100 Ohm Heater Resistor Wiring

[Okbill]

Every time I build a new amp I wonder why Fender et al connect the two 100 ohm grounded resistors to the heater string *at the pilot lamp* as opposed to somewhere else thats easier to reach. Such as the last preamp tube in the heater string. There is always room on the chassis to install them there, plus with only one heater wire per solder lug on the last preamp tube it would be a lot easier to solder on the resistors there than back on the pilot light fixture. Is there a theoretical consideration I’m missing?

[johnnyreece]

Maybe there's a problem with ground currents there? I dunno...

[Stevem]

With the those two resistors carrying the full load of the total heater current you don’t want to so to speak drag ( impress ) that current to the other side of the chassis and in turn chance 60 hz noise issues.

You want to keep it all localized close to where the B+ center Tap is landed .

[sluckey]

Is there a theoretical consideration I’m missing?

The first preamp is usually the most sensitive stage in an amp. Those two 100Ω resistors will act as two 60Hz antennas. Maybe you'll hear it, maybe you won't. Why tempt fate? Put them at the PT end of the chassis with all that other dirty stuff. Keep the preamp area tidy.

[sluckey]

With the those two resistors carrying the full load of the total heater current you don’t want to so to speak drag ( impress ) that current to the other side of the chassis and in turn chance 60 hz noise issues.

The total heater current DOES NOT flow through those resistors. The only current that flows through those two 100Ω resistors is 6.3V / 200Ω = 0.0315A

[wpaulvogel]

It’s my opinion that the best location for the heater center tap is grounded at the same location as the input jack. Using an artificial center tap could be different because there is actually current through it and it’s really only a balance. It’s not the same concept as far as the transformer is concerned with a physical center tap. I probably should have just stayed out of this one but if you have a center tap for the heater, put it at the input jack. Sounds silly probably but it’s quiet.

[pdf64]

Same as a heater winding CT, It’s purely a reference, 0 current flows from the heater circuit to 0V common (or whatever reference point is used).
As it’s a reference, it would be nice it it was a clean one.
It’s worth bearing in mind that the HT CT carries the dirtiest current in an amp (to 0V common).