Heater Hum Reduction

[Structo]

I was reading the other day about hum reducing techniques and was reading about heater circuits.

One article talked about connecting the center tap of the filament winding to the cathode resistor of the power tubes to elevate the voltage.
This is on a cathode biased amp.

Has anybody here tried that and how does that work?

I assume you connect it to the high side of the resistor, the side that is connected to the cathode?

I can't wrap my head around how that works?

Anybody?

Currently I just have the center tap grounded to the main power supply ground.

Also on my amp which is dual 6V6, cathode biased, I have a single cathode resistor and cap.

Is that the norm or should I have one resistor and cap for each cathode?

[Jana]

Connecting the center tap of the filament winding to the cathode of the Power tubes, which are bypassed with a cap, is a cheap way to get a positive voltage source to elevate the heaters. There is no voodoo magic here because it is the cathodes or anything, it is just a place to find a +voltage of about 20 to 30 volts.

[collinsamps]

That trick has been used by many. It just lifts the ground reference for the CT or 100 ohm resistors without a CT above ground potential, making it less likely to form a loop, but still allowing a close enough reference to ground it.

Here's an article talking about DC heaters but actually just lifting ground reference by tying it in at the PS node and not above the cathode resistor on the power stage.

http://people.cornell.edu/pages/mt24/Am ... aters.html

[Andy Le Blanc]

It's what they used to call a humdinger. You can power all kinds of thing
off the cathodes of power tubes. A pair of 6l6 or a quad of 6v6 will light the
heaters of a couple 12ax7 strung series, I saw an old Alamo that powered a
transistor verb recovery that way.

Lately I saw a reference that suggested putting coupling caps to ground off
both legs of the heater string, Ive used this on a center tap of a fil. winding
but not both legs. DC heaters are nice but you have to plan ahead to have
about twice the amps, bridge rectification only gets you .61 the current.

[rfgordon]

Andy, I was in an old amp that did the same thing: ran the other filaments in series as the Rk for the power tubes.

[dehughes]

DC heaters are nice but you have to plan ahead to have about twice the amps, bridge rectification only gets you .61 the current.

Right...so 6.3v AC would end up at around 5v. I was just reading that over at the Valve Wizard site. So then, one couldn't really convert their 6.3v AC to DC by just tapping in, huh...

[skyboltone]

DC heaters are nice but you have to plan ahead to have about twice the amps, bridge rectification only gets you .61 the current.

Right...so 6.3v AC would end up at around 5v. I was just reading that over at the Valve Wizard site. So then, one couldn't really convert their 6.3v AC to DC by just tapping in, huh...

Take another look, that's not what he said.

Paulster has a wonderful board......complicated but not expensive, that makes 6.3VDC out of 6.3VAC. Shotky diodes etc.

[billyz]

Take a look at the Ampeg B15n schematic. I believe that is what they did.

http://www.schematicheaven.com/ampegamp ... taflex.pdf