Measuring Triode-Connected Pentode Curves - diyAudio
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Old 1st April 2006, 06:04 PM   #1
rdf is offline rdf  Canada
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Default Measuring Triode-Connected Pentode Curves

Hi all. I was half way through measuring triode data for a pentode when it occured to me there are two possible methods. The question relates more to standard practices, which manufacturer data sheets don't make clear. Do published plate curves for triode-connected pentodes include the screen current? Since screen current passes through the OPT and contributes to the output my measurements will include it but what am I seeing on data sheets? Thx in advance.
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Old 1st April 2006, 07:19 PM   #2
sajti is offline sajti  Hungary
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If You connect together the anode and the screen, You are not able to separate the currents anymore. So screen, and anode current makes the "triode anode current" together.
If You think this is the good way, because both electrodes are connected to the OPT

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Old 2nd April 2006, 01:35 AM   #3
rdf is offline rdf  Canada
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Thx Sajti, that's the method I went with. Just curious if it's the one manufacturers use. The datasheets typicaly label the vertical simply 'plate current' and make no mention of screen.
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Old 2nd April 2006, 09:07 AM   #4
lndm is offline lndm  Australia
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I think the point was that when the screen is connected to the plate, it is the plate, electrically. IMO it makes no sense to measure the current of a triode connected pentode at a point between the plate and where the plate meets the screen, and to only measure that branch. I have seen ambiguities before, and this one seems more obvious to me than some.
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Old 3rd April 2006, 07:40 PM   #5
ilimzn is online now ilimzn  Croatia
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Plate current on triode conencted graphs DEFINITELY mean pentode plate + pentode screen total current, as the two conencted together represent a triode plate.
It would, however, be nteresting to know how the currents are actually distributed between pentode plate and screen in some applications, in particular where it may be suspected that screen dissipation is a limiting factor, or when attempting to run a tube outside published specs(*).

(*) When triode curves are not published, the safe approach is to limit plate voltage at the maximum published spec screen voltage for the pentode. This, however, may not be a true maximum rating in triode mode as there are other factors to consider - case in point being tbelab's experiments.
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Old 3rd April 2006, 09:02 PM   #6
rdf is offline rdf  Canada
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Hi ilimzn. You illustrate the ambiguity perfectly because the graphs are definitely not labelled 'plate + screen current' and designers as a rule don't tie them directly together. A resistor separates them at a minimum. I'll admit to likely overthinking where a simple answer exists and did my curves using the combined screen/plate currents separated by the planned 1000 ohm resistor.

You bring up another great point I wanted to ask. My understanding is the maximum screen voltage is specified to prevent flashover to the grid. Is that correct? If so it appears to provide a guidline for using a tube in circuits in which not all parameters are stressed. Transmitter tubes are a great example. Let's say the max allowable grid swing is specified at +-100, screen voltage as 400, plate voltage at 1200. This implies a peak permissible grid-screen voltage of 400 (screen V) - 100 (grid bias) -100 (grid max neg swing)=600. If used for a low power audio amp requiring +-20 volts grid swing, does this raise the permissible screen voltage to 600-20-20=540 volts? This analysis also ignores that transmitter tubes are spec'd at tens of megahertz. Audio's greatly reduced bandwidth further implies less hammering of the screen-grid capacitance, though it might not be a significant consideration. Wishful thinking?
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Old 4th April 2006, 12:05 AM   #7
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I have never thought about this!

Most tube data sheets state that screen is tied to plate. I have not noticed that a resistance is inserted in the screen line - not for my data sheets for KT66, KT88, 6L6 or EL34. In practical circuits it is usually about 100 ohm, and I am not sure that this is not often just a pious gesture. I grew up with tubes but cannot recall that anybody ever mentioned parasitics as a result of not putting a resistor in. But for the cost of a resistor.....

Inserting appreciable resistance would bring G2 transfer linearity into the picture, and with that resistor tied to plate the matter is not simple. I have never seen Vg2/Ig2 characteristics vs. G1 voltage. It is known that for UL operation of EL34s distortion is lower with a G2 serie resistor of 1K, but that is the only tube where the data sheet specifies it. Many UL circuits for 6L6, KT66, KT88 use no screen resistor.

Screen dissipation is part of the limiting factor. In pentodes, where maximum screen current occurs when the plate is at the lowest part of its swing, it is of greater importance than in triode connection where the screen goes down with the plate.

Also, tube data sheets are interesting. Maximum voltages for the 6L6GC is 500V plate and 450V screen, but the latter is 500V under UL conditions. For the 7027 it is 600V and 500V respectively, but with G2 spec. raised to 600V under UL conditions. For the EL34 the max. plate voltage is 800V and screen 425V, but at zero plate current the ratings rise to 2KV and 800V respectively. (This seems to be an indication of maximum swing.)

I once discussed this with a friend who previously worked at the Philips Tube Manufacturing Plant at Eindhoven, Holland, where he tested tubes. He said that data sheet maximum voltages are very conservative, for worst conditions. They may be safely exceeded depending on the application and operating conditions. But by how much.......? With some uncertainty involved in the quality of present-day tubes, such a decision is not easy.

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Old 4th April 2006, 12:51 AM   #8
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I have just read another thread "Diode Trick", which members reading here may find interesting - it is broadly allied to this subject.
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Old 4th April 2006, 07:58 AM   #9
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Hi folks,

I have traced quite some pentodes as triodes, look here.

Also, I have studied and compared factory sheets with own measurmensts. So maybe I can contribute a bit from practice.

1. The plate current shown in manufacturers sheets for triode(d) op actually always is the sum of Ia + Ig2, measured at the junction of plate and g2 feeds, regardless of a 100 ohms resistor present between g2 and that junction, or not.

2. As for precision, a 100 Ohms resistor used between g2 and plate realy doesnīt make a difference at all. The voltage drop of at most few volts at g2 is absolutely neglegible, you will see no difference at all between a trace made w/ or w/o a 100 Ohms resistor between g2 and plate.

3. In measuring (and amp building) practice, I strongly suggest to always use the 100 Ohms resistor indeed, tied as close as possible to g2 pin. Also, always use a g1 stopper when tracing curves. If you donīt, be aware that especially with medium and high gm tubes you almost inevitably will get parasitic oscillations - you may not even notice them except maybe for "strange" or changing current readings, because they might occur so deep into the RF band (two-to-three digit MHz figues) that even your cool 50Mhz scope might not resolve them! You have been warned.

4. If accessible at a separate pin, always tie g3 to cathode directly. Tying g3 to plate can give considerably different curves (usually higher mu and higher gm, linearity usually gets considerably worse).

4a. If the tube you are tracing has internal shielding accessible at a separate pin, use it tied to cathode, too.

5. Give tubes, especially NIB ones, *plenty* of time to stabilize and settle before tracing, and then some more. Previously unused indirectly heated cathodes need time to develop their active cathode surface, indeed. This is simply due to the way cathodes are manufactured. They wonīt stabilize when only being heated, so you have to apply reasonable plate current + voltage to stabilize new tubes, too. Usually, I run them at about 50%-75% max Pd during stabilizing, to give a figure. Donīt underestimate this hint! While some small signal pentodes might take only 1/2 an hour to reach stable long term conditions (that is what you want to trace!), bigger types might take much longer. I had some NOS 807 that took almost 2 days of simmering until they stopped to drift! They probably had old style inter-layer constructed cathodes.

5. Always, I said always, measure several samples, or you wonīt know if you are just tracing a "typical" one or an "untypical" (or bad) one. You donīt need to make complete traces to be sure, just pick a few (say, 3 or something) intelligently chosen op points and compare those among all your samples. If in doubt, use the tube for tracing that comes closest to the average results of the "healthy" ones out of your samples.

Tom
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Old 4th April 2006, 12:32 PM   #10
SY is offline SY  United States
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Nice work!

Tom, what test setup did you use to generate the curves?
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