ruffrecords
Well-known member
I have been contacted several times over that last few months and asked to explain what elevated heaters are and why they are necessary so I thought it might be useful to post an outline description here.
Heater elevation becomes necessary because of certain circuit topologies and tube specs. Most tubes have a specification for the maximum voltage that is allowed between the heaters and the cathode. In many designs, like a conventional common cathode amplifier, where the heater is just a few volts above ground, this is not an issue, but in designs where a cathode can be 100V or more above ground, such as a cathode follower or a mu follower or an SRPP stage, it needs to be considered.
For a mu follower like the poor man's tube gain make up stage, or the SRPP output stage stage in the Eurochannel, where one tube sits on top of the other, the top cathode is at about half the supply voltage or around 140V above 0V. If the heaters were connected to 0V then they would be 140V below the voltage of the top cathode. We also need to think about signal conditions. At the maximum output of about 20V rms, the peak voltage on the top cathode would rise to 140V + 20*1.414 = 168V. This may exceed the heater to cathode voltage spec of many tubes. However, if we raise the heater voltage to about 80V above ground, then we reduce this to 88V and remain comfortably within the heater cathode voltage spec of most tubes.
The specifications for maximum heater to cathode voltage vary widely between tube types and even between versions of the same type of tube or between manufacturers of the same tube type. The maximum voltage the heaters can be above the cathode is often different to the maximum voltage the heaters can be below the cathode. Some double triodes even have different specs for each triode within the envelope.
The way heaters are normally elevated is by using a simple potential divider connected across the HT supply. The bottom leg of this pot divider is usually decoupled so the heaters at 0V for ac signals. There is often a specification for the maximum resistance between the heaters and the cathode so this needs to be taken into account in designing the pot divider.
I have attached a snippet from an RCA tube mixer power supply from the 1940s showing how they did it. The pot divider is 4R3 and 4R4 and the decoupling capacitor is 4C3. Notice the pot divider is connected to the hum dinger pot for the heaters.
Cheers
Ian
Heater elevation becomes necessary because of certain circuit topologies and tube specs. Most tubes have a specification for the maximum voltage that is allowed between the heaters and the cathode. In many designs, like a conventional common cathode amplifier, where the heater is just a few volts above ground, this is not an issue, but in designs where a cathode can be 100V or more above ground, such as a cathode follower or a mu follower or an SRPP stage, it needs to be considered.
For a mu follower like the poor man's tube gain make up stage, or the SRPP output stage stage in the Eurochannel, where one tube sits on top of the other, the top cathode is at about half the supply voltage or around 140V above 0V. If the heaters were connected to 0V then they would be 140V below the voltage of the top cathode. We also need to think about signal conditions. At the maximum output of about 20V rms, the peak voltage on the top cathode would rise to 140V + 20*1.414 = 168V. This may exceed the heater to cathode voltage spec of many tubes. However, if we raise the heater voltage to about 80V above ground, then we reduce this to 88V and remain comfortably within the heater cathode voltage spec of most tubes.
The specifications for maximum heater to cathode voltage vary widely between tube types and even between versions of the same type of tube or between manufacturers of the same tube type. The maximum voltage the heaters can be above the cathode is often different to the maximum voltage the heaters can be below the cathode. Some double triodes even have different specs for each triode within the envelope.
The way heaters are normally elevated is by using a simple potential divider connected across the HT supply. The bottom leg of this pot divider is usually decoupled so the heaters at 0V for ac signals. There is often a specification for the maximum resistance between the heaters and the cathode so this needs to be taken into account in designing the pot divider.
I have attached a snippet from an RCA tube mixer power supply from the 1940s showing how they did it. The pot divider is 4R3 and 4R4 and the decoupling capacitor is 4C3. Notice the pot divider is connected to the hum dinger pot for the heaters.
Cheers
Ian