WHY DO TUBES NEED MORE VOLTAGE????

[nero1985]

hey guys, whats the standar VOLTAGE a tube needs to work??,, and why do they need that voltage and not the classic 9v as transistors and opamps?....

[aaronkessman]

http://www.svetlana.com/docs/tubeworks.html

different tubes need different voltage.

[The Tone God]

I notice that when people talk about using tubes a major thing that many seem to forget is that unlike solid state parts you need to deal with heater filmants. They need juice too.

Andrew

[Paul Marossy]

I notice that when people talk about using tubes a major thing that many seem to forget is that unlike solid state parts you need to deal with heater filmants. They need juice too.

Yep. 12AX7 type preamp tubes need 300mA of current just for the tube heaters...

[Peter Snowberg]

Tubes work by using electrons that are literally "boiled" off the cathode and fly through space (a vacuum) to hit the anode. The difference in electrical potential between the cathode and the anode is what makes the electrons move through space.

You can get a tube work on low voltage, but you end up with something less than good performance.

In a TV tube for instance, the voltage required to make those electrons fly from one end of the tube to the other is typically between 15 and 30 thousand volts.  :shock:

In a 12AX7, the space is much smaller so the required voltage for good performance is more like 150-250V.

Tubes are just a different animal and they work very differently from semiconductors.

Every tube design is different and they take different voltages to do their thing depending on the amount of current the tube has to handle and the sensitivity of a given tube for a given function.

[puretube]

IIRC, "standart" power supply voltage is 90V and 150V;
those were the standart plate batteries about 85 years ago....
there was no "mains" (widely available) at that time;

9V popped up in the mid-sixties, when the "quality" of a radio was measured according to the number of transistors it incorporated....

[Lonestarjohnny]

don't forget Bais Voltage, without the correct bais voltage the process that Peter described will not take place.
Johnny

[puretube]

to be more clear: the bias voltage (negative compared to the cathode potential), hinders the electrons on their way...
so the bias voltage takes care of controlling the amount of current;
(the more negative the bias, the less current through the tube:
with no bias (0V), the current is at max ("saturation");

the mere function Peter described, will work without bias (e.g.: diodes).

In low power circuits (preamps...), bias is almost always "derived" from the HV, so no extra battery or transformer winding is needed.