cl60 thermistor in BA3 preamp

fwt1712

Member
2012-07-22 10:02 am
Newby question here.

I am starting to build a ba3 preamp and am wanting to clarify the use of cl-60 thermistor as inrush protector and as last link to earth from power supply ground.

I am using block 50va 2 x 24 transformer and want to know if it makes sense to use a cl-60 as an inrush protector by connecting between the two primary windings (I am using a tubecad 24v regulated supply).

Just wanting to know if its a good, bad or unnecessary step. I presume slowing inrush doesn't hurt, but there is not much of a capacitance bank in a preamp so maybe it is not needed.

From the earthing perspective I understand the CL60 gives a useful last link to earth, offering some resistance in normal (no short etc.) situations but gives a reliable path to earth if needed. In the case of not having access to a cl60 I have seen high wattage 10ohm resistors used as this last step from power supply 'ground' to earth, is one preferred over the other.

Thanks in anticipation of your assistance,
Nigel.
 
....................From the earthing perspective I understand the CL60 gives a useful last link to earth, offering some resistance in normal (no short etc.) situations but gives a reliable path to earth if needed. In the case of not having access to a cl60 I have seen high wattage 10ohm resistors used as this last step from power supply 'ground' to earth,..............
Connect the Protective Earth (PE) wire direct to the Chassis.

Connect the Main Audio Ground (MAG) to the Chassis. This can be a direct wire connection, or via a Disconnecting Network.
This link from MAG to Chassis may have to pass Fault Current approaching or even exceeding kA

NEVER use a "high wattage 10ohm resistor" between your MAG and Chassis.
It will blow up and could leave your faulty equipment with a live connection waiting to be "touched" by the operator wondering why the equipment has suddenly gone quiet !
 
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.................I am using block 50va 2 x 24 transformer................
The normal fuse one would use for this transformer would be
Fuse rating = 3times 50VA/230Vac ~ 652mA
A T615mA, or T800mA, or T1A could be used to start this transformer direct on line.

If you want to try a close rated fuse, then use Fuse rating = 50VA/230Vac ~ 217mA
Try a T200mA, or T250mA, fuse but you may need to use a soft start.
Would it be worth the effort?

I use a few 3VA, 5VA or 6VA to power auxiliary circuits in my equipment. I use an F500mA to start these direct on line.