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Separate transformer for heaters. How to reference this to ground?


2013-03-11 10:52 am
Hey guys.

I replaced the output tubes in an ECL86 amp with PCL86, but in order to do this, I had to install a separate transformer to get the necessary heater voltage.

The transformer's two output wires are twisted tightly, and directly connected to the PCL86's heater pins (4 and 5), and the hum is aggressive.

I assume I am doing it wrong, but I'm not 100% sure on how to rectify this. Other than literally rectifying it to dc, that is. ;)

Am I supposed to connect one of the output wires to ground?


Paid Member
2010-12-11 12:39 am
The easiest way to create an earthed heater supply is by adding two resistors in series across the transformer taps.The centre point of the resistor is then connected to ground . The resistors can be 100 ohm each and a wire wound type of 2 to 5 w should be used. It is prudent to measure the secondary voltage of the transformer under loaded conditions to see that it is at the correct level,as it is common to find a higher voltage with an under run item.
For a PCL86 the resistors should be a bit bigger than 100 ohms, otherwise you will waste too much power.

As a quick check, ground one of the heater wires. This will at least establish a DC reference voltage - heaters should never float. Balancing (via secondary centre taps or resistors) is a refinement; DC elevation is a further refinement.