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electric guitar - BeamTetrodeAmplifier

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BEAM TETRODE AMPLIFIER BASICS

Tetrodes have got 4 electrods. But standard TETRODE with electrodes named: (1) cathode, (2) anode and (3,4) two grids have dynatrone effect and negative resistance effect and cannot be use in audio.

Contructors and designers gave long time ago special plates for eliminate this effect.
It's special five electrode. But it is not a pentode and we cannot spare pentodes with tetrodes like we want 1:1.
Great number of audiophiles  very like tetrodes in their favourites amps.

Tetrode is bulb (a valve tube) with 5 electrodes, anode, cathode, first and second grid, and special plates as 5th electrode.

Electrone current float inside bulb from kathode to anode by the grids and plates. Name cathode and anode is the same like in chemistry. But there is no ions but single electrons floating inside vacuum of tube.

Effect of floating is amplified by giving hot into area of cathode a heat. Heat is generating indirectly or directly. Indirecly is meaning to the cathode we put circuit of external voltage DC or AC or current source to. Cathode [in this type of heating] has two outputs outside. From one end we give one potential from high voltage and low voltage for heating, for second we need only one potential from heating. High voltage between cathode and anode aplificated by heating directly cathode giving an internal electrone current.

In directly method we need internal coil with output outside bulb. Heat is giving into cathode by infrared.

Impacts between electrodes in tetrode:

Plates in tetrode is for inhibition of electrons floating inside bulb. Something like in standard CRT monitor valve. It's function near to grid g3 in standard pentode (we speak about pentodes in next time). In standard we connect plates into cathode. Many types of tetrodes have this element inside construction. Connection is not required.
Grid 2 is named screening grid. This grid decrease electric capacitance betweek anode and cathode like in mathematical formula for series connection between capacitors 1/C = 1/C1 + 1/C2  it's the same like: C = C1*C2 / (C1 + C2). Another changes is inside resistance. According to grid as object inside trajectory of electrons the internal resistance is grow. Also the amplification factor is grow.

Grid no 1 is input of amplifier built of tetrode.


It's a symbol of BEAM TETRODE

POLARIZATION OF BEAM TETRODE:

S2 (G2) has great potential to ground Us2. If Us2 < Uzas - Uzas - is supply voltage -wee need to input the Rs2 into circuit. At this coincidence grid is connect into alternate current signals by Is2 . Resistor Rs2 is calculate by equation drawn at schematic  above.  For S1 we give voltage at negative potential for ground.


Audio amp with beam tetrode

Amp need polarisation, It was depend on parameters of beam tetrode. This Configuration of values is equal and design by internal parameters of tube. The most important parameter is internal resistance - it signed by greek symbol "Rho".

Above we have got standard polarization of speaker amplifier.

How to equal elements on diagram:

It's easy:

Resistance of Rk is near to 150Ohms we can calculate it from Rk= Us0/Ia. This resistor limits current in this circuit. Capacitor Ck reactantion could be equal to 10% of resistance Rk for frequency 100Hz. Xck = 1/(2piFCk) = 10%*Rk

Practical values is between  20 up to 100uF.

This amp have got very big output impedance and couldn't power the speaker directly. it could spoilt the speaker and give too much Anode Current. We can use special old type of headphones. But it is not sucerity. At this situation we have 300-400V on head... Because we have not got any protection if isolation of cables will be cut.

The best way for test it is use transformer (TRAFO). It is special magnetic machine for transform alternate current and voltage - and this can change the visible resistance for output of amplifier. Have 2 or more outputs. Outputs also have minimum 2 pcs of endings. For this specific easy invention wee need only 2 output transformer with 4 pins. Transformer can change voltage by the ratio of number of windings in endings of trafo. U1/U2 = n

n it's meaning as transformer ratio. It's proportional for voltage on input and output of transformer and inversely proportional for currents. In ideal transformer power's on endings is the same U1*I1 = U2*I2

That machine converts the resistance like n^2 between input and output.
If the internal resistance of tube is 5kOhm, the Ra is ~0,5 to 1kOhm.  Ra= ~Rho/(5...10). Factor 5...10 it's experimentally designed for best sound :D

Robc in above schematic is impedance of speaker. Standard in guitar combination is 8 or 16 Ohm. In special we have got 4Ohms. Equations for it is above in left-up corner of schematic.

As opposed for pentodes tetrodes have very low internal resistance. Specially the tubes from soviet union. For it the n for tetrodes is lower than pentodes.

Cw is special capacitor, it's for stabilisation of amplifier for not electrical excitation.Sometimes we need to ad  Resistor Rw Rw is equal to Ra. Rw = Ra. 

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