Tube Question/ Mesa BB750

[MoStevens]

I was digging around in my Pops' garage and uncovered a BB750 that's been sitting there for years untouched. I brought it home and fired it up; of course, it smelled dirty but sounds great to my ears (I only have a PF210 to run it through).

My question is this? I took the cover off to dust it out (again, really dirty) and afterwards turned it on again to hopefully get rid of the smell (much better now). I noticed only one tube glowed brightly and the others were very dim. Is this normal?

 

[agedhorse]

Normal, the heaters are different styles in different tubes.

 

[MoStevens]

Thank you!

 

[MoStevens]

I just realized... they’re all (4) 12AX7’s. Still not the same as far as “glow?”

 

[mikeswals]

As long as they're all glowing they're fine.
The manufacture process is not perfect... Some tubes you'll see the tips better than others.

 

[agedhorse]

I just realized... they’re all (4) 12AX7’s. Still not the same as far as “glow?”

Maybe not. Even identical brand/model of tubes will have manufacturing differences. Different brands and different versions/models of seemingly the same tube certainly contain differences.

 

[beans-on-toast]

In standby, what you see is the result of a glowing heater filament. A heater is necessary for the tube to work, it heats the cathode to the point where is boils off electrons so it gets quite hot. If your preamp tube is working, the heater is working. The amount of heater glow is unimportant.

The heater is shielded by the plate (anode) and the cathode. Some tubes glow brighter because the heater is less covered. It depends on the design of the tube. In general, preamp tubes glow less brightly than power tubes.

A dissected tube.

 

[808State]

usually a larger plate pre tube covers the heater more
so they look dimmer.