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Old 1st Oct 2019, 2:05 pm   #1
ValvoStef
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Default EABC80 question

Hi all,

I seem to have a problem understanding with the operation of a valve as a diode. Say, for example, take the diode section of an EABC80. I apply the heater voltage, then pin 2 and pin 3 act as a diode. Surely, there must be a voltage drop between anode and cathode. Data sheet does not mention this. In comparison to a silicon diode e.g. 1N4148 I should expect the A-K voltage drop of a thermionic diode will be significantly higher or is my assumption wrong?
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Old 1st Oct 2019, 2:56 pm   #2
kalee20
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Default Re: EABC80 question

Yes there will be a voltage drop. Usually, the detailed data gives the info.

Unlike the 1N4148 where forward voltage is about 0.6V and is pretty constant, for a thermionic diode the voltage drop is much more dependent on current. It follows a 3/2 law whereas the silicon diode is exponential. See attached plot for the EABC80, where for 20mA current the voltage drop is about 4.2V (note the diodes in this valve are not all the same!)

The good news is that at low currents, the thermionic diode far outperforms silicon and has a very low voltage drop.
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Old 1st Oct 2019, 3:03 pm   #3
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Default Re: EABC80 question

Thank for that, kalee20. Very informative, I learned something new.
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Old 2nd Oct 2019, 3:24 pm   #4
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Default Re: EABC80 question

At extremely low currents a thermionic diode can have a small negative voltage 'drop'. This may or may not be an advantage.
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Old 2nd Oct 2019, 4:59 pm   #5
kalee20
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Default Re: EABC80 question

They can indeed - which implies they can actually SUPPLY a few microwatts of power, rather than dissipate it.

However, the curve in Post #2 does not seem to show much of this - though the scale doesn't give much away at the microamp level.
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Old 2nd Oct 2019, 5:17 pm   #6
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Default Re: EABC80 question

Yes, it may just be a few tenths of a volt at a microamp or so. It can be enough to disrupt a high impedance AM detector on small signals. A possible solution is to run the valve heater a bit low so the cathode is cooler, but not enough to mess up the triode in the same envelope.
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Old 2nd Oct 2019, 6:23 pm   #7
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Default Re: EABC80 question

The diode current starting point, Vd max (-ve)) for an Id of 0.3uA for the EBC41 is the same as its grid current starting point in that valves data, probably the same for the AM detector diode in the EABC80.

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Old 2nd Oct 2019, 6:58 pm   #8
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Default Re: EABC80 question

That few tenths of a Microamp of bias caused by the fizziness of electrons around the cathode can come in handy though - specially in low-signal-level triodes, where - with something like a 10M Ohm grid-leak resistor - it can provide all the bias you need.
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Old 2nd Oct 2019, 7:20 pm   #9
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Default Re: EABC80 question

ISTR that the triode section of the EABC80 under discussion was indeed frequently used and maybe even specified for contact biassing- the clue when puzzling over a circuit that seems to be otherwise short of bias arrangement is that afore-mentioned high-meg resistor.
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Old 3rd Oct 2019, 11:17 am   #10
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Default Re: EABC80 question

Yes, useful when you want low hum (the cathode can be directly grounded) or low cost (no cathode components needed). The downside is that you need a lowish impedance source, or small signals, or accept some nonlinear distortion due to grid current.
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