Biasing Theory for Tubes and Transistors

[walters]

transistor baising

Why do you have to set Baising for a Tube or transistors?

What is the Operating Stage for a Tube?

What is the Operating State for a transistor?

How do i Bais a Tube?

How do i bais a tranistor?

I see Trim pots in preamps and power amps for tubes or transistors im really
scare to move these trim pots because most Tube preamps and power amps
are 300Volts and up and if i move the BIASING trim pot it can "over bias" the power tubes or "under bias" them what voltage is a good start so should i hook the oscilloscope up to a power amp what should i look for in BAISING a power amp?

How do i Bais a Power amp?

How do i Hook up a Oscilloscope up a the output of a power amp ? and what should the Oscilloscope display is the power amp is Baised ?

 

[SkyHigh]

transistor biasing theory

walters,

Why do you have to set Baising for a Tube or transistors?

Biasing is required for any transistor that is designed to function as an amplifier, not a switch. Biasing sets the DC operating conditions, operating region of amplification and AC swing.

What is the Operating Stage for a Tube?

I don't quite understand what you meant by operating stage here. I presume you are refering that which stage of amplification a transistor can be found. Transistor can used in pre-amplification or signal amplification or 1st stage, gain or power amplification or 2nd stage, and lastly an output or buffer stage.

What is the Operating State for a transistor?

Depending whether you are refering to MOS or BJT. For MOS, Saturation region is used for amplification. For BJT, Forward Active Region is used for amplification.

How do i bais a tranistor?

Refer to textbooks, for example Sedra & Smith, "Microelectronic Circuits".
You can have Class A and AB for pre-amplification, Class B for power amplification. You can have Class D but non-linear.

How do i Bais a Power amp?

Trim pots in power amp are not much different from the ones in pre-amp. There is a likelihood that a current mirror is used, either in the current source or current sink. This current mirror uses a smaller transistor to mirror a larger current that flows into the power amp via the larger transistor by means of K times proportional width compared to the smaller transistor. Usually a PMOS is used at current source, NMOS at current sink. Therefore a trim pot is found at the drain or either NMOS or PMOS transistor (smaller transistor).

The analogy is simple. Nobody directly trims a larger transistor that carries large current. It's dangerous! It is a good practice that engineers a circuit that uses a smaller transistor to control a larger transistor. Therefore you adjust the trim pot at the smaller transistor that will in turn proportionally adjust the current flowing through larger transistor.

How do i Hook up a Oscilloscope up a the output of a power amp ? and what should the Oscilloscope display is the power amp is Baised ?

You need several steps to do it so that adquate electrical protection and safety is ensured for you and your equipment because you are working with 300 V!
Are you working with 300 Vdc (300 Vav) or 300 V~ (also known as Vac)?
1. You need a potential divider or buck/step-down converter to proportionally reduce the voltage level you are working with. Usually voltage at 25 Vdc or lower are safe. Most people to work with 16 or 12 Vdc for instrumentation or measurement with Oscilloscope, preferably less than 1 Ampere of signal input current.
2. You use can setup bridges or another potential divider such that one resistor is a large resistor B, with 2 smaller resistors A and C connected on each side of this large resistor. To increase the accuracy or reduce percentage error, use B such that 10 times the value of A and B. Then use a difference amp across resistor B and use the oscilloscope to probe the output of the difference amp.

Using the formular of difference amp, the potential divider, and the step-down size of buck converter, you can calculate the actual magnitude of the power signal.

To check if your power amp is biased such that the it doesn't saturates is straightforward. Trim your pots such that the oscilloscope shows respective changes. If the signal reflected on the oscilloscope is not making changes as you trim, it means your power amp has saturated and you MUST trim it down to set it back before saturation.

 

[walters]

how to use an oscilloscope to bias a tube amp

Thanks SkyHigh for writing all this out for me

When working with 300VDC to 500Vdc how do i work with voltage like this
if i touch it i would get shocked pretty bad?

How does a tech. work around voltage like this?

I have a Hard time Biasing a Power amp what should the cathode resistors voltage be and what should the biasing look like on a oscilloscope? GOOD VS BAD oscilloscope waveforms to know if the power amp is biased right?

Tube Heater voltages are 6 to 12Volts "AC"? or are they DC

Cathode Biasing? what does this mean really ?

There is a Negative Voltage going to the Tube to set the Biasing what should the
negative voltage be mostly?

Added after 1 hours 11 minutes:


Biasing Transistors:
To keep the transistor operating within this useful range, resistors are used to establish a predetermined potential difference between emitter and base and base and collector. These resistors are called 'bias' resistors. Sometimes a resistor is also used in the base circuit to limit the flow of current into the base. There are a variety of ways to provide the correct 'bias' to a transistor, one of the most common is presented below:
a 'voltage divider' which
establishes correct bias and ensures a 'linear' response

 

[walters]

biasing tubes

Do you know how much Postive voltage on the annode is it? whats the
Is the voltage AC or DC on the annode?
is the voltage around 300volts?

a positive voltage on the screen
Is the voltage AC or DC on the sceen?
is the voltage around 300volts?

a negative voltage on the cathode
Is the voltage AC or DC on the cathode?
is the voltage around 300volts?

a negative voltage on the control grid.
Is the voltage AC or DC on the control grid?
is the voltage around 300volts?

Whats the voltage for a Cathode resistor?
is the voltage AC or DC on the Cathode reistor?
is the voltage around 300 volts?