Edson Brusque wrote:
>      Can a speaker's magnetic field interfere with tubes/valves and
> damage it?

No. But vibration does.

> 
>      Let me try to explain the situation. I've made some combo guitar
> amps. They are just prototypes I'm making while studying tube circuits.
> They uses a pair of 6L6 in the power section and gives close to 40W
> maximum output power.
> 
>      Almost all the 6L6s (3 pairs) I've been using have developed tuble
> rattle. I still have to make a simple single ended power amp to
> individually test the tubes and discover wich ones are really
> compromised but it seens all 6 have different levels of rattle. Tapping
> the tubes with a finger while holding it near the ear, I can hear
> something vibrating inside the tubes. The frequency/volume of the
> vibrations changes from tube to tube and also depending on where I tap.

The word is "microphonic" when it creates sound through the amplifier.
All tubes have it to some degree. Negative feedback reduces the effect
of it.

> This is really intrigging me. The power section works at about 450V
> on the tubes plates. Plate current is about 45mA (each tube). Screen
> grid voltage is about 420V and current is bellow 2mA. The power
> dissipation seens to be well bellow the maximum allowed. I don't have
> any fan blowing air to keep the tubes cool, but there's some airflow by
> convection.

Turn the lights off and look at the plates. If they glow, they are too
hot. Otherwise they are OK.

A little blue is typical above 475 volts. At 450 volts blue means a
gassy tube.

> Now, one the power tubes are very near one of the speakers (it's
> two 10 inches). The physical layout, side-looking, is something like:
> 
>                      .
>     +--+  +--+      /|
>     |V1|  |V2|     / |
>     |  |  |  | .--|  |
>     '--'  '--' |  |  |
>                '--|  |
>                    \ |
>                     \|
>                      '

Magnetism is not a problem. But definitely try to reduce vibration, to
extend the life of the tubes.

Cheerful regards,

Bob

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