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Old 5th June 2014, 11:17 AM   #1
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Default E88CC / 6922 Datasheet Limiting Values

In the Mullard E88CC datasheet, there are a number of different limiting values specified for Anode voltage:

Anode voltage, Va0 max 550V
Anode voltage (zero cathode current) Va max 400V
Anode voltage Va max 220V
Anode voltage (max anode dissipation 0.8W) Va max 250V

Can anybody explain to me what the first of these is?

The other three seem fairly self explanatory although I'd be grateful if any of you could confirm that my understanding is correct based on the following. In my design, I have the quiescent anode voltage set at about 220V with a cathode current of 4.5mA, giving an anode dissipation of 990mW. With full input signal level, the anodes swing up to 350V, when the instantaneous cathode current is 3.7mA, and down to 80V, when the instantaneous cathode current is 5.4mA. Am I operating the valve outside its rated limits?
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Old 5th June 2014, 11:32 AM   #2
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The VaO is the maximum voltage that can be safely placed on the Anode without fear of damage by arcing. Most double triodes of this design operate happily with 400V HT available. Valves are great, over current and the Anode heats up and the current reduces and causes that pleasant compressing/limiting effect. The only real issue with over current is if the Cathode is at the wrong temperature, too cool, that will strip the Cathode.
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Old 5th June 2014, 11:39 AM   #3
nl3prc is offline nl3prc  Netherlands
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VAO means that the voltage can not be greater than 550 volts when the tube is still cold ( not heated yet )

VA max means max voltage on the anode when the tube is heated and oparational
Measured between cathode and anode
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Old 5th June 2014, 12:14 PM   #4
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So if the anode voltage swings to 350V with a current of 3.7mA at maximum signal excursion, am I at risk of damaging the valve?
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Old 5th June 2014, 12:32 PM   #5
nl3prc is offline nl3prc  Netherlands
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The given is in WATS zo VOLTS * CURRENT = Watts

Zo when you have 250 v and .003 A this is 0.75 Watts

250*0.003=0.75

Always make sure that you don't go over the max Watts given in the datasheet

Last edited by nl3prc; 5th June 2014 at 12:43 PM.
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Old 5th June 2014, 12:48 PM   #6
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Plate dissipation in datasheets is average, so you can allow for peaks higher than max as long as the average is not over max limit.
These tubes (6922) are low voltage tubes, good idle state plate voltages around 90-150V. Sounds to me you need to change to 6SN7 or something similar.
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Old 5th June 2014, 01:19 PM   #7
DF96 is offline DF96  England
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Quote:
Originally Posted by JonSnell Electronic
Valves are great, over current and the Anode heats up and the current reduces and causes that pleasant compressing/limiting effect.
How does overheating the anode cause a current reduction?
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Old 5th June 2014, 01:39 PM   #8
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When the Anode starts to glow dull red, it is less efficient at absorbing electrons and so the flow slows down and the current is limited.

Under biasing an EL34/6L6 show what happens on a grand scale. The Anodes glow. If you use an 813 in class C and overload it, the Anode glows but as they have carbon Anodes, they continue to draw excessive current and eventually the glass envelopes implodes. I have done it!
Carbon doesn't emit electrons.
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Last edited by JonSnell Electronic; 5th June 2014 at 01:43 PM. Reason: Extra description
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Old 5th June 2014, 08:14 PM   #9
DF96 is offline DF96  England
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OK. I have never heard of that effect. Are you saying that heat enhances secondary emissionfrom a metal? Does it form a space charge next to the anode?
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Old 7th June 2014, 09:54 PM   #10
Mikey R is offline Mikey R  United Kingdom
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Looking at the ECC88 and E88CC datasheets, it looks like you can get huge output swing at very decent current levels.

Im a little surprised these valves arent used more regularly in LTP phase inverters capacitively coupled to power valve grids - is there a reason why other valves are favoured for the PI, such as the ECC81 or ECC83? Im more familiar with guitar amp schematics so maybe in the world of hifi, different valves are preferred.

Sorry for dragging this a little off topic.
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