though they do
require a much higher load impedance, as well.) A tetrode improves amplification and
high frequency response by isolating the anode
from the control grid by the screen grid (G2)- normally
connected to a high potential- at the expense of some of the
output current and additional noise. A pentode (fig.4)
recovers most of the current by using yet another grid, the suppressor
(G3),situated between (A) and (G2) and usually connected to
a low voltage. This prevents secondary emission (high
speed electrons bouncing off the screen and being redirected to
the anode.)
The power supply for the anode to cathode circuit is usually 250-
350 Volts, while the heater of a double- triode electronic tube
may consume 0.3 Amps at 6.3 Volts AC- these specifications make
battery operation and portability largely impractical.
Hence, notwithstanding those differences, the triode looks like an n- type depletion MOSFET. Note that there is no electrical connection between the control grid and cathode. (In depletion devices,an anode current flows even if there is no potential difference between the control grid and cathode. This current will be several tens of milliAmps, which is a lot in terms of low power valves. The negative grid to cathode bias required to switch off the device varies, even between valves of the same type, but a maximum value- in magnitude- of -20 Volts may be appropriate. The anode current is substantially proportional to the anode voltage. This is in contrast to enhancement devices- like bipolar transistors- whose collector current is zero in the absence of a suitable base- emitter forward bias; as long as the device is out of saturation, the collector current is considerably independent of the collector voltage.)
But there are further differences: Though there are n- and p- type MOSFETs, there are only 'n- type' tubes; If the anode is not connected to a higher potential than the cathode, electrons will not be attracted to the former. Tubes are high voltage, low current devices- the exact opposite of what low impedance loads, like loudspeakers, need. Lastly, it would make a lot of sense to isolate any external load from the dangerous high voltages present in valve circuits.
Therefore, the final stage of a (power) valve amplifier comprises two valves (a so called push- pull stage), fed in antiphase, and each valve is intended to mainly amplify either the positive or the negative portion of the signal- their outputs are combined via the primary winding of a transformer (whose center tap provides one of the connections to the power supply.) The secondary, having fewer turns than the primary, then feeds the low impedance loudspeaker, and also isolates it from the high tension. (On the other hand, semiconductors have a low ouput impedance, are availiable in both n- and p- types, and can operate from safe, low voltages, so an output transformer is totally unnecessary in this case).
Important note: Valve projects are certainly not intended for inexperienced constructors- there are hazardous, even lethal high voltages involved. Flimsy construction techniques are definitely out- they sould never be present in the first place. The finished board must be operated within a closed box (with ventilation grills); even so, access to the high tension must be restricted. Extreme care ought to be taken during testing/ maintainance of the unit. It is safer to deduce the anode current (and possibly the anode resistor voltage) by a measurement of the low voltage across the cathode load.
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