Chris asked:
"will it kill it right away. What is the max filament voltage any of you
have put on this tube and made it last and preform to your satisfaction?
thanks for any help you can give"
There are two separate questions. First, it will not kill it immediately.
It depends upon the tube and how much over-voltage is applied. Many tubes
today are run above design voltage. However, these are not expensive, high
power tubes but rather tubes in old radio receivers. They are operated
above design voltage because line voltage has increased since the radios
were designed 50 years ago. The radios may last for decades with slightly
higher voltage.
However, the same problem arises for vacuum tube filaments as 120 volt
lamps. The higher the voltage, the less time they will operate. The
difference, however, is that a light bulb can be replaced for less than a
dollar. A new final tube might cost $700 or more. Even the old 4-250
might cost $50.
I understand a few Henry amplifiers applied 5.9 volts to a 5 volt tube.
The tubes could last for years. However, they will fail and they are
likely to fail substantially before you want them to. Filament life span
will decrease rapidly if the voltage is raised above the specified voltage.
In the broadcast industry, it is common to set the filament voltage below
design voltage. Voltage is set by emission required to operate at a
desired input. As the filament is used up, voltage is increased to retain
desired emission.
Inrush current can be damaging to some - but not all tubes. Limiting the
starting current may be advised if the transformer is capable of
substantially more current than the tube needs. The reason for this is
that when the filament is cold, it draws more current. Too much current
can warp the filament.
In short, you can exceed filament voltage, screen voltage or plate voltage.
The tube will continue to work for some time, depending upon what the
excess is. But, why? If you want the 4-250 to operate as a flashbulb, put
240 volts on the filament.
73, Colin K7FM
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