ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

previous: Ricky yes, Iguess I forgot to mention it ... -- 4/18/2000 11:18 PM

Re: proper way to wire AC heaters?

4/19/2000 2:15 AM
Steve A.	Re: proper way to wire AC heaters? You might try taping off the CT for the filament windings and using two matched 100 ohms resistors to create a "virtual" centertap. Transformer center taps may not be exactly in the middle of the windings. Maybe you can elaborate on your "two-point" star ground scheme... What works really well is to run the PT CT to the initial filter cap(s) negative return and then run a wire from that point to the star ground point. You can also try tying all of the grounds associated with a particular stage together and then running a wire from that point to the star ground. For example, for the first stage run the input jack ground wire to the same ground point that your cathode resistor and Ck cap go to. As you follow the audio signal path, when you pass through a coupling cap (or tone stack) then start the next intermediate ground point. For an example of what not to do, I thought I'd "improve" the grounding of a post-PI MV (which had been going to the back of the dual pot) so I ran the ground wire from the pots to the grounded tail of the mid pot in the tone stack... talk about oscillation! You could adjust the pitch by tweaking the controls and it sounded more like a theremin than a guitar amp... A DC filament supply for the first preamp tube helps too, and if you are thinking about adding in some relays you can tap 6vdc from the 6vac supply going to the other heaters by looking at the Mesa Boogie Mark III schematic (two birds with one stone and all of that). --Good luck! Steve Ahola

4/19/2000 2:15 AM

Steve A.

Re: proper way to wire AC heaters?

You might try taping off the CT for the filament windings and using two matched 100 ohms resistors to create a "virtual" centertap. Transformer center taps may not be exactly in the middle of the windings. Maybe you can elaborate on your "two-point" star ground scheme...

What works really well is to run the PT CT to the initial filter cap(s) negative return and then run a wire from that point to the star ground point. You can also try tying all of the grounds associated with a particular stage together and then running a wire from that point to the star ground. For example, for the first stage run the input jack ground wire to the same ground point that your cathode resistor and Ck cap go to. As you follow the audio signal path, when you pass through a coupling cap (or tone stack) then start the next intermediate ground point.

For an example of what *not* to do, I thought I'd "improve" the grounding of a post-PI MV (which had been going to the back of the dual pot) so I ran the ground wire from the pots to the grounded tail of the mid pot in the tone stack... talk about oscillation! You could adjust the pitch by tweaking the controls and it sounded more like a theremin than a guitar amp...

A DC filament supply for the first preamp tube helps too, and if you are thinking about adding in some relays you can tap 6vdc from the 6vac supply going to the other heaters by looking at the Mesa Boogie Mark III schematic (two birds with one stone and all of that).

--Good luck!

Steve Ahola

Replies:

Ricky my two-point star ground is as desc... -- 4/19/2000 3:14 AM