filament DC reference

[Tonewood]

I found the following on the Paul Ruby site Paul Ruby Amplifiers

"Warning!! All amps have some form of DC referencing of the filament winding from the PT. If you're trying to get rid of hum in an old amp, you MUST find the existing DC reference connection and determine it's type. If you decide to use an alternate referencing method, the existing reference MUST be disconnected or you risk toasting your PT!! "

I have both a Kalamazoo model 2 and a Vibro Champ with LF hum and who's schematics show no center tap. I would like to try the 2 x100 ohm resister "center tap" but I don't see the above mentioned existing "DC reference". Can someone point it out to me?

Thanks
scott

[Wilder Amplification]

I found the following on the Paul Ruby site Paul Ruby Amplifiers

"Warning!! All amps have some form of DC referencing of the filament winding from the PT. If you're trying to get rid of hum in an old amp, you MUST find the existing DC reference connection and determine it's type. If you decide to use an alternate referencing method, the existing reference MUST be disconnected or you risk toasting your PT!! "

I have both a Kalamazoo model 2 and a Vibro Champ with LF hum and who's schematics show no center tap. I would like to try the 2 x100 ohm resister "center tap" but I don't see the above mentioned existing "DC reference". Can someone point it out to me?

Thanks
scott

Ground is the most common "DC reference" used on heater windings, either via the heater winding's own center tap or via the 2 x 100R resistor false center tap.

[defaced]

Per the schematic, there isn't a DC reference. I can think of three ways that are typically used to DC reference the heaters: center tap, 2 x 100 ohm resistors, or grounding one side of the heater. I'd open up your amp and verify there's no ground reference, but it looks like you're safe to add the resistors.

[Tonewood]

Thanks you guys, I'll add the "center tap".

[olddawg]

Can't you also elevate the dc reference for the center tap?

[defaced]

Yea, and actually I do that via a virtual center tap. My setense is worded kinda funny, but the CT/virtual CV options weren't implied to be grounded, just referenced.

[guitician]

100 ohm wirewound potentiometer

Do they still make those open frame wirewound pots that were used to null out the filaments hum? The center wiper was grounded to the mounting bushing and you would attach the non-center tapped filament winding across it. They seemed to work pretty good from what I remember.

[Tonewood]

Can't you also elevate the dc reference for the center tap?

This may be what you are talking about- from Paul Ruby site:

"A better, but slightly more difficult, solution than grounding the CT is to connect it to some clean, positive DC voltage source. If the power stage in your amp is cathode biased with a nice big bypass cap on the cathode resistor, you're set! Just tie the CT to your power tube's cathode. There's somwhere between 9VDC and 35VDC at this node. It's just as free and easy as ground, so use it instead of ground if you have it."

[Wilder Amplification]

Do they still make those open frame wirewound pots that were used to null out the filaments hum? The center wiper was grounded to the mounting bushing and you would attach the non-center tapped filament winding across it. They seemed to work pretty good from what I remember.

Don't even need anything that big. With a 200 ohm pot, the pot would only dissipate close to 200mW so a 1/2W 200R trimpot should work just fine.

[guitician]

So they don't make them anymore? That's what they used, I guess it must have been for simplicity, and cost wasn't an issue in them days.

[Wilder Amplification]

So they don't make them anymore? That's what they used, I guess it must have been for simplicity, and cost wasn't an issue in them days.

I don't know if they make them anymore or not. I don't use them in any of my builds so I wouldn't know. I'm just saying it's not needed and you can get away with using a 1/2 watt 200 ohm trimpot.

[guitician]

I'm just saying it's not needed and you can get away with using a 1/2 watt 200 ohm trimpot.

I knew that already. I would prefer the WW over most of the lousy film pots made today though, regardless of wattage.

[bigsmitty]

What if your amp is fixed bias?

[Bruce / Mission Amps]

DC elevated filament supply with virtual center tap

Super simple and very effective:

[Wilder Amplification]

What if your amp is fixed bias?

Make a voltage divider comprised of a 470K and a 47K or a 56K resistor from the screen node to ground. Strap a 10uF or higher @ 100V filter cap across the grounded resistor. The node where the two resistors connect together is your heater CT reference.

The 470K gets connected to the screen node...the smaller resistor gets connected to ground, then the two connect together. Use 2 watt resistors for both.