[Amps] EIMAC 3-400 vs. 3-500 how to ID??

[bearlabs at netzero.net]

Due to lack of personal experience over the years with these tubes
I don't know the answer, do you?

 - Is there a reliable way to ID an EIMAC 3-400 vs. a 3-500 beyond
reading the silkscreened info? Assume it is rubbed off completely.

 - Q2. is there *any* reason in a standard ham two hole linear amplifier to *not run a 3-400 with a 3-500?

 TIA,

     _-_-bear wb2gcr

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