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Old 26th March 2009, 07:54 PM   #1
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Default Pentode grid question...

Hi all,

A quick question for anyone that has time to answer.

In the picture I have uploaded is the input pentode for a preamp I am building. The screen grid is shown to be connected to ground and the cathode, and the suppressor grid appears to be unused.

Is it common for the screen grid to have the same polarity as the cathode? I understand it will push secondary electrons back to the plate, but will the pentode still function correctly, or should miss out the suppressor and the screen all toghether and just treat the EF86 as a triode?

Any help is much appreciated

Cheers


Charlie
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Old 26th March 2009, 07:58 PM   #2
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the convention in symbols is for the suppressor to be drawn at the top, the screen below that, and the control grid at the bottom.

the EF86 has suppressor internally connected to cathode (works best for 'normal' applications), and this is usually shown in its drawing.

Your screen is connected to a decoupled supply, which is also normal.
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Old 26th March 2009, 08:14 PM   #3
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I was unaware that it is connected internally.

Thanks very much for your reply
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Old 26th March 2009, 08:35 PM   #4
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Having said that, I just looked at the data sheet and found it is not in this case!

You have to connect pin 8 (g3) to pin 3 (k) or normal pentode operation.

many pentodes do connect internally, and I rashly thought this one did too...


always check the data sheet!!!!
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Old 26th March 2009, 08:39 PM   #5
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Quote:
Originally posted by Rod Coleman
the convention in symbols is for the suppressor to be drawn at the top, the screen below that, and the control grid at the bottom.

the EF86 has suppressor internally connected to cathode (works best for 'normal' applications), and this is usually shown in its drawing.

Your screen is connected to a decoupled supply, which is also normal.

The connection between cathode (Pin 3) an suppressor grid (Pin 8) must be done externally.

Regards Andreas
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Old 26th March 2009, 08:41 PM   #6
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Yeah I had checked the datasheet, i was just a bit thrown by the suppressor being connected to the cathode also.

I shall connect the pins externally

Thanks
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Old 26th March 2009, 08:51 PM   #7
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Default Re: Pentode grid question...

Quote:
Originally posted by Charlie Slee
Is it common for the screen grid to have the same polarity as the cathode?
It's not only common, but mandatory. What counts is not the screen voltage to DC ground, but rather the screen voltage to the cathode voltage. If you fix the screen voltage with respect to the DC ground, and there's any AC on the cathode, the Vsgk will not be constant. That'll cause an effect like Ultralinear operation, and you won't get the results you're expecting. If you bypass the screen to the cathode, then both will move together, and give a constant Vsgk. They seem to have gotten that right with the schemo you showed. Otherwise, somebody goofed, and forgot to connect the suppressor grid to the cathode. You can never go wrong by making that connection externally if it hasn't been done internally.
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Old 26th March 2009, 09:16 PM   #8
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Thanks Miles, very helpful
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Old 26th March 2009, 10:33 PM   #9
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from the link it shows that the ef86 would need to be linked from the outside.

<http://www.r-type.org/exhib/aai0113.htm>
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Old 26th March 2009, 10:51 PM   #10
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Thanks pointy,

This is one of the datasheets I have come across. Think its just the schematic that threw me.

Cheers guys, most helpful
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