Biasing Theory for Tubes and Transistors

[walters]

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 scared 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 ?

What voltage should the Cathode Resistor be mostly around what voltage?

 

[walters]

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

 

[Styx]

here we go again

 

[tavib]

I think you want to say bias not bais.
this is a general nomination for seting of static point of functioning for an electronic device. how this is make depend by type of device and application.
generally tubes are biased with a positive voltage on anode(0 - 1000 V) and a negative voltage on gate(0 - 100 V) relative to cathode. small NPN transistors are biased with a positive voltage on collector(0 - 50 V) and a pozitive voltage on base(0 - 1 V) relative to emitter.

 

[walters]

adjust the bias voltage? what range is the voltages?
for a Tube power amp? its (-)38vdc or vac?
for a transistor power amp?

Tube power amp- is the voltage measured AC or DC
Transitor power amp- is the voltage measured AC or DC


Using the bias test point and bias adjustment pot next to the corresponding tube socket for measuring and adjusting

Where would the Bias Test Points be at?

Where Should i put the probe at? Should i put it on the cathode resistor ?
what voltage range would the cathode resistor be?

 

[Wrighty]

biasing is done using DC. The point of biasing is getting your signal (audio) to 'sit' on a dc level, giving the transistor maximum swing for both half cycles (class B).

if you over or under bias a transistor, you will only distort your output signal by clipping the waveform, so experiment.

by tubes, i hope you dont meen Traveling Wave Tubes! if you mean valves then there is always a strong positive voltage on the annode, a positive voltage on the screen (if used) and a negative voltage on the cathode and control grid.

Quick lesson on valves=

the cathode is heated by a element. This warms the Cathode to free up the electrons that are sat on it. The positive potential on the screen starts to attract the electrons toard the Annode, which itself attracts the electrons. The control grid, which has your signal on it changes how many electrons are atracted to the annode which will be proportional to the signal, thus amplifing (and inverting) it.

 

[walters]

Thanks for the information

mean valves then there is always a strong positive voltage on the annode

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?

 

[Wrighty]

ALL BIASING IS DONE WITH DC! It depends on what valve it is. A valve im currently working on has +8.5KV on the annode, +1.4Kv on the screen and -250 on the grid and about -25V on the cathode.

what do you mean the cathode resistor? is this the resistor your going to use to set up the biasing voltage? If so just use ohms law.

 

[Wrighty]

oh its probibly worth addding that that valve is in the final stage of a 10KW Tx!!!!!!!!!

 

[walters]

So the screen,cathode,control grid are all measured in DC voltages?

To measure the Heaters of the Value is measured in AC voltages?

 

[spuffock]

To adjust the bias pot in a tube guitar amp, connect a resistive load in place of the speaker and connect the oscilloscope to the output. Feed a sinewave to the amplifier input, preferably to the "effects return" socket. Turn up the gain until the wave on the scope is clipped top and bottom, then down until the clipping disappears. At this point the bias can be adjusted. You won't damage the amplifier by taking the pot to the end of its travel for a short time. At one extreme of the pot's travel, there will be a kink on the sinewave where it crosses zero. This is overbiassed. now turn the pot until the kink has just disappeared. You will need to adjust the gain as well, to stop the clipping at top and bottom. This is the correct bias point.

 

[walters]

How do i measure the Tube sockets i see like 8 pins are the tubes
measured in DC voltages or AC voltages?
the voltages are around 300-450volts DC i think because there is a
tube rectifier