H&K Grandmeister 36, Black spots on Power Tubes

[Bonepark]

Hi all,

New to this forum, hoping to hear comments for this problem:

I discovered black spots on the power tubes, EL84, the spots most probably are due to heat. The middle ones have these spots, not the outer ones (GM36 has 4xEL84).
Question is why they look like that? I have been using the power soak which switches exactly these middle tubes off, can that "idle" of the tubes cause this? Or is it due to the tube type?

Otherwise the tubes are working perfectly, all 4 give the same 12 flashes when tested with the TSC. Also no other signs like purple color or red glowing in the plate cannot be seen.

First picture is from normal looking tube (on the side) and second (both middle tubes) has the 3 black spots inside the glass.

[Jon Snell]

That is caused by overdriving them, age or just too hot on the bias. The outer pair or inner pair can be switched off when required.
The cathodes will spray their coating on the inside of the glass where there is a hole.
Perfectly normal on older worn valves.

[vintagekiki]

Grandmeister_36_Head_Servicedokument_1A.pdf

[Bonepark]

Hi,

Thank you for the answers! So I'll start to prepare to change the tubes, good to know there isn't anything wrong with the amp.