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Old 25th October 2006, 02:29 AM   #1
jarthel is offline jarthel  Australia
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Default max plate voltage rating in tube datasheet

is this max B+ voltage that can be supplied to the plate?

thanks for the help
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Old 25th October 2006, 02:37 AM   #2
Tweeker is offline Tweeker  United States
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Tubes have guidelines rather than it will break neigh instantaneously if exceeded ratings, but yes. This voltage is relative to cathode.
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Old 25th October 2006, 02:40 AM   #3
jarthel is offline jarthel  Australia
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Quote:
Originally posted by Tweeker
This voltage is relative to cathode.

just to clarify on that statement.

to measure plate-cathode voltage:

1. do I measure voltage from plate to ground and cathode to ground?

OR

2. measure voltage from plate pin to cathode pin?

thanks again
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Old 25th October 2006, 02:54 AM   #4
Tweeker is offline Tweeker  United States
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Well, I really wouldnt worry about it too much.

I mentioned that because if you were floating the cathode at say +250V over ground you could run the plate of a tube rated at 450V at 600V above ground, as it would still be seeing 450V across the tube. Heater to cathode issues would be another matter.


Another issue to be aware of is that new (or even some NOS) production tubes dont necessarily match all that closely to the old datasheets.
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Old 25th October 2006, 03:10 AM   #5
arnoldc is offline arnoldc  Philippines
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jarthel,


let's say my 2A3-

voltage on the 2A3 plate is 303V, voltage on the cathode is 46V.

303-46 = 257V which exceeds the 250V "maximum"

The tubes are still alive though- Sovtek 2A3 (I really wouldn't care if exceeded) and RCA bi-plate 2A3

ps.

I think it would be the plate dissipation one has to be worried with.

Again, take my case 257V * 56ma (46V / 880R) = 14.4W which is below the 15W max plate dissipation of the 2A3.
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Old 25th October 2006, 03:15 AM   #6
jarthel is offline jarthel  Australia
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I wasn't really concern on longevity. I just want find out how to calculate plate-to-cathode voltage

thanks again for the help
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Old 25th October 2006, 05:44 AM   #7
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Jartel,

What the tube sees is the potential difference between its plate and its cathode. It has no way of knowing what the absolute potential of each electrode is. So if the maximum plate voltage is defined, example, as 250v, it means there should be no more than 250v difference between plate and cathode.
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Old 25th October 2006, 01:10 PM   #8
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Quote:
is this max B+ voltage that can be supplied to the plate?
My answer is no.

In addition to the cathode voltage that must be accounted for mentioned in other posts, there is the voltage drop across the plate resistor. ( assuming there is one, and its not transformer coupled ). But bottom line is max voltage refers to voltage from plate to cathode.

But, having said all that though, I have seen spec sheets that do specify a maximum B+ supply voltage, not a maximum plate to cathode voltage. I have always assumed this was to protect against high plate voltage in cases when there is low or no plate current being drawn. Or is it just a way of "playing it safe" ?
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Old 25th October 2006, 01:43 PM   #9
SY is offline SY  United States
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1. The datasheets usually give two numbers for max plate voltage, one for a warmed up tube and one for a cold tube.

2. The numbers assume that the cathode is grounded, or equivalently, the numbers are given with respect to the cathode.

3. One has to consider the voltages at turn-on and turn-off to make sure that they're within the appropriate ratings.
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Old 25th October 2006, 01:55 PM   #10
dhaen is offline dhaen  Europe
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Another reason the answer is no, is that many ratings assume an inductive load. That means that with full signal swing, the anode voltage is nearly doubled. It is therefore acceptable to run some valves in some instances at higher voltages.

Eg a 2A3 with a maximum rated anode voltage of 300v operated in a class A transformer output stage is subject to about 500v peaks. If I wanted to use it in another application with a resistive load, I could choose a voltage higher than 300v without worries.
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