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proper way to wire AC heaters?

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4/18/2000 7:23 PM

proper way to wire AC heaters?

what is the proper way to wire up AC heaters? I recently finished my 2-6v6 amp and have noticed some hum. I'm wondering if maybe I didn't wire it up the best way to remove hum. the heaters are basically wired in paralel to one another, on the 6v6's the two #2 pins are tied together with the same side of AC,same way with pin #7. Is this wrong? should I have made them alternate, with pin #7 going to pin #2 of the other 6v6 and vice versa? how about the two 12ax7? one is the PI and the other is the two gain stages. I'm especially confused about these two since they're like two tubes in one. they are currently wired similar to the 6v6's, withe one side of the AC connected to pins 4 and 5 on each tube and the other side of the AC connected to pin 9 on each tube.
I've spent most of my time modifing and understanding amps' circuits but don't know much when it comes to heaters, so, if someone could explain how to wire these heaters up, I would greatly appriciate it.
-Ricky

4/18/2000 7:43 PM

Don't fret about which side of the AC goes to which heater pin on which tube.

The two important things are keeping a good tight twist on the heater wire pair, and keeping that pair as far away as practical from your signal wires. Make the heater/signal crossings more-or-less right angled, and that's about as well as you can do WRT heater-induced hum.

General lead dress (lower-level signal wiring away from higher-level signal wiring) becomes important.

R.G.'s tube debugging FAQ has some stuff about hum... I think.

Good luck!

4/18/2000 8:39 PM
Ricky	yeah they're twisted real tight, at 45 degree angles, I made sure of that. also, I have a real thick, 3" chassis so, I bent the heater wires up and out of the way of all signal leads (and everything for that matter)I just thought I might have unitentionally hooked it up wrong sreies-parallel wise, and possibly created some excess noise. Ricky

4/18/2000 9:49 PM
Stuart	Do you have a grounded centre-tap on the heater winding? That helps a lot.

4/18/2000 10:03 PM
Troy	Ricky: If you don't have a center tap wire on the filament wires of your PT, then install 2-100ohm resistors off of each leg of the filament to ground. I like to do this at the pilot lamp assembly, yet it all depends on where your ground will go and if you're going for star grounding. Troy

4/18/2000 11:18 PM
Ricky	yes, Iguess I forgot to mention it but I'm using the hammond 270fx (which has a fillament CT). Input and out jacks are isolated. I'm also using a two-point star ground scheme. the power supply has a 5H hammond choke in it. the amp doesnt have excessive hum just some and i wasn't sure abut the heaters. Ricky

4/19/2000 2:15 AM

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

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