since July 2000 

Heater Concerns

This Issue

Subject: 6.3v or 12.6v
   I am well aware of the virtues of the regulated B+ be it split rail or single ended, but what about an article on a few good heater supply schemes. All this emphasis on circuit topologies and so forth and so on is useless without proper heater control and decoupling. Over heated or under heated cathodes cannot maintain the proper emission level for the tube to operate at its specified rating, especially in the preamp stage. A fancy circuit without a good heater supply scheme is like a muscle-bound man with extraordinarily skinny legs. Can you prove me wrong??

   I agree wholeheartedly. Thus, this month's topic: tube heaters. We will look at both directly heated and indirectly heated tubes, current sources, and fixed regulators for the heaters.
    Remember, if you have a request or suggestion of your own for either an article topic or circuit explanation, please e-mail: 

   "I like everything about tubes, the history, the sound, the look, the glow, everything actually, except dealing with the heater." This is what a friend once told me was his view on tubes. He argued that the heater was the single largest liability to the vacuum tube. He then listed hum, heat, high current demands, failure, heater-to-cathode voltage limits, and the sonic imprint imparted by the choice of heater supplies. I did not argue with him. Taking the headache out of the heater is the aim of the following topics.

Getting the Right Voltage
    The heater is a low resistance length of wire that is either coiled or folded upon itself inside the cathode structure. When attached to a voltage supply it conducts enough current to heat to a glowing hot state. This is necessary to bring the cathode's temperature up to what is needed to ensure electron emission. Once this point has been achieved, however, any further voltage increase will only shorten the tube's life expectancy.
   You might say: true enough, but since the heater string is usually attached to a 6.3 or 12.6 AC transformer winding, what is the issue? The problem is that the 6.3 volt winding often puts out more than 6.3 volts! How is this possible? Two factors join together to increase the 6.3 to as much as 7 volts. The first is that historically wall voltages have been slowly on the rise in the United States. If you scan old schematics you will see sometimes 110 VAC or 112 VAC or 115 VAC specified. Today, 117-120 VAC is the standard the power company aims for and usually it is closer to 122 VAC or 123 VAC.

In This Issue


Heater Concerns
  Free DC Power Supply
  Free Transformers (Sort of)
  Current regulation for Heaters
  Using a 6SN7, 8SN7, or 12SN7
  Directly Heated Filaments
  Voltage regulators for the 2A3
Publishing Information
Glossary of Audio Terms Articles
Classic Magazine articles

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