Blumentritt Amplification Texas Tone 50 Owner's Manual 9
Tubes: Why (And When) To Replace Them:
Tubes are made of many fragile mechanical components that are vacuum sealed in a glass envelope.
The longevity of a tube depends upon several factors, including how hard and often the amplifier is
played, vibration from the speakers, road travel, repeated set up and tear down, etc. Any time you
notice a change in your amplifier's performance, check the tubes first.
If it has been a while since the tubes were replaced and the sound from your amplifier lacks punch,
fades in and out, loses highs or lows or produces unusual sounds, the power tubes may need replacing.
If your amplifier squeals, makes noise, loses gain, starts to hum, lacks dynamic sensitivity, or feels as if it
is working against you, the preamplifier tubes may need replacing.
The power tubes are subjected to considerably more stress than the preamplifier tubes. Consequently,
they usually fail/degrade first. If deteriorating power tubes are not replaced, they will ultimately fail.
Depending on the failure mode, they may even cause severe damage to the audio output transformer
and/or other components in the amplifier. Replacing the tubes before they fail completely has the
potential to save you time, money, and unwanted trouble. Since power tubes work together in an
amplifier, it is crucial that they are replaced by a matched set. If you are on the road a lot, we
recommend that you carry a spare matched set of replacement power tubes and their associated driver
tubes.
After turning off the power and disconnecting the amplifier from the power source, carefully check the
tubes (in bright light) for cracks or white spots inside the glass or any other apparent damage. Then,
with the power on, view the tubes in a dark room. Look for preamplifier tubes that do not glow at all or
power tubes that glow excessively red.
Whenever you replace the power tubes:
The output tubes of the Texas Tone 50 are biased with an adjustable fixed bias. When changing the
output tubes, it is important to validate and possibly adjust the negative bias voltage. The output
section of the Texas Tone 50 is designed for long tube life. When the output tubes are replaced, we
recommend that you replace the phase inverter tube as well. The phase inverter tube determines the
shape and amplitude of the signal applied to the power tubes and must work almost as hard as the
power tubes. The phase inverter on the Texas Tone 50 is a special design that yields wide bandwidth
and gain characteristics when used with a 12AX7 tube. A 12AT7 may be used for less output drive.
You can check your preamplifier tubes for microphonics by turning the amplifier on, turning up the gain
and tapping lightly on each tube with a chopstick or other light wooden dowel. You will be able to hear
the tapping through your speakers, which is normal. It is not normal for a tube to ring like a bell after it
is tapped. If it does ring, then it is microphonic and should be replaced. The first stage preamp tube in
shock-mounted to reduce noise and microphonics. Remember to use only high quality, low microphonic
tubes in the preamplifier section. Even though power tubes are rarely microphonic, you should check
them anyway. The power tubes can be checked for microphonics just like pre-amp tubes.
