Elevated heater question

[joesatch]

Greetings friends and colleagues. When working with a PT with no heater CT and want to use the elevated heater circuit the common method is to run two 100ohm resistors into a junction off each heater line (artificial CT). This junction would go to a voltage divider off the B+ or to the cathode of a power tube. On the design i'm working with the cathode of the power tube is going straight to ground. If i connect the resistor junction to this cathode as opposed to ground what is the difference? Also would this not be considered "elevated" as it is going to ground?

[sluckey]

Connecting the junction of the two 100Ω resistors directly to chassis ground is perfectly fine. But that is considered the ground floor, or zero volts. If you want to elevate above the ground floor in a fixed bias amp, you'll need to build a voltage divider attached to B+.

Read this...

     http://www.valvewizard.co.uk/heater.html

[joesatch]

Thanks Sluck yes the info i provided is directly from that site.  Not a fixed bias circuit i'm working with. Rob's mini 800 cathode biased. His scematic has the voltage divider but with a PT center tap connected to it. The valve wizard states i can accomplish the same by connecting the artifical CT to cathode but this cathode is direct grounded. This would not be elevated then?

[sluckey]

Not a fixed bias circuit i'm working with. Rob's mini 800 cathode biased.

Jose, the layout you just posted is absolutely a fixed bias circuit.

[joesatch]

Not a fixed bias circuit i'm working with. Rob's mini 800 cathode biased.

Jose, the layout you just posted is absolutely a fixed bias circuit.

my mistake i will do more reading. I thought amps without a bias pot were considered fixed bias and those with a bias pot are not fixed bias.

Starting to understand

"In cathode bias a small value resistor is placed between the cathode and the ground so it develops negative voltage on the cathode thus achieving bias. As you hit a note the tube calls for more power and negative voltage on the cathode decreases briefly changing the tube's bias. This gives cathode biased amp their awesome sustain. In fixed bias a separate circuit provides negative voltage to the grid of the power tube and the cathode is simply grounded. Since the grid is negative relative to the cathode thus bias is achieved. Since the voltage applied to the grid does not change the bias is “fixed”. This is what give fixed biased amp a crisper stiffer response."http://carlscustomamps.com/cathode-vs-fixed-bias

[sluckey]

my mistake i will do more reading. I thought amps without a bias pot were considered fixed bias and those with a bias pot are not fixed bias.

That's a very common mistake with newbs. Fixed bias simply means a negative voltage is applied to the grid. This negative voltage may be adjustable so you can easily set the bias point. But once you set the pot, the bias is fixed, meaning it does not change regardless of how hard the tube conducts.

OTOH, cathode bias voltage is developed across a cathode resistor in series with the cathode. The grid is usually referenced to zero volts. As tube current increases, the voltage across the cathode resistor increases, and this causes the tube current to decrease. This situation quickly stabilizes as the tube warms up. For this reason, cathode bias is also called self bias.