"Ground" and "Center Tapped" Power Transformer Secondary in guitar amp

Center Tap in the power transformer secondary. This symbol indicates that the Center Tap is the "source" of all the DC electrons that the amp will need. (See the gray lines with arrows in the second attachment.)f
  • #1
TL;DR Summary
Trying to understand how a "Center-Tapped" power transformer secondary is the "source" of all DC electrons that come from "ground" (a.k.a. "the earth") when there is no connection to earth.
Hello - thank you for reading this, and thank you in advance to anyone who replies and clarifies the mystery of "ground."

I have finally gained a full understanding of how a full-wave rectifier, two-power tube electric guitar amplifier works in a push-pull manner, but one thing remains after I "thought" I understood everything. I read that "ground" is the source of all DC electrons that will be used in the amp, and that they re sourced through the Center-tapped power transformer secondary. I have two conflicts with this:

1. My understanding is the 120 volt "AC" flows from my wall socket into the amp where it hits the power transformer primary. Magnetic flux induces the AC across to the Center-Tapped "secondary" of the power transformer, where the Center Tap allows for division of stepped up and stepped down voltage to power certain things in the amp: 5 volts of stepped-down AC to heat the full wave rectifier cathode; 6.3 volts of stepped-down AC to heat the filaments of the preamp tubes that will heat the cathode that has a "chemical paste" that will release electrons to the anode plate; and...325 volts of stepped-up DC power to the plates of the rectifier, which then sends that power to the power vacuum tubes of the amp.

2. There is no physical connection of any part of a guitar amp chassis or its components to actual "earth"..."the ground". The amp chassis sits in a wood speaker cabinet with rubber feet. Some amps use what is called a "ground buss" to which everything is connected that needs to "go to" ground. I have been told that the Center Tapped secondary of the power transformer goes to "ground", and that the Center Tap is the "source" of all DC electrons that the amp will need, and gets those electrons from the ground...literally "the ground", which is of course impossible. But my understanding that was all of the electrons that the amp would need, came from the AC in the wall that cross from the primary to the secondary of the power transformer, and was split to various places and functions (see below). So...

The only thing that I can think of that I have not read in any of the ten texts on the subject, is that perhaps that sleepy little, previously ignored "ground wire" in my 120 volt AC wall socket, is somehow supplying electrons to my amp, but...how, especially when that power is AC (or is it DC?), and the guitar amp must have DC power to run itself like all appliances and devices.

If you do not know, the electric guitar signal that comes from the guitar pickups, is in fact an "AC" signal that flows into the pre-amp tube grid which disrupts the flow of electrons flying off the cathode moving toward the anode plate. The signal is "gained up" in the first and second stages of the dual triode preamp tubes. So: the amp operates on DC power, but it processes an AC guitar signal all the way through the circuit.

So...I am to believe/understand that the Center Tap acts as the source of DC electrons because it is supposedly "grounded" (to earth...literally earth?), and that remains the confusing mystery to me. Again, the only thing referred to in a guitar amp for "ground" is a Ground buss, or occasionally a chassis screw to which anything and everything that actually needs to be grounded is in fact connected to that screw, and the metallic amp chassis acts as "ground"...but, in my mind, it can't possibly be the source of DC electrons to power the amp. Thus my confusion. I am missing a big piece of the puzzle. The Center Tapped in the power transformer secondary, seems NOT to be really connected to the "ground" (earth), but it is supposedly the source of all DC electrons, but from where?
Thank you for your help and comments.
  • #2
I didn't read your whole post, but perhaps the use of "ground" in this context just means the common return line for the output power rails, and nothing to do with Earth ground itself.

It would help immensly if you could upload the schematic of the circuit you are asking about. Use the "Attach files" link below the Edit window to upload a PDF or JPEG copy of the schematic please. Thanks. :smile:
  • #3
Thanks for your quick reply. I have attached three examples, one in particular showing the flow of electrons and the all of it flowing from the Center Tapped secondary of the power transformer...the schematic will the gray lines with arrows (second attachment).
Thank you
  • #4
Thinking about electrons flowing is not a very good way to understand electronics, but to work out the answer to your question look in the middle picture at the ground symbol at the head of all the arrows to the other ground symbols. There is a wire connected to this ground symbol from the centre tap of the transformer with an arrowhead pointing towards ground. Keep following the arrowheads back "upstream" and you will see where they come from
The total number of electrons inside the amplifier must remain constant otherwise it would build up charge until it explodes in a flash of lightning!
  • #6
Each of those schematics shows the secondary center tap connected to the "ground" that the mains power supplies. Although the 2nd one is complicated. That "ground switch" implies some sort of dumb-a## common mode filter, I guess. But the point is it wouldn't be there if that wasn't intended to be associate with the mains ground.

EE's will often refer to "ground" as a common reference point which may not be related much to the dirt you stand on. For example, in a car which sits on rubber wheels. It is often a chassis, or maybe a Cu spike pounded into the ground, sometimes a big sheet of Cu inside a PCB, but the functional currents that make the circuit work won't actually flow away to those locations. Excluding subtleties, those circuits would work just as well if all of those ground symbols were removed and replaced with wires.

I did many designs with schematic symbols for "Analog Ground", "Digital Ground", and "Chassis/Earth Ground". It was just a way of conveying to readers the separation of current flow paths into separate regions. In the real world they were just wires or traces/planes on a PCB.

So, maybe I don't understand your real question, or you are putting more meaning into "ground" than is necessary to understand the basic function of the circuit. There are some practical but esoteric reasons that EEs include the mains ground in their designs (primarily EMC and safety issues), but I think that is beyond the scope of your question.

For an amplifier, just think of "ground" as a frequently used connection that things are referenced to. It's primarily a convenience for the designer and the people reading the schematic to refer to the most common, most significant, or a special net in the circuit.
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  • #7
The second schematic has the well known 'death switch' that weakly couples the chassis of the amp to hot or neutral to help combat hum. It's a controversial thing. Some people swear the only thing preventing the death of a guitar player is they just never happen to be touching anything else to complete the circuit. In reality if the 600 volt capacitor doesn't fail shorted little will happen. Nonetheless, not a wise move.
Here's a link to an old thread about safety of guitar amps. It turned into a train wreck rather quickly.
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