Linear microZOTL User Manual page 7

microZOTL Preamplifier User Guide
When replacing the tubes, the two smaller ones are the input tubes. They must be
matched so that the gain of each channel is the same (you could possibly correct for
unmatched tubes using the balance control). The power tubes typically do not have
to be matched in gain. For the input tubes, any 12xxx equivalent tube can be used.
Typically 12AT7s are used (they have a gain of 60) or 12AU7s (gain of 20) to lower
the gain. There are many variants like 6201s, 5751s, 5814s, etc. that all work well
and if one set of tubes sounds better, use it. (12AX7s have a gain of 100 and is
typically too much). 12SN7s are sonically identical to 6SN7s. Since few amplifiers
use them, they are considerably less expensive than 6SN7s.
Also, even though the amplifier uses 450V on the tubes, it is at a different frequency
and current than the tube rating. So, 350V and even 250V rated tubes can be used.
In short, don't worry about the tube voltage rating.
When removing the tubes, use a gentle rocking motion as you pull out the tube. When
replacing in the socket, make sure the pins are straight and aligned correctly with the
hole pattern.
When replacing tubes, in rare cases the tubes might have a short. This typically
results in blowing the fuse on the amplifier board. The replacement is a 5mm x
20mm 2.5 amp fast blow, preferably ceramic (glass will work fine) fuse. The other
issue is tube noise. With no input to the amplifier connected, you should not hear
any static or rushing sounds. Sometimes tubes will make a little noise as they warm
up, as this is normal, but should disappear after a couple of minutes.
Technology:
In 1996 David Berning developed a new and technically advanced vacuum tube
architecture designated as ZOTL. ZOTL stands for Zero-Hysteresis Output-
Transformer-Less amplifiers. The ZOTL architecture eliminated the problematic
sound quality issues that audio-output transformers caused, as well as provides
many other advantages such as better frequency response and accuracy, longer tube
life, less tube heat and far better coupling from the tube to the output device.
Since ZOTL technology was introduced in 1996, the MicroZOTL amplifier has the
lowest power output of all ZOTL amps so far. Hence the name.
The ZOTL amplifier uses radio frequency to change the voltage-current transfer
characteristics of the output tube from its normal impedance plane to one suitable
for driving a dynamic loudspeaker (or headphones and power amplifiers). The
radio-frequency remapping is implemented using special high-frequency power-
conversion techniques. The high-voltage, low-current tube impedance plane is
remapped to the high-current speaker impedance plane through special
transformers operating at a constant RF carrier frequency of 250kHz. Because the
audio signal is riding on a carrier, it is not subject to parasitic elements of the
transformer that would distort the audio signal. Unlike the conventional audio-
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Linear Tube Audio