  I agree with what you said.  I don't mean to take this thread's focus off Tom's beautiful (and completed! what a concept) preamp.   I just was thinking about how I said that and thought well, I could see cases where, in a grounded cathode amp, the cathode is only sitting a few volts above ground.  There I wouldn't bias up 1/4 B+ obviously or you are in fear of violating the heater/cathode max to the positive side.   In this case, not biasing up would be ok because it is only a couple volts or your can bias up those couple of volts.