Vacuum tube - Electrode Insulation importance for audio?

[Pegasus123]

I am looking to buy some NOS vacuum tubes for audio purposes and there is more than one version of the vacuum tube.

What is Electrode Insulation and does it have any purpose for audio purposes?

Electrode Insulation (min)
Grid to all other elements: 500 Megaohms
Plate to all other elements: 500 Megaohms

Versus

Electrode Insulation (min)
Grid to all other elements: 100 Megaohms
Plate to all other elements: 100 Megaohms

 

[OldHector]

What tube?
What application?

 

[Pegasus123]

5814 and 5814A

12AU7/ECC82 type

 

[OldHector]

I was testing a batch of EBL21 earlier this year, and a few showed measurable resistance heater to cathode. When I looked up the criteria for failure it was quite a low resistance, something like 200K, so I think if they test ok it is nothing to fret about.

 

[Pegasus123]

After digging around I think it only affects vacuum tube tuners. Not much of a concern for vacuum tube preamps or amplifiers.

 

[Osvaldo de Banfield]

It is the resistance between pins with tube cold. The higher, the better. But in any case, external resistances are usualy much lower, say 2megaohms, so in the parallel both 100 and 500megaogms are negligible.

 

[Tubelab_com]

I had not heard of this spec until I saw this post. I have three different data sheets for the 5814, the generic GE GL-5814 sheet where this is not mentioned, the Sylvania 5814A sheet where 500 megohms is called out, and the Raytheon MIL spec 5814WA where 500 meg is spec'ed before their "torture test" and 50 meg is spec'ed after the test. This got my curiosity up, so I went through some other data sheets and found the Tung Sol 5751WA, a Mil spec 12AX7 variant. Here the test conditions for the "insulation of electrodes" is called out. For the grid test the grid is biased 100 volts more negative than the other electrodes, with negative 300 volts called out for the plate to other elements test. The heater is hot during both tests. They also refer to MIL-E-1C section 4.8.2.

I could not find anything relevant by Googling that spec, but I didn't dig too deep. Those conditions might be relevant to an instrumentation amplifier or something else that ran at very high impedances but are not important in an audio amp since you may not be able to find tube sockets that good today.