Penthode used as triode.

[RuudNL]

In condenser microphones I see different methods how a penthode tube is used as a triode.
In most cases (like the U67) I see g2 connected to the plate and g3 connected to the cathode.
In some microphones g2, g3 and the plate are simply connected together.
In the M-audio 'Sputnik', g3 and the plate are not even connected, but g2 is used as a substitute for the plate...
Are there disadvantages doing this? In the last case, a grid is used to take over the function of the plate.
It feels strange anyway!

 

[ruffrecords]

There is a thing in the tube bible about using a triode wired B7G pentode (EF92?) with its plate grounded and g2,g3 wired together as a plate. The reason given is the physical layout aids hum reduction.

Cheers

Ian

 

[rock soderstrom]

There is a thing in the tube bible about using a triode wired B7G pentode (EF92?) with its plate grounded and g2,g3 wired together as a plate. The reason given is the physical layout aids hum reduction.

Cheers

Ian

That is interesting! However, if you do something like this, you should consider the maximum permissible voltages for the individual grids. These vary under certain circumstances (and for different types of tubes) and are usually smaller than the max. voltage of the anode.

Which book do you mean by "tube bible"?

 

[kingkorg]

In condenser microphones I see different methods how a penthode tube is used as a triode.
In most cases (like the U67) I see g2 connected to the plate and g3 connected to the cathode.
In some microphones g2, g3 and the plate are simply connected together.
In the M-audio 'Sputnik', g3 and the plate are not even connected, but g2 is used as a substitute for the plate...
Are there disadvantages doing this? In the last case, a grid is used to take over the function of the plate.
It feels strange anyway!

Tbh i'm equally confused by u67 wiring now that you mentioned it. G1 controls the flow of electrons between cathode and G2. But what is going on with G3 being at lower potential, sandwiched between anode and G2? Is there some feedback going on as well?

 

[ruffrecords]

That is interesting! However, if you do something like this, you should consider the maximum permissible voltages for the individual grids. These vary under certain circumstances (and for different types of tubes) and are usually smaller than the max. voltage of the anode.

Which book do you mean by "tube bible"?

Radiotron Designers Manual 4th edition:

http://www.tubebooks.org/books/rdh4.pdf

see page 832 of the above pdf. Turns out to be a 6AU6/EF94 tube and the plate and suppressor grid are grounded with G2 used as plate. The pinout is ideal because G3 and A are on either side of the heaters. I think I saw a physical implementation somewhere that fitted a physical screen across the tibe socket from G3 to plate.

Cheers

Ian

 

[RuudNL]

Tbh i'm equally confused by u67 wiring now that you mentioned it. G1 controls the flow of electrons between cathode and G2. But what is going on with G3 being at lower potential, sandwiched between anode and G2? Is there some feedback going on as well?

If I remember correctly: the positive voltage on g2 is meant to speed up the electron flow after the effect of the negative voltage on g1 and the voltage on g3 is inteded to prevent electrons from 'bouncing back' from the plate because of the high velocity they have. (That is what I remember from my electronics teacher after more than 50 years...)

 

[Tubetec]

Ive read a few different references to screen grid used as plate , mainly in low voltage preamp stages . The plate itself if grounded can sheild the grid wires from external magnetic fields , but thats not usually an issue with tube mics , where the psu is remote and everything is already very well screened .
I checked one of the references relating Ians post ,
https://www.bbc.co.uk/rd/people/d-e-l-shorter?page=3Lots of great audio topics here ,
I couldnt find the reference to the portable mic preamp in RDH4

 

[doqmemory]

The writeup I have used for reference is Tom Schlangen's Pentodes connected as Triodes, which gives lots of interesting information. G3 is connected to cathode is some pentodes, such as the 5240, so the usual option in that case is G2 to anode and G3 to cathode (since it already is connected internally).

In microphones, I always see EF86 wired with G2 to anode and G3 to cathode (like Poctop did), but EF800 is often wired with both G2 and G3 to plate (as Oliver did with U47).

I have not (yet) seen a "grid stopper" resister used between G2 and anode in a tube microphone, so it is probably not needed.

 

[ruffrecords]

If I remember correctly: the positive voltage on g2 is meant to speed up the electron flow after the effect of the negative voltage on g1 and the voltage on g3 is inteded to prevent electrons from 'bouncing back' from the plate because of the high velocity they have. (That is what I remember from my electronics teacher after more than 50 years...)

The screen grid is there to screen the grid from the plate. That is what seriously reduces plate/grid capacitance and got rid of the need to neutralise triode amplifiers (at rf). it is decoupled to 0V which is what achieves the screening but it is more positive than the grid because you want electroncs to travel in the direction of the plate. This is a tetrode. As you increase plate current. electrons boounce off the plate and are collected by G2 which results in a dip in plate current - the well known tetrode kink. G3 connected to 0V stops bounced electrons reaching G2 and removes the kink. This is a pentode.

Cheers

Ian