Rob Robinette Bassman Micro - heater centre tap wiring

[Yazoo]

I'm building the Bassman Micro. There is one query I have about the  centre tap for the heater supply. On the schematic, this is connected to ground but on the layout it is connected via the 820 ohm resistor connected to the cathode of the 12AU7 tube used as the power tube. Could someone clarify this for me please?

The schematic and layout are available here:

https://robrobinette.com/Bassman_Micro.htm

[ThermionicScott]

A positive DC bias on the heater supply helps the cathodes reject hum:  http://www.valvewizard.co.uk/heater.html

[PRR]

Either way has worked.

If power cathode is convenient, the small DC voltage may reduce one form of heater-cathode leakage hum, making first-tube selection a little less fussy.

[amptramp]

A positive DC bias on the heater supply helps the cathodes reject hum:  http://www.valvewizard.co.uk/heater.html

The article does bring up the issue of heather-to-cathode voltage in stages like an inverter or cathode follower where the heater-to-cathode voltage limits may be exceeded but there is another hum mechanism: heater emission.  The heater usually overhangs the cathode by a bit although some tubes keep the heater contained within the cathode but the heater itself can emit electrons above the top spacer of the tube and below the lower spacer.  This emission can go directly to the plate but since it is modulated by the AC voltage on the heater, it introduces hum.  If the heater voltage is elevated, the control grid post will usually cut off this current flow because it is negative with respect to the cathode.  By elevating the heater, you make it more negative and this prevents the electrons from reaching the plate.

[ThermionicScott]

A positive DC bias on the heater supply helps the cathodes reject hum:  http://www.valvewizard.co.uk/heater.html

The article does bring up the issue of heather-to-cathode voltage in stages like an inverter or cathode follower where the heater-to-cathode voltage limits may be exceeded but there is another hum mechanism: heater emission.  The heater usually overhangs the cathode by a bit although some tubes keep the heater contained within the cathode but the heater itself can emit electrons above the top spacer of the tube and below the lower spacer.  This emission can go directly to the plate but since it is modulated by the AC voltage on the heater, it introduces hum.  If the heater voltage is elevated, the control grid post will usually cut off this current flow because it is negative with respect to the cathode.  By elevating the heater, you make it more negative and this prevents the electrons from reaching the plate.

Interesting, I hadn't considered direct heater emission to the plate.  And I wasn't really concerned about heater-cathode voltage ratings, since we're only talking about a couple of volts of offset and preamp tubes are usually rated at 90+ heater-cathode volts.  You don't need to damage the heater insulation for there to be coupling (capacitive and resistive) between the heater and cathode that can pass hum.