Good evening everyone,
If I need to use an "external" filament transformer, is there any advantage to buying one with a center tap and grounding the center tap.
Thanks
Ray
If I need to use an "external" filament transformer, is there any advantage to buying one with a center tap and grounding the center tap.
Thanks
Ray
It depends on your circuit, your tubes, etc.
Give us more information.
A schematic with details is helpful.
"All Generalizations Have Exceptions"
Give us more information.
A schematic with details is helpful.
"All Generalizations Have Exceptions"
It is usual to ground the centre tap to ensure a valve with a leaky cathode-heater does not hum.
Some like to elevate the heaters to a positive voltage, there are no technical advantages of doing this as long as the valves do not exceed their safe working limitations but either way centre tap is better than a virtual tap using 100R resistors.
Some like to elevate the heaters to a positive voltage, there are no technical advantages of doing this as long as the valves do not exceed their safe working limitations but either way centre tap is better than a virtual tap using 100R resistors.
Only if it is a directly heated rectifier. There are 5V heater rectifiers which are indirect and then you take the DC from the cathode (which is connected to one side of the heater). Taking DC from the CT in this case will add a little 50/60Hz hum to the 100/120Hz ripple.
Here are more considerations, which is why we ask:
How are you going to use the xfmr?
If you are going to use a DHT in circuit, you either need a Very well constructed tube and a center tapped transformer, or:
If your tubes are not good enough, it will not “balance out” the 60Hz filament voltage.
As a result, you will get hum. In that case, put a shrink tube over the center tap, and
use a potentiometer to “balance” the tube hum.
The wiper becomes the needed (“almost center”) tap.
There are many ways to do this. Also, a couple of fixed resistors in parallel with the pot will prevent the filament circuit from floating if the pot wiper opens. Some circuits give more range, some give a finer setting of the exact balance for a particular tube.
An additional problem with AC on DHT is that some tubes still hum. That is the action of AC current through the filament causing a magnetic field, with the filament attracted to the steel
plate (often attracted more on one side than the other side due to imperfect spacing (not symmetrical).
With AC filaments and DHT, there is the intermodulation that occurs as the grid voltage goes to zero. One end of the filament goes positive, while the other end of the filament goes negative.
With the grid at zero, and one end of the filament negative, the grid draws grid current (we just
clip before the grid goes to zero, but at a rate that is 2 times the AC line frequency (i.e. 120Hz for 60Hz power).
Don’t get me wrong, I like DHT, and have used them both ways, AC and DC filaments.
DC has its own problems, but that is a discussion for another thread.
How are you going to use the xfmr?
If you are going to use a DHT in circuit, you either need a Very well constructed tube and a center tapped transformer, or:
If your tubes are not good enough, it will not “balance out” the 60Hz filament voltage.
As a result, you will get hum. In that case, put a shrink tube over the center tap, and
use a potentiometer to “balance” the tube hum.
The wiper becomes the needed (“almost center”) tap.
There are many ways to do this. Also, a couple of fixed resistors in parallel with the pot will prevent the filament circuit from floating if the pot wiper opens. Some circuits give more range, some give a finer setting of the exact balance for a particular tube.
An additional problem with AC on DHT is that some tubes still hum. That is the action of AC current through the filament causing a magnetic field, with the filament attracted to the steel
plate (often attracted more on one side than the other side due to imperfect spacing (not symmetrical).
With AC filaments and DHT, there is the intermodulation that occurs as the grid voltage goes to zero. One end of the filament goes positive, while the other end of the filament goes negative.
With the grid at zero, and one end of the filament negative, the grid draws grid current (we just
clip before the grid goes to zero, but at a rate that is 2 times the AC line frequency (i.e. 120Hz for 60Hz power).
Don’t get me wrong, I like DHT, and have used them both ways, AC and DC filaments.
DC has its own problems, but that is a discussion for another thread.
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It is usually not needed. You can use a low resistor value with center tap to take advantage of a non-center tapped transformer.
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