| Negative
Bias Supply
Power tubes need to be biased at a constant current,
to achieve optimal performance of the amplifier. Both
cathode and fixed bias methods suffer from the fact
that the current depends on tube age and mains variations,
not to mention the aging of other passive components.
As a solution, servo circuits have been proposed in
many publications, but the bias these supply all depends
on the music signal, hence the bias current drifts with
the music played, which is unwanted.
The
new Tentlabs / Vanderveen circuit offers rock solid
bias, independent of tube aging, music signal or mains
power voltage changes. The modules biases up to 4 tubes
at exactly equal currents, with the setting of only
1 trimmer. All classes of output stages can be biased,
A, AB or B, triode, tetrode or penthode, single ended
and push-pull.
Result
areas are:
• better imaging and stage due to lower hum land
noise evel
• better bass due to near perfect (<1% error)
current matching in push-pull stages
• Better resolution and imaging due to optimized
left / right channel matching
Technology
The module contains an integrated negative bias supply
(including power transformer), which is properly filtered
and stabilized to achieve a clean bias voltage. A measurement
section for each tube measures the actual current around
a very small window around the bias hence is not affected
by any music signal.
The
maximum output voltage is -160V, enabling the use of
virtually any tube up to a 211 or a 845. Every tube
can be biased up to a maximum of 175mA. The outputs
of the module provide linear and low output impedance
to avoid interaction with the preceding driver stage
driving the power tubes. Each of the 4 outputs can sink
and source 1,5mA, allowing the use of relatively low
grid resistors, if required.
The
module has an integrated high voltage detection circuit
and a 45sec switch on delay whereafter the output tube
current gently increases, after the heaters or filaments
have had their chance to heat the cathode. This avoids
clicks and plops, and enhances tube life.
Application info
Download the Application
Note (pdf)
Application examples
Menno Vanderveen describes the application of the automatic
bias module in the
UL40-S2 amplifier at his website
Specifications
Electrical
• Output voltage range to grids -2V to -85V or
-2V to -160V
• Output sinc & source current max 1.5mA
• Input mains voltage 115Vac or 230Vac (+6% /
-10%)
• Input range high voltage sense: 100V to 500V
Mechanical
• Total size 100*100*40mm (W*L*H, standoffs excluded)
• Hole pitch 91mm * 91mm (symmetrical)
• Total size separated boards 49*100*40mm (W*L*H,
each board, standoffs excluded)
• Hole pitch separated boards 91mm * 40mm (symmetrical,
each board)
• Hole size 3.5mm
• Weight 375 gram
Warrantee
The module is warranted for 5 years.
Safety
Though this is a product which if handled properly is
save, it is surrounded by high voltages. Working with
high voltages requires safety measures and professional
behavior. Tentlabs / Vanderveen take NO responsibility
for accidents resulting from improper usage or handling.
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