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Why so different plate size for same model?
Why so different plate size for same model?
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Old 11th December 2019, 05:51 PM   #1
Elerion is offline Elerion  Spain
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Default Why so different plate size for same model?

Hi everyone.

Does anyone know why the plates of these 12AX7 are so much different?
I have a third one around, which is closer to the one on the right (image attached).

According to analytical studies, the amplification factor of a triode increases as the distance between grid and plate increases. But the depth difference of the plates of these two models is just huge. I didn't find any specific information about this. Any comment is very much apprecited.

This is not exclusive of 12AX7s. Same thing happends with a pair of 12AU7.
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Old 11th December 2019, 06:07 PM   #2
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Plate size depend of plate disipation, but as this tubes usually work well under their maximum ratings, perhaps decreasing plate size saves cost.
Compactron 6U11 has two units in a single 12 pin tube: a 12AX7 plus one unit from 12AU7, but wastes only 600mA of heater, so one unit is "free" saving 300mA.
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Old 11th December 2019, 06:10 PM   #3
SpreadSpectrum is offline SpreadSpectrum  United States
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I bet geometry is similar between the two. Looks like they rotated the cathode/grid 90 degrees in the second one. I'd bet the other dimension is about the same as what the picture on the left is.
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Old 11th December 2019, 06:19 PM   #4
Elerion is offline Elerion  Spain
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Quote:
Originally Posted by SpreadSpectrum View Post
I bet geometry is similar between the two. Looks like they rotated the cathode/grid 90 degrees in the second one. I'd bet the other dimension is about the same as what the picture on the left is.

And the grid support rods seem to be placed in the same orientation.
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Old 11th December 2019, 06:30 PM   #5
SpreadSpectrum is offline SpreadSpectrum  United States
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Quote:
Originally Posted by Elerion View Post
And the grid support rods seem to be placed in the same orientation.
Are they? I couldn't tell from the pictures. I do have many later manufacture 12ax7s that are rotated 90 degrees and don't have the welded plate seams in line with the grid support rods, so I thought this might be the same.
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Old 11th December 2019, 06:34 PM   #6
GoatGuy is offline GoatGuy  United States
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Little known fact…

Most of the mu of a valve depends on the inter-grid spiral spacing and the distance from the shell-of-the-cathode to each concentric grid. The anode can be near or 3 as far (on the scale of cathode-to-grid-(to-grid)*-to-anode measure), and won't change mu much at all.

However, anode-being-farther-away increases the maximum tension (i.e. anode-to-cathode voltage) that is allowable for a given design.

You can see this especially with high-tension TV flyback rectifier tubes.

The cathode is an itty-bitty thing, but the anode is large, and even has physically rounded edges, to suppress arcing … at 35+ kilovolts of blocking tension. If made smaller in radius, arcing would happen. The larger anode can also serves — probably obvious — to dissipate more waste-heat power.

Always good with flyback rectifiers.

So… so long as the mu and transconductance curves of the given 12AX7 (or any other family) design are adhere'd to, then whether the plates are larger, or smaller, doesn't much matter.

The exception would be if you're driving the AX7's at high tension, where the larger, further anodes would have a better chance of tolerating the tension without BLAM degeneration.

Just Saying,
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Old 11th December 2019, 06:44 PM   #7
N101N is offline N101N
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Go for a large plate if sound quality matters.
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Old Yesterday, 08:06 AM   #8
Chris Hornbeck is offline Chris Hornbeck  United States
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It's an excellent question, and one that interests lots of people, but I don't think you'll get any useful answers on an Internet website, even this one. Valves were built from proprietary experience and largely ad hoc, and all before any ideas of computer modeling.


The idealized 3/2 power function comes from infinite parallel planes with equal charge everywhere, among other gross simplifications. If any big semiconductor manufacturer were to spend the Big Bux to adapt their proprietary design software to vacuum valves, amazing new valves could be designed. (not *built* - that's another Big Bux to spend just to duplicate ancient tech) But that can't possibly be paid for, so can't happen.


Our current knowledge of valve design isn't very different from archeology. The same will soon enough be true for our current semiconductor processes. You don't believe me now, but half a century will tell the tale.


All good fortune,
Chris
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