Mesa/Boogie Vacuum Tube Audio owner manual Headphone Output, Auto Bias

5 - 3 HEADPHONE OUTPUT: (Continued)

what to listen for when you resume listening to your speakers. If you wish to mute your main speakers not to listen to Headphones but while a second pair of speakers is active in a multi-room application via the Preamp Output, we recommend adjusting the multi-room volume with the Level Set attenuator, leaving the Master Volume control at the lowest possible setting. This avoids running the load

FRONT PANEL: TIGRIS	resistor of the Headphone circuitry unnecessarily hot. Should	REAR BAY:	TIGRIS
	you own Headphones so efficient that the usable gain range is

	restricted to just a few “clicks” on the Volume control, select
	one of the higher stages of Feedback and/or more treble activ-	SPEAKERS
	ity. This procedure decreases overall output and widens the
V A C U U M T U B E A U D I O
	usable range of the Attenuator.
		HEADPHONES

WARNING: Similar to the warning signs in your car’s review

HEADPHONE

mirror ( objects are closer than they appear ), volume levels during headphone listening are often higher than they might appear. This is especially true after prolonged listening sessions, when listener’s fatigue reduces

our sensitivity and we compensate by increasing the volume. Abnormally elevated playback levels during headphone listening can permanently damage your hearing!

5 - 4 AUTO BIAS:

The TIGRIS uses cathode biasing for its power tubes and thus no adjustments are ever required. This is one of the invisible features we mentioned in the Introduction and it is attractive because it removes the need to monitor and maintain the biasing of the power tubes. Cathode biasing is so effective it is also known as self biasing or Auto Biasing. For the technically minded who want to know more about Bias techniques, here is a brief explanation:

A tube’s operating conditions are established by striking a balance between attracting and repelling forces. Remember, unlike charges attract, like charges repel. At the center of the tube is the cathode which, when heated, emits a cloud of electrons. Possessing an inherently negative charge, these electrons are strongly attracted to the plate (or anode) which is charged to a high positive voltage from the main power supply. Located in between the cathode and the plate is a spiral of thin wire called the control grid. In order to restrict the flow of electrons from the cathode to the plate, the plate is made to be more negative than the cathode. Because like charges repel, negative electrons from the cathode are both repelled by the more negative grid and attracted by the highly positive plate. A small change in the negativity of the grid can cause a large change in the amount of electrons flowing to the plate. And that’s the essence of amplification: a small change in the grid voltage causing a large change in the plate current.

But first, a proper zero-signal, static state current flow must be established and that is accomplished by achieving the proper relation- ship of voltages between the cathode, plate and grid.

In a fixed bias system, such as the BARON, the power tube cathodes are tied directly to ground at zero Volts. A negative voltage from a separate small power supply is then applied to the control grids and that voltage ( called the Bias voltage ) is set such th at a predetermined idle current flows through the power tubes under zero-signal conditions. The more negative the grid is with respect to the cathode, the less idle current will flow through the tube to its plate.

Cathode	...........................................0
Plate	+ 450
Grid	- 50 (voltage generated by
	separate power supply)

In a self-bias system, such as the TIGRIS, the power tube cathodes are tied to ground - not directly - but through resistors. When current flows through the tubes, a voltage drop occurs across these cathode resistors and this provides a positive voltage on the cathodes. The control grids may then be operated at a zero D.C. voltage - but they will still be negative with respect to the cathodes because the cathodes are now slightly positive.

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