41MP, triode connected tetrode, please explain

I have a 41MP triode, but looking at the interior, it is a tetriode. The second grid is internally connected to the anode via a small spiral. may be just to prevent mechanical stress or it may be a resistance. Can anybody tell me what the reason is for this construction?

• interior (103 KB)
• valve (108 KB)

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You are right, the second grid is tied to the anode.

Here is the explanation by the manufacturer:

In fact, with this trick the anode get's closer to the control grid and the claimed Mutual Conductance
of 7.5 mA/V is achieved.

41MP

Kind regards.

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I don't understand what's the advantage to mount a second grid close to the control grid and tie it to the anode, then mount the anode itself just as close to the control grid ?

Best Regards,

Jacob

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The advantage of this construction may be threefold.

1. No new design for trode or tetrode is necessary.
2. Reduced capacity between grid and anode unlike a triode having its anode in the distance of the 2nd. grid.
3. Heat dissipation can be increased while the first grid stays cooler.

Best regards,

Dietmar

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Dear Dietmar

I respomd to you, but really like to thank all of you for the answer. The reasons seem clear and indeed, the valve has a very high transconductance. Perhaps I like to try the 41MP one day in an amp to see how it behaves, interesting

regards

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Another advantage -due to the high Mutual Conductance- an additional lf-preamplifier may be obsolete.

On the other hand,
as already stated in the Manufacturers Info, no large output can be obtained.

The reason for this is given in the Technical Data below:

Because of the little distance of the second grid (at anode level) in respect to the control grid, the anode voltage is limited to 200 volts!

Kind regards

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Since this design was not continued in newer types, it may not have fulfilled as was expected. As only 12 models used this tube, it could be considered as a non-starter.

The reduced capacity between grid and anode may not be of much importance for an audio output triode.

While the heat dissipation of the anode can be increased, the thin windings of grid # 2 are even more endangered for overheating, hence grid # 2 is connected to the anode via a spiralised resistor wire as current limiter.

Best Regards,

Jacob

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