Heater Elevation in Fixed-Bias Amps

[dwinstonwood]

Hi all, I may be missing something obvious to everyone else here.

In cathode biased amps I have attached artificial center taps (with 100 Ohm resistors) to the power tube cathodes. It's simple, and I'm convinced it reduces hum.

So, where do folks elevate the heater center tap in fixed-bias amps? To a preamp tube cathode? Or, to a point in the bias voltage circuit?

Thanks!

[sluckey]

I would make a voltage divider that will give about 60v to 80v. Better to tap into the screen B+ node. You need two resistors and one filter cap.

[dwinstonwood]

Thank you sluckey! What do you not know?

[sluckey]

Thank you sluckey! What do you not know?

I don't know. You'll have to ask my brother. He's got that covered.   

[bmccowan]

In cathode biased amps I have attached artificial center taps (with 100 Ohm resistors) to the power tube cathodes. It's simple, and I'm convinced it reduces hum.

Hey DW, I am curious. Do you also do this when the PT has a center tap; taping off that tap and using the artificial, or do you connect the PT center tap to the cathodes? I ask because I usually experiment with both directly grounding the center tap and tying it to the power tube cathodes. I find the results vary. Sometimes better, sometimes worse, sometimes no difference. Most vintage Gibson amps have the tap tied to the cathodes and I always retain that.

[dwinstonwood]

Hey bmccowan, I've only elevated the voltage on three amps so far. Two use the Hammond 270EX which doesn't have a 6.3V center tap, so I did the "two 100 Ohm resistors" thing. My Vox AC4 uses the 270CAX which does have a center tap. All three amps are cathode biased, so I grounded the 6.3 at the power tube cathodes.

With the 270CAX, I used the PT's center tap and did not add an additional artificial tap. So, I've never tried using both.

My AC4 is in a small 11" x 7" chassis, and did have some hum when I first built it. Later, I grounded the 6.3 CT at the power tube cathode, and redid the grounding scheme with just one connection at the input jack (other than the power cord earth ground). For a single ended combo in a small chassis it is almost silent now.

[bmccowan]

Thanks for the info. Heater center tap and chassis grounds in general seem to often defy logical thinking. But, I tend to think that's a good thing.