how to connect an EL34 pentode into a triode and 100ohm resistor between plate and g2

[jarthel]

here's the Mullard datasheet: http://www.mif.pg.gda.pl/homepages/f...129/e/EL34.pdf

here's the Phillips datasheet: http://www.mif.pg.gda.pl/homepages/f...030/e/EL34.pdf
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1. According to Philips (top of page 7), I need to connect grid2 to plate and I'll get a triode.

BUT according to mullard (page3), to get a triode connection, it's grid2 to plate and grid3 to cathode.

so which is right? It is possible as well that I misread the datasheet.

ps. I've seen replies were people have recommended the Mullard way but Philips must know something if they are recommending it differently.

2. I've seen replies where people have suggested a 100ohm resistor between plate and grid2. But I was looking at the mullard and philips datasheets and it seems I only need a piece of wire to connect the two pins.

what is the purpose of the 100ohm resistor?


Thank you for the help

[HollowState]

Hi jarthel,

You asked:

so which is right? It is possible as well that I misread the datasheet.

Actually, they're both correct, and you did not misread anything. The fact is that Philips just didn't mention the suppressor grid (G3) being connected to the cathode as it should be. Either they assumed one would already know this, or there may be versions where it's internally connected.

In any event, to make an EL34 a triode, connect G2 (screen) to the plate. And G3 (suppressor) to the cathode. Very technically, a straight wire between the screen and plate is all that's needed. However, in practice it's very common (and generally preferable) to use a low value resistor of 100 ohms to link the two. Doing so provides a small amount of decoupling as well as current limiting for the screen. No resistor is used between the suppressor and cathode. Just a piece of wire.

Incidentally, the suppressor and cathode should always be connected together whether in triode or pentode mode. By keeping G3 negative, with respect to the anode, it fulfills it's duty of repelling (or suppressing) secondary emission from the plate.

Victor

[jarthel]

much appreciated. thank you

[Tubes4e4]

Hi Victor,

However, in practice it's very common (and generally preferable) to use a low value resistor of 100 ohms to link the two. Doing so provides a small amount of decoupling as well as current limiting for the screen.

Sorry to say so, but I think you have a double misconception here; but never mind, they are very common ...

Neither the decoupling of g2 is wanted to make a triode out of a tetrode or pentode (instead, g2 shall just swing along with anode; effectively adding its area to the anode area - but when it is "decoupled" (even a small bit - think UL!), it just ain´t triode mode anymore), nor does a typical 100 ohms resistor between anode and g2 do any current limiting whatsoever (and for what reason to boot with? Certainly not because g2 dissipation limits would require) - just do the very simple math and you will see.

The *only* purpose of this resistor is to act as a (screen-) grid stopper. Hardly needed when the anode connection is available at a base pin as with usual audio pentodes; but can make sense with top-capped tubes, like 807 and such.

Tom

[jarthel]

Originally posted by Tubes4e4
Hardly needed when the anode connection is available at a base pin as with usual audio pentodes

Tom

It seems to me that you're suggesting the resistor is not needed. am I right?

[HollowState]

Hello Tom,

I read with interest your response and do agree with your words to a large part. Unfortunately, I am not an experienced writer and may sometimes use incorrect syntax to convey what I really mean. Decoupled may have been a poor choice of terms. But I do fully understand the concepts of this circuitry.

If you consider how a stopper resistor works, the main thing it does is to slightly desensitize the element by decoupling it from the circuit and lowering the Q to prevent oscillation. Decoupling can be a subtle event, or more direct as in branch-out circuits. It's really a matter of degree. At least that's how the term is used here in the U.S. Prehaps Europeans have a different concept of the meaning.

As far as the current limiting goes I agree that, pragmatically, it is of little value. Especially since the screens are at anode potential (not higher as with UL operation) dissapation should not be an issue. But strictly speaking, by virtue of the fact the resistor was physically and electrically in the circuit, there would be some current limiting. Albeit only a tiny amount.

And btw, I like your icon. I'm a cat person too.

JARTHEL,

You do not need to use this resistor. But you can if you want to. Many designers and builders use it anyway as a preventive just in case. It's up to you.

Victor

[Tubes4e4]

Hi Jarthel,

Originally posted by jarthel
It seems to me that you're suggesting the resistor is not needed. am I right?

Right, it is not needed when you want to get triode characteristics from a pentode or beam power tube.

But it may help preventing parasitic oscillations in certain cases.

In this sense, a resistor of the usual 100 ohms value does not hurt, but definitely does not serve the purpose of "partly decoupling" nor "current limiting".

Tom

[bear]

IF by connecting the screen to the plate the dissapation of the screen is exceeded, the tube's life will be decreased considerably. In such cases the resistor should be scaled to keep/prevent excessive screen dissapation at full/high power levels.

It seems that the current crop of non-NOS EL34/6CA7 tubes do not have the same screen dissapation capabilities as the NOS did. For that matter, none of the import tubes (eg 6550, 6L6GC, etc.) appear to really equal the NOS tubes in that respect.

_-_-bear

[Tubes4e4]

Hi Bear,

Originally posted by bear
IF by connecting the screen to the plate the dissapation of the screen is exceeded, the tube's life will be decreased considerably.

Can you please give an actual, real world audio duty operation point example for your "if-clause" with EL34 in triode mode?

In such cases the resistor should be scaled to keep/prevent excessive screen dissapation at full/high power levels.

By a resistor that is of a value high enough to be able to "limit" (nah, better lets say "lower") g2 current, hence g2 dissipation, significantly, you would have left triode mode by far already.

Tom