Aphex
Thermionics
- Division of Aphex Systems Ltd.
7247, 7318, 7728, 7729
8.5 Tube Life
It is unlikely you will ever have to replace the
tubes in your Model 1100 MkII. Here’s why.
The Model 1100 MkII regulates the filament
supply voltage very tightly to about 20% less than
the tube rating. This extends the filament life
indefinitely. More than sufficient cathodic emis-
sion is still generated for use by the RPA circuits.
The only failure mechanism will be expansion and
contraction fatigue when the power is switched
on and off. There will be a small percentage of
tubes that fail earlier but most will survive indefi-
nitely.
Another design feature that extends the tube life
is the very low plate dissipation of an RPA circuit.
Virtually all the heat generated by the tube is from
the filament temperature alone. In conventional
circuits, the plate power dissipation is consider-
able and that is what eventually causes a weak or
gassy tube. In our RPA circuits, a well manufac-
tured tube will probably never become weak or
gassy even in 100 years of use.
Strangely, a benefit of how gracefully the RPA
method embraces the tube function is that a
tube testing too weak for a conventional circuit
will probably work just fine in the Model 1100
MkII unless it has somehow developed excessive
noise. You are probably asking, “How is that
possible?”
The answer is that the Aphex patented Reflected
Plate Amplifier takes full advantage of thermionic
tube amplification at a relatively low plate volt-
age and current. With the RPA, the plate voltage
is held essentially constant while the low plate
current is extracted, reflected, and magnified by
a linear current mirror. It is the reflected plate cur-
rent that generates the output voltage. The tube
runs much cooler yet still generates the tradi-
tional transconductance curve. In contrast, tradi-
tional tube stages have to operate at a high plate
voltage to permit a useful signal swing to develop
as a function of the tube’s plate resistance, plate
load resistor, and transconductance.
With the RPA, the tube’s control grid receives the
input signal and controls the electron flow by the
law of the tube’s transconductance curve just like
a traditional stage. The tube’s dynamics still exist,
giving us the great tube sound from a much more
efficient circuit.
8.6 Relevant Base Diagrams
The tubes that work in the Model 1100 MkII
follow the industry standard base diagrams 9A
and 9AJ. You will see that the only difference
is the filament center tap at pin 9. We use the
center tap provided on 12 volt tubes to allow
their use with a 6 volt filament supply. When you
change the filament switch from 6 to 12 volts,
the two filament halves are placed in parallel, cre-
ating a 6 volt filament. This is a widely accepted
practice having no deleterious effects.
Tube Base Diagrams
12 Volt Tubes
6 Volt Tubes
Page 8-2
Tube Topics
