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'[EE]Power tubes near speakers could cause tuble ra'
2009\05\12@002711 by Bob Blick

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

No. But vibration does.

{Quote hidden}

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.

{Quote hidden}

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

Cheerful regards,

Bob

2009\05\12@085748 by olin piclist

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Bob Blick wrote:
> Turn the lights off and look at the plates. If they glow, they are too
> hot. Otherwise they are OK.

Not for a 6L6.  The plate will glow a low red-orange during normal
operation.  When you can see the plate obviously glowing when the lights are
on is when you start to worry.

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

All 6L6 I've ever seen had a little gas.  A faint blue glow with the lights
off is normal.

Note that what I'm saying is specifically for a 6L6, which is a beam power
pentode.  What you say is applicable to most other tubes.


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