DC Heater Question

[markr14850]

Find some space in your chassis to bolt one of these in. Power your 12ax7's at 12v, and never need to think about it again.

http://www.alliedelec.com/search/produc ... U=70069560

[bal704]

Can you run 12AX7's at 12V? I thought that was too high.

Maybe use a voltage divider to get the 12V down to 6V?

[markr14850]

The "12" in the name indicates that it was designed to run on a 12 volt heater supply. But it is also center tapped, for use with typical 6 volt heater supplies.

To use it with 12v, you connect to pins 4 and 5, leaving pin 9 disconnected.

When run at 12v, each tube only requires 150mA. So, you could run quite a few of them off the 1.3A of the supply I linked to.

[bal704]

Thanks. Would a 12V DC wall wart work just as well (or even a 6V DC wall wart for that matter)?

I've probably got a 12V DC 1 Amp wall wart around the house somewhere.

[markr14850]

Would certainly be fine for debugging. For longer term use, I would worry about how securely it could be mounted.

Note that you still need to reference the heater supply to ground somehow. (This is one big pain in the butt if you try to have a mix of 6.3VAC and rectified 6VDC off the same transformer tap. Where is the ground ref? Can only have one side grounded. Accidentally measure across it, and you smoke the rectifiers.)

You'll have to determine if your wall wart comes with some internal reference to ground.

With the unit I posted, the output is floating, so you can connect it's negative output to ground to set the reference.

[bal704]

My PT has 2 heater taps. I'm only using 1 for the DC heaters. It's the one for the preamp/PI tubes. The power tubes have their own heater tap.

I thought the heaters could be referenced to something other than ground, such as the case where the heater CT is elevated?

In my case, I'm running the 2 heater taps to the ~ inputs on the rectifier, and using the + and - rectifier outputs for the heater voltage. I've left the heater CT hanging out in space, so I suppose that heater tap isn't reference to anything right now.

Should I ground the Heater CT (or hook it to the power tube cathode)?

[markr14850]

My warning was just to be sure that you don't try to set a reference on both sides of the rectifier.

If you are sure that there is no ground reference on the AC side, then I would suggest setting the ground reference on the DC side, either at ground, or at some elevated level.

For noise reduction, the important thing is that there be no signal happening between the heater and the cathode. Setting the reference on the DC side accomplishes this. Setting it on the AC side - not so much.

[bal704]

to set it on the DC side, just hook up the - output on the rectifier to ground (or power cathode)?

Doesn't seem right, but that's why I'm asking.

[Tillydog]

In my case, I'm running the 2 heater taps to the ~ inputs on the rectifier, and using the + and - rectifier outputs for the heater voltage. I've left the heater CT hanging out in space, so I suppose that heater tap isn't reference to anything right now.

Should I ground the Heater CT (or hook it to the power tube cathode)?

Ideally reference it to a +ve voltage (PT cathode might be OK, or build a divider off B+ to give you ~~30-50V)

[bal704]

My PT cathode is about 25V DC, so I'll hook the CT up to it.

How about the DC side of the rectifier? Would I hook up the - output of the rectifier to the PT cathode as well to elevate it?

[markr14850]

My PT cathode is about 25V DC, so I'll hook the CT up to it.

How about the DC side of the rectifier? Would I hook up the - output of the rectifier to the PT cathode as well to elevate it?

If you tie the AC side to ground, or any voltage, via the center tap, then you must not tie the DC side to ground, else the rectifier will die.

Setting the ground/positive reference at the CT and letting the rectified side "float" will still leave a signal relationship occurring between your cathode and heater.

[dorrisant]

What about the way Mesa does it on the DC-3? Just for V1.

http://schems.com/manu/mesaboogie/mesaboogie.htm

I think it is on page 7... the switching matrix sheet.

Seems much simpler. I don't know what diodes they are using, but I was thinking of trying it out in my X10 clone.

You guys see any reason it would not work?

Tony

[Tillydog]

If you tie the AC side to ground, or any voltage, via the center tap, then you must not tie the DC side to ground, else the rectifier will die.

^ +1 to that

Setting the ground/positive reference at the CT and letting the rectified side "float" will still leave a signal relationship occurring between your cathode and heater.

^ I don't follow this part, though.

[markr14850]

Setting the ground/positive reference at the CT and letting the rectified side "float" will still leave a signal relationship occurring between your cathode and heater.

^ I don't follow this part, though.

Well, if you measure the DC output of the rectifier/filter, it will have very little AC on it - depending on how you filter/regulate it. But that measurement is taken relative to the plus and minus of the output. Between those two points, yes, DC.

Imagine that this was on an isolated winding, and nothing in the circuit grounded. What is the relationship between the DC and the cathode? Undefined, right?

If the DC side is then tied to ground, then the relationship between heater and cathode is clear - zero signal.

However, if it's the AC side of the rectifier that is tied to ground, then the relationship between that steady DC output and ground is going to be pushed up and down. This is kind of the worst of both worlds. You go through the work to feed DC to the heater, but it's not DC *relative to the cathode*, which is all that matters.

[Tillydog]

OK, I see your point, but I thought we were talking about the CT of the winding, not either end (so notionally at a constant voltage, but accepting that nothing is perfect...).

Better to leave the AC side floating, then, and elevate one of the DC legs?