Why does 6BM8 have so high max plate voltage?

[vax9000]

Comparing with other tubes, 6BM8 does not look like having extra distance between its plate and other parts. Why is it different then? Could I use other tubes labeled with 300V in circuits with 400 or 500V plate voltage?

[Tubes4e4]

Hi vax9000,

maximum allowed (statical) plate voltage under operating conditions is 300V for for the pentode section of ECL82/6BM8.

Maybe you mixed that up with the maximum allowed plate voltage without any cathode current going (tube shut down completely, or not heated), which is 550V, indeed. This voltage is usually designated as "Va0" in the spec sheets.

Regards,

Tom

[HollowState]

Curiously enough the 6BM8 does have some unusually high voltage ratings. According to my Amperex tube manual (Vol. 3 Semiconductors and Special Purpose Tubes) for the pentode section Vp w/o current is 900v. Max Vp is 600v. Peak pulse Vp is 2500v at 4% of one cycle or 0.8 milliseconds max. And peak inverse Vp is 500v. Max Vg2 is 550v without current & 300v with.

For the triode section Vp w/o current is 550v. Max Vp is 300v, and peak pulse Vp is 600v. These are absolute maximum ratings and typical operating levels are much lower for both sections.

The 6BM8/ECL82 is a one envelope triode/pentode designed for applications in medium power Hi Fi amplifiers. It is also suitable for vertical deflection in TV applications. In vertical deflection there may be some inductive kickback issues during retrace which would require a high peak rating.

As tough as this tube seems, I would not run it anywhere near these maximums.

Victor