Aphex
Thermionics
- Division of Aphex Systems Ltd.
Page 8-1
Tube Topics
8
Model 1100 MkII Mic Preamp - Owner’s Manual
8.1 Tube Choice
The design of the Model 1100 MkII centered
around a Russian tube called the 6N1P having a
6 volt filament. We found that the more readily
available American made 6DJ8 performs identi-
cally in our listening tests and for all measure-
ments. Therefore we reserve the right to ship
units tubed with either type as their availability
shall dictate.
Should you decide to experiment with different
tube types, you will find we have made that very
convenient for you. There may be differences in
sound between different types and we feel some
people will select a favorite. We invite you to
experiment with tubes and tell us about your
experience.
8.2 Filament Voltage
From a practical view, it only made sense during
the design phase to accommodate as many tube
types as possible in the Model 1100. The future
supply of tubes is not completely certain, and
we have recently seen some high demand tubes
come into scarce supply such as the omnipresent
12AX7. To accommodate as many tube types as
possible, we supplied a tube filament switch for 6
and 12 volts. This allows you to put many 6 or 12
volt types into the same socket.
Set this switch as appropriate when replacing or
exchanging tubes.
Having the switch in the wrong position will
simply result in no light-up but will cause no
harm.
8.3 Consideration of Tube Types
We selected the 6N1P and 6DJ8 types because
of their low noise, rugged construction, and low
microphonics. They are readily available in pre-
mium quality from new old stock and not very
popular for audio equipment making their con-
tinuing supply well assured.
Bear in mind, as already stated, the difference
between tubes will be sonically very subtle, not
as great as heard when substituting tube types
in other equipment using conventional circuitry.
For that reason, we expect that future tube
replacement will not be an agonizing judgment
call. Simply replace with any suitable type that
is currently available. Of course, we recommend
attempting to obtain the original type if possible.
We maintain a stock of qualified tubes at the
factory, and we recommend letting us supply
replacements to you if ever needed.
8.4 Suitable Tube Types
We prefer not to comment upon the relative
merits or sonic differences between tubes since
those judgments are mainly subjective. Many
people will probably detect no difference at all.
We therefore offer these suggestions only on their
merits as technically operable in the circuit.
List 1: Known To Work
6DJ8, 6N1P, 12AT7, 12AU7, 12AX7, 5751,
5814A, 6679, 6680, 7025
List 2: Believed To Work
6BC8, 6BK7, 6BS8, 6BX8, 6BZ7, 6BZ8, 6FW8,
12AV7, 12AY7, 12AZ7, 12BZ7, 12DM7, 12DW7,
12U7, 6072, 6211, 6679, 6829, 6851, 6955,
6V
12V
Figure 9-1 Filament Voltage Switch
The Model 1100 MkII contains two dual triode tubes, one per channel. One triode of each tube is used
in the low noise second stage and the other is used in the impedance balanced output stage. Thanks
to the Aphex exclusive Reflected Plate Amplifier (RPA) circuits, you can substitute different tube types
with equal success.
Representative Tubes