Project
by Bob Armstrong
The Stereo 6T9
This Month”s
Projects
The Stereo 6T9 . . . .50
LED Night Light . . . 57
Enigma Machine . . . . 64
The Fuzzball
Rating System
To find out the level
of difficulty for
each of these
projects, turn to
Fuzzball for
the answers.
The scale is from
1-4, with four
Fuzzballs being
the more difficult
or advanced
projects. Just look
for the Fuzzballs in
the opening header.
You’ll also find
information included
in each article on
any special tools
or skills you’ll
need to complete
the project.
Let the
soldering begin!
Building a Vacuum Tube Amplifier
There’s a magic in building audio
amplifiers that I’ve never found in a
thousand microprocessor projects and
— if that amplifier happens to glow in the dark
with the soft light of vacuum tube filaments —
so much the better. This project is a simple
introduction to vacuum tube amplifiers.
No one will accuse it of being HiFi, but it’s
easy to assemble using a printed circuit
board, uses only a handful of readily available
components, and — best of all — the vacuum
tubes and transformers used are cheap
enough to not break the bank, even if you
have to buy them brand new.
Output power from this stereo amplifier is
approximately 4 W per channel. If you mount
it in a nice enclosure and give it a pair of
efficient speakers to work with, it makes a
pleasant amplifier for a PC sound card
or a portable CD or MP3 player. Yes, it is
anachronistic to use a vacuum tube amplifier
with a PC and your coworkers will probably
think you’re slightly off balance, but that’s
part of the appeal.
The safest way to work on this circuit is to
always disconnect the power before making
any changes or connecting any test probes.
Make sure that you are well clear of the circuit
and any test leads before applying power. If
you must work on it while it is on, always think
about where both of your hands are! Follow
the old radio man’s rule of keeping one hand
in your pocket at all times.
Remember that high voltage capacitors
such as C1 and C2 can store lethal voltages
for hours after the power is removed. This circuit
has a bleeder resistor — R3 — which normally
drains the charge from these capacitors
within a few seconds, but never assume that
it’s working. Resistors can fail, so always
check the voltage on these capacitors or short
them out before working on the circuit.
Finally, power tubes — such as the 6T9 —
get very hot in normal usage. You can easily
get burned by touching the 6T9 envelope!
Circuit Description
Warnings
Unlike battery-operated transistor or IC
circuits, vacuum tubes can be dangerous. Not
only is this project powered from the AC
power line, but it contains voltages as high as
500 volts. These voltages can be lethal!
Figure 1. The Stereo 6T9 with small MP3 player.
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Figure 2 shows the schematic for one
channel of the Stereo 6T9 amplifier; the other
channel is identical. With the exception of
V1/V2 and T1/T2, all parts numbered 1xx
(e.g., R101, C104, etc.) are part of channel
one and parts numbered 2xx (e.g. R201,
C204) are the corresponding parts for
channel two.
The circuit used is a classic two stage
amplifier. The triode section of V1 serves as a
voltage amplifier with a gain of approximately
10-15x and the pentode section is a power
amplifier with an audio output of around
4 watts. If you do not know anything about
how vacuum tubes work, you might want to
read the sidebar before reading further.
Capacitors C101/C201 AC couple the signal
to the grid of the triode section and block any
possible DC offset from the signal source that
might affect our bias. Resistors R101/R201
are “grid leak” resistors — they ensure that the
grid has a DC path to ground. Of all the
electrons that pass from the cathode to the
AUGUST 2004
