• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

GM70 lifetime

Thanks for the reply. Just these days I was reading Kevin's posts about the GM70, and he wrote that he is replacing the GM70's every 1000 hours, to avoid nasty surprises.

Do you have a more specific link?
Thanks! Erik
No, but I remember Kevin saying he was getting 1500/1700 hours on his SET amp with no soft start, I suppose with softstart, dalay and 60W dissipation it can reach 3K hours.


Paid Member
2008-06-03 5:18 am
Big Boss at Eimac say :

"The key to extending the life of a thoriated tungsten filament
emitter is to control operating temperature. Emitter
temperature is a function of the total RMS power applied
to the filament. Thus, filament voltage control is temperature
control, because temperature varies directly with voltage.
Figure 8 shows that useful tube life can vary significantly
with only a 5% change in filament voltage.
NOTE: If the filament voltage cannot be regulated to
within ± 3%, the filament should always be operated at
the rated nominal voltage specified on the data sheet.
It should be noted that there is a danger to operating the
emitter too much on the “cold” temperature side. It may
become “poisoned.” A cold filament acts as a getter; that
is, it attracts contaminants. When a contaminant becomes
attached to the surface of the emitter, the affected area of
the emitter is rendered inactive, causing loss of emission.
Should this happen, recovery is possible by operating the
filament at full voltage for a period of time. Closely monitored
operation of the filament at slightly below the rated
nominal voltage, however, can extend tube life, if done
Note that these filament management techniques should
not be applied to oxide cathode tubes, such as the
3CX1500A7/8877. Running oxide cathodes too cold will
result in internal arcs; and once that happens, an oxide
cathode tube is not recoverable.
Of equally great importance to long tube life is the temperature
of the other tube elements and of the ceramicto-
metal seals. Element temperatures can be held within
proper limits by observing the maximum dissipation ratings
listed in the tube’s data sheet. Tube seal temperatures
should be limited to 200°C at the lower anode seal under
worst-case operating conditions. As tube element temperatures
rise beyond 200°C, the release of contaminants
locked in the materials used in manufacturing increase
rapidly. These contaminants can cause poisoning of the
filament, and in turn loss of emission.
When a new power tube is first installed in a transmitter, it
must be operated at rated nominal filament voltage for the
first 200 hours. This procedure is very important for two
reasons. First, operation at normal temperature allows the
getter (a device that chemically binds tube contaminates)
to be more effective during the early period of a tube’s
life, when contaminants are more prevalent. This break-in
period conditions the tube for future operation at lower
filament voltages. Secondly, during the first 200 hours of
operation, filament emission increases. It is necessary for
the life extension program to start at the peak emission

In another doc :

"Filament Voltage
Proper filament v o I t a g e and the allowable departures therefrom are usually specified in the tube data
sheet. In general, quick-heating thoriated tungsten filaments used in the larger power tubes may be operated
over a range of plus or minus 5-percent of the recommended voltage. Slower heating cathode-type
filaments used in small power tubes usually have a filament operating range of plus or minus 10-percent
of the recommended voltage. External anode tubes have a filament voltage range of plus or minus 5-percent.
Some variation in power output must be expected as the filament voltage is varied in this range.
Lower than normal filament voltage will impair the power output of the tube, and higher than normal voltage
will cause critical parts of the tube to run at an excessive temperature, and may even cause damage
to the grid structure in extreme cases. In passing, it should be noted that an inexpensive a.c. type meter
of plus or minus five-percent accuracy can tell the operator little about filament voltage, when the voltage
must be held to the same value of accuracy. Use a good filament voltmeter of known accuracy."

Eimac Extending.jpg