wires mounted with spring suspensions to mica supports pressed into the glass envelope.
The fragile heater wires can break very easily under shock or over-tension, especially
when they are glowing or when they are still hot.
D T-s are fragile devices and must be handled with exceptional care!
Never pull out a D T out of its sockets as long as it is still hot!
Never move or even transport a D T as long as it is still hot!
Moreover, the heater wires are able to swing very easily. As a result, severe vibrations of
the glass envelope will be converted into audio signal and passed on to the speakers. This
effect will be especially pronounced when very high-efficiency speakers are to be driven
by the amps. In view of this effect called tube microphony, amplifiers using DHT’s need
more considerate placement than amplifiers with the more widely used indirectly-heated
tubes in order to prevent sonic artifacts from entering the audio signal due to power tube
tube microphony.
Is is strongly advisable to place D T amplifiers on a rigid rack, shelf or platform,
carefully decoupled from the floor or wall, and as far away from the speakers as
possible, in order to prevent sonic artifacts from entering the audio signal due to
power tube microphony.
300B POWER TUBES
In our amplifiers the control-grid bias voltage cannot be trimmed in any way to adjust the
idle current of an individual power tube. This is a very reliable and stable way to achieve
grid bias, but it makes it especially important to use tubes that meet tight specifications.
The average 300B tube draws an anode current Ia of 70mA for an anode-cathode-voltage
of 400V when the grid bias is set to 90 volts. Allowing for a 10% tolerance in anode
current with respect to this quiescent point, only tubes with values Ia between 63 mA and
77 mA should be used to equip the amplifier. Although matching the power tubes is not
very critical, a difference in the Ia values of the two tubes of a matched pair should be
preferably not more than 5 mA. The use of 300B tubes with questionable tube data and/or
poor quality will lead to inferior sound quality or in (extreme cases) even to serious
damage to the amplifier. If there is any doubt that a certain 300B power tube actually
fulfills the stated requirements, the idle current can be easily determined by Ohm’s law
by measuring the DC voltage-drop over the winding resistance of the output transformer
primary while the amplifier is powered on.
In view of the high voltages involved, measurements within the powered-on
amplifier circuitry should always be carried out by an experienced technician only !
When the power tubes are replaced for whatever reason, it is strongly advised to use only
high quality matched pairs tested for the above specs supplied either from the
manufacturer or a “decent” tube supplier.
