new tubes

[patm]

Built a Bottlehead system 2 years ago. Started with the FP III. It worked amazed myself. Moved on to 300b Paramounts. Like many I tweaked as I went along. Goldpoint's for the FP III, Mondorff SO caps for all. pullout Telefunkins for the Paramounts, nos cv4003,s for the FP III, Mikes nickel for the  Paramounts, Svetlana winged C 300b's from Conus. I have the soft start upgrade coming for the Paramounts and I ordered a pair of Emission Labs 300b mesh plates to use when the mod is finished. The EML's came on Friday and they are fantastic looking tubes. Build quality is amazing. I lost it sometime on Saturday morning. Rainy cold dreary day so I decided what the heck. Pulled the winged c's out and plugged in the EML's and set the hum bias (more on this later), WOW what a difference. Absolutely every facet of the sound is vastly improved, clearly the biggest change to my system yet. More sonic change than the Iron upgrade, it's close but the EML's win by a nose. More detail, more and tighter bass warmer cleaner

[Doc B.]

You are not adjusting cathode bias with the hum pot. You are nulling out AC hum. And yeah, in theory it could get down to zero with respect to the resolution of your DMM if the tube filament is well balanced.

[Grainger49]

Pat,

Take a look at the schematic.  The heater for the 300B is fed by DC.  That DC is not grounded at the power supply.  In the schematic you will see the hum balance pot adjusts where the ground reference is connected relative to the heater DC.  It goes through a 22 ohm resistor on each side of the heater, through the 10 ohm pot then from the wiper through the paralleled 1k ohm resistors to ground.  So neither side of the heater supply is hard grounded, the center is grounded through the hum balance pot.

It is a characteristic of each individual tube that the heater isn't exact.  The hum balance pot allows you to achieve the lowest hum possible by adjusting the heater relative to ground.

[patm]

Thanks Doc and Granger, I knew I was adjusting hum I just used the wrong word. I should have said balance not bias.