What's the benefit of 2 100 ohm Resistors on the Pilot Light?

[coreysan]

Could someone teach me something, please?

In Fender amps, the schematics show the power transformer heater wires (green) going to the pilot light,
then to the heaters. There's no 100 ohm resistors going from the pilot light to ground.
My guess is because the transformer used has a CT.

I have a transformer with no heater CT, so I'm being told to include 100 ohm resistors.

What does that accomplish? What's the point of those 2 resistors? What would happen if
there were no resistors, and no CT?

I'd be grateful for anything you might share.
Thanks so very much in advance!

[PRR]

> What would happen if there were no resistors, and no CT?

It would work, probably, for a while.

But leaving stuff "floating" (no connection to common) is bad practice. Stray leakage can pull-up the heater lines to any voltage, AC or DC (in addition to the 6.3VAC *across* the two wires). This can cause trouble.

A CT or two resistors is cheap and effective.

It does not have to be "on the pilot light". Anywhere along the heater wiring will do. The 6V pilot light is usually the least-crowded place to put resistors. 

[HotBluePlates]

What would happen if there were no resistors, and no CT?

You will hear a strange, loud, buzzy hum. I know because I forgot to include them on a 5E3 build once. Added the resistors, and the noise went away.

It does not have to be "on the pilot light". Anywhere along the heater wiring will do.

Right. On the 5E3 build, everything was "in the way" once fully wired. The open spot was at V1. So I took the 2 resistors, crimped & soldered 1 lead each into a ring terminal, bolted that down with one of the screws for V1's socket, and soldered a lead a piece to pin 4/5 & pin 9 on the socket. Cured the noise.

The ground reference can be anywhere along the string; it does the same job and looks electrically-same to the circuit regardless of where you add them.

[Mike_J]

In addition to the reduction in hum mentioned by HBP I had a small piece of solder from the preamp tube socket to ground I did not see when going over one of my amp builds.  There was no center tap on the OT for the heater supply so I used the 100 ohm resistor method of balancing the heater supply.  By using this method the 100 ohm resistors acted like fuses and burned through.  I fixed the solder problem, put two new 100 ohm resistors in and the amp worked great.  If I made the same mistake with a center tap I doubt that my heater supply would have survived.

Thanks
Mike

[coreysan]

Gentlemen, thank you all so very much. It makes more sense now!
This is my first build, and if it all works well, it will be because of you all!

Again, thanks!

Corey

[eleventeen]

Incidentally, these 2 R's form what's commonly called a "synthetic center tap". That's the lingo.


On tube amps, most older power transformers included a heater CT which was grounded as a routine matter. In some amp designs, the 6.3 volt CT was in some cases lifted above ground by a DC source by 25-50 volts in order to reduce heater-to-cathode voltages if the designer used some circuit where a cathode was 100 or more volts or so above ground. Tubes in earlier days were thought to be vulnerable to heater to cathode shorts and indeed, they *do* have maximum ratings when it comes to heater-to-cathode volts.


Most importantly, it has been found that about 95% of tube amps will hum pretty badly if the heaters do not have a ground reference. In days of yore, the CT connected to gnd did the deed. If the PT has no CT, the 2 qty 100-ohm R's do the job just fine. Actually, the 100-ohm R's are *preferable* to the CT because they can act as fuses in certain fault situations which can and do burn up heater windings on PTs.


Where they are physically mounted means little. They can be anywhere in the heater chain: on a tube socket in the middle or end of the filament chain, on the pilot lamp ass'y, anywhere convenient.

[tubeswell]

Keeps each side of the pilot light filament in balance. ;-)