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Old 2nd April 2007, 10:02 PM   #1
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Default Questions on Triode Strapping

On some Pentode datasheets (E180F), it says that to turn the triode into a pentode, once should connect G2 to the anode. On others (EF86) it says to connect G2 to the anode, and G3 to the cathode. Does this just mean that on some pentodes G3 is connected to the cathode internally?

Also, even though there is no mention of it in the datasheets, is it a good idea to make the connection fro G2 to the plate via a 100R resistor, even on these small signal pentodes? And, if yes, what wattage does this resistor need to be?
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Old 2nd April 2007, 10:08 PM   #2
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Yes, many pentodes strap g3 to k internally.

Although it's never stated as an explicit parameter, you will find that pentodes tend to have a fixed ratio of anode to g2 current under normal operation. Typically, small-signal pentodes have a ratio of 4:1. Since your g2 stopper resistor is so small and the current is so low, a carbon type (less inductive) is fine. For a power valve such as KT66, an I2R power calculation might be required to confirm resistor suitability. I've never bothered with g2 stoppers on small-signal valves.
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Old 2nd April 2007, 10:44 PM   #3
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Now that you mentioned about using carbon resistors as gridstoppers because they are less inductive, I want to ask if its their non magnetic nature that is their true benefit. I have actually had metal film picking up fields by nearby cabling and introducing very low level zzz when installed in output valve pins as gridstoppers. Using carbon same value resistors in their place, killed that.
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Old 3rd April 2007, 09:51 AM   #4
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Carbon resistors are less inductive because the bulk resistivity of carbon is higher than that of the metal films (nichrome etc) used for metal film resistors. As a consequence, metal film resistors need more turns of the helix cut between the end caps to achieve a given resistance, and that increases the inductance.

Most resistors have steel end caps and there's no law that says carbon resistors might not have them.
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Old 3rd April 2007, 10:29 AM   #5
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The leads are copper in the Kiwame I used. And this must have played a role?
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Old 3rd April 2007, 10:43 AM   #6
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No, it is the reduced number of helix turns needed by carbon that reduces their inductance (inductance is proportional to the number of turns squared).
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Old 3rd April 2007, 10:53 AM   #7
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Reducing their magnetic profile I meant, due to copper leads. Valid thought?
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Old 3rd April 2007, 11:03 AM   #8
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No. Afraid not.
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