Heater circuits

[Paul P]

Suppose the following power supply :



I've read about different ways of reducing hum from the heater supply. One
is to add a balancing pot between the two legs of the supply (and I presume
disabling the heater coil center tap) :



Another way is to add an offset voltage to the center tap :



Or do both :



These last two were taken from the RCA RC-30 manual and I believe it would provide
about 50 volts of offset at 1ma through the dividing resistors in the case of
a blackface Deluxe Reverb.

Are these circuits worth it ?

Paul P



-

[MWJB]

If you have a centre tap use it, if not use 2x100ohms (or a pot) as a virtual centre tap.

Neither of the 2nd 2 courses of action should be necessary on a well laid out amp.

[Bruce / Mission Amps]

Suppose the following power supply :

I've read about different ways of reducing hum from the heater supply. One
is to add a balancing pot between the two legs of the supply (and I presume
disabling the heater coil center tap) :

Another way is to add an offset voltage to the center tap :



These last two were taken from the RCA RC-30 manual and I believe it would provide
about 50 volts of offset at 1ma through the dividing resistors in the case of
a blackface Deluxe Reverb.

Are these circuits worth it ?

Paul P



-

The number three picture is the way I do all my custom amp builds but I don't use those exact resistor values and I always put a small 22uF@100v cap across the grounded resistor (47K in the example) as that new DC tap is right off the rectifier, isolated from the main filter caps and you might pick up some B+ rail ripple hum.

[Paul P]

I just noticed that in the hi-fi amp circuits in the RCA manual they take their
voltage off of B+ right after the choke, which should be pretty smooth. You
can't do this with a standby switch though, so I see the necessity of the
cap across the grounded resistor.

Paul P

[Steve Conner]

The two different methods address two different causes of heater hum. The centre tap (real, virtual, or hum bucking pot) is for electrostatic coupling inside the tubes and in the heater wiring. I once built a high gain amp with the hum balance pot, and it made a big difference to the dirty channel. The setting that cancelled the hum wasn't in the middle, either!

Adding a positive DC voltage to the heaters helps to combat emission from the heaters. Much as the tube maker tries to stop this, the heaters sometimes get a little cathode material on them, and even if they are clean they probably still give off some electrons anyway.

So you can use either method or both together.

[rathmann]

Could the second problem (that of heater emission induced hum) also be resolved by referencing the heaters to the cathode in a cathode biased amp?

Also, the nice thing about using a pair of 100ohm resistors is that you know how balanced they are. Might not be so lucky when using a center tapped heater winding.