Circulation:

59072

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??
Rowan

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:

Editor

"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

1
3
5
6
7
9
10
11

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
E-mail
Publishing
Information
Glossary
of Audio Terms
Glass-Ware.com
Articles
Classic
Magazine articles

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