how to use a filament transformer without center tap

[WU2D]

Sorry guys, I was still stuck on the grounded transformer with no center tap but I did not quote it so it looked like I was reacting to the diode comment. Even looks that way to me. Here is the schematic of The GSB-201 Linear. Notice how the filament transformer has no center tap and that one side is grounded. That is the situation which can cause hum. Since we are talking linears - Grounded Grid - I redrew the schematics and added the diodes.

Mike WU2D

[AB2EZ]

It is difficult for me to understand why the two-diode approach will not cause hum.
Non-linear circuits are difficult to analyze but...

1. The two-diode circuit will steer the plate current through one diode or the other as the filament transformer's voltage swings between positive and negative. I.e. if the filament voltage is across the back-to-back diodes, and if the filament voltage is not zero, then only one diode can be conducting. This behavior is similar to that of a mixer.

2. When the plate current is passing through the left-side diode, then a portion of the right-to-left AC voltage across the filament will add to the bias voltage. When the plate current is passing through the right-side diode, then a portion of the left-to-right voltage across the filament will add to the bias voltage.

3. Since the switching between the diodes is caused by (and therefore, synchronized with) the AC filament voltage... the result will be that the voltage added to the bias will be a rectified (approximately) 60 Hz signal. Thus it will contain harmonics of 120Hz (assuming everything is symmetrical.. e.g. the diodes are matched).

Stu

[KE6DF]

The two diode circuit is basically a full wave rectifier -- without a filter.

You will end up with a 120 Hz pulse train -- with each successive pulse on opposite ends of the filament.

But, if we are talking 5 volt tubes like 3-500z's then the pulse train is only 5 x 1.41 volts in amplitude and perhaps you won't hear it as it's only a small percentage of the 3KV plate voltage.