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73.242.169.18
standard Hammond filament transformer (with proper 1.2A rating)?
Be a volt short of specification, but...
Really like the idea of AC heaters Even with a bias voltage of 30+ volts, with DC filament there will be 7.5 V difference in bias from one end of the filament to the other. Seems a lot.
Plus, hum would not be an issue as this amp will be used for 500Hz and up.
First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass
Follow Ups:
15% under voltage is quite a lot and I'm sure many will recommend against it.
From Eimac.
"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 properly.
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 point."
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."
But, Hammond makes a 14VAC 2A filament transformer with a center tap so using one half we should get 7.5VAC +/- not very much.
First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass
Rod is now on V9 of his regs and it's excellent. The older ones are very good too. I use the 10Y in filament bias, but that's as an input stage, not output.
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