Thermionic Vavles - Questions

[whiz115]

Hi!
I have no special knowledge on electronics but i like Thermionic valves very much...i think they are more than an electronic component, they are art! or magic! like the "magic eye valve"

i need your help to understand how they work... although i know some very basic things, i need to know more... i wrote some things i know hopping they can help you understand my level, so you can answer more effectively my questions.

* I know valves are dangerous!
* I know Diodes, Triodes,Tetrodes, Pentodes, double Triodes etc
* I know that the "control grid" it's the equivalent of the base on a transistor
* I know that the "Screen grid" helps to reduce the capacitance of the valve
so we can use it in higher frequencies
* I know what is anode cathode how it works and the use of the heating filament
* i Know that they need high voltage and very low current.

And i need to know:

1) what is the "suppressor grid" exacty?
2) How the "screen grid" helps reduce capacitance?
3) what is the range of voltage and current we give at the "control grid"?
4) Where does "suppressor grid" and "screen grid" connected on a cirquit?
5) how do we measure a bad thermionic valve?
6) how two thermionic valves exist in one glass?! (double triodes)
7) i have EL84 ecc85 ech81 eaf801 can you help me build a small amp with
them?


Thank you!

 

[JimB]

OK, I will bite on a few points.

1 The suppressor grid is a grid close to the anode and is used to suppress secondary emission from the anode. ie electrons "bouncing back" off the anode.

2 The screen grid creates a faraday screen between the anode and control grid. The screen grid has a +ve DC potential, but is earthed at signal frequencies by a capacitor.

3 Depends on the individual valve and what it is being used for.
Generally 0volts to -10 volts (with respect to the cathode) for a small signal valve.
Generally there is no grid current, except in the case of RF power amplifiers operating in class C, then there can be several mA of grid
current.

4 Have a look (via Google) for some typical circuits.
The suppressor grid usually connects tho the cathode.
Screen grid, see my answer to (2).

5 Depends on the mode of failure.
If the valve envelope has a milky white appearance inside, the valve has lost its vacuum.
If the filament is open circuit, it wont light up. Measure the filament continuity with a meter.
If the valve has in internal short circuit, there could be internal sparking, or the valve anode is glowing bright red.
Generally, measure the circuit voltages around the valve, if they are vastly different from those on the service sheet for the equipment, you have a problem. But beware, in older valve equipment, the fault is more likely to be a resistor gone high resistance or a capacitor gone short circuit or "leaky".

6 Why not? A Vacuum is an insulator, so the two valves can co-exist no problem.

7 Google is your friend, use him.

JimB

 

[whiz115]

Thank you very much for your answers!

1 The suppressor grid is a grid close to the anode and is used to suppress secondary emission from the anode. ie electrons "bouncing back" off the anode.

"bouncing back" means that anode emits electrons if yes to where?

2 The screen grid creates a faraday screen between the anode and control grid. The screen grid has a +ve DC potential, but is earthed at signal frequencies by a capacitor.

as far as i know faraday cages cancel electromagnetic emissions by
grounding them or the energy is spend over the faraday cage.
what is +ve? if i understood correctly screen grid always goes to the ground through a capacitor?

4 Have a look (via Google) for some typical circuits.
The suppressor grid usually connects tho the cathode.
Screen grid, see my answer to (2).

i don't understand how it blocks anode from "bouncing back" electrons...

7 Google is your friend, use him.

yeah google is my friend i already know it...everything i know is from
google! the problem is that it's not customizable to my needs!! i need
a human to answer my questions

 

[JimB]

"bouncing back" means that anode emits electrons if yes to where?

Eventually back to the anode. The suppressor grid, having a negative potential repels the electrons back to the anode. Without the suppressor grid, they can end up at the screen grid.

as far as i know faraday cages cancel electromagnetic emissions by
grounding them or the energy is spend over the faraday cage.

Perhaps the expression faraday cage is a bit strong, I should have said electrostatic screen.

what is +ve?

Short hand notation for positive. The Screen grid usually takes its supply through a resistor from the same supply as the anode.

if i understood correctly screen grid always goes to the ground through a capacitor?

Usually yes.

i don't understand how it blocks anode from "bouncing back" electrons...

The SUPPRESSOR grid has a negative potential and drives the electrons which bounce off the anode, back to the anode.
The flow of bouncing electrons is usually referred to as Secondary Emission.

i need a human to answer my questions

Who started the rumour that I was human!

JimB

 

[whiz115]

Thanks for everything jimb!

is it possible for someone to give me simple examples on how we connect
the valves so i can understand better everything that jimb told me?

Who started the rumour that I was human!

 

[whiz115]

is there any chance someone to help me with my questions?
thank you!

 

[crashsite]

Too Tough of a Request!

is there any chance someone to help me with my questions?

Wow! That's difficult. As you know, "valves" (vacuum tubes as we Yanks call 'em...yikes...never thought I'd every get the chance to casually refer to myself as a Yank).

Anyway, as you know...vacuum tubes were in common use for about 75 years and are still used in some apps even today. And, they were used in all sorts of devices...each with a different use and requiring different designs. So, how do you explain how to use a tube?

I guess my recommendation would be to look up the operation of the classic "all American five" radio of the 1950s since it uses vacuum tubes in a variety of ways (power supply rectifier, oscillator, RF/IF amplifier, detector and audio amplifier). Tube type TVs would add VHF/UHF amplifiers, oscillators (and synchronizing circuits) used for sweeping the CRT and an FM detector as well as the tube used in the high voltage circuit. And, of course, the cathode ray tuhe itself.

Since this was the era of radio and TV repair shops, there are boundless books that describe the circuits the tubes are used in to great detail and it should be relatively easy to still find some of them.

You can also check out old Heath manuals, of that era, online as they had some pretty good circuit descriptions for hi-fi gear, test equipment and radios that all used tubes.

You mentioned, magic eye tubes. Check out this link: https://www.akh.se/tubes/eyes.htm