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Rob11966 18th July 2008 06:33 AM

Heater circuit - to twist or not to twist
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Hi Folks just a very quick one -

I a currently building a fender 5e3 guitar amp. The schematic has the heater circuit completed with one loop of the circuits (for all but the rectifier tubes) via ground. In other words, one wire coming out of the transformer is grounded, with the other wire looping across the heater elements with the other pin of the heater grounded ,thus completing the circuit.

In my audio amps, I have always run two heater wires to each tube and twisted them tightly and I was planning to do this for this amp. Then I began to wonder why the fender people did it as described above. Was it simply a money saving exercise and how did they get away with it without hum? Indeed, should I twist or should I stick to the original design?

Input appreciated.



PS, I have attached a picture of a slightly different model showing the layout just to demonstrate what I am talking about

DigitalJunkie 18th July 2008 08:22 AM

I assume they did it because it was easy,and cheap.
If I were going to run separate wires,I would definitely twist them to help keep hum down.

Rob11966 18th July 2008 09:18 AM

Thanks DigitalJunkie,

I probably was not clear enough. Of course, if I were to run separate wires I would twist as per usual, my question should have read - do I have to run separate wires at all - does it make a difference to this amp?

My initial thoughts were that Fender designed it the way they did to cut costs and labor. My thoughts were to run separate twisted wires to reduce hum, but not being a guitar amp person (I am making it for my son), It then occurred to me that there might be (although I can't think what) some functional reason to use the chassis as one path of the heater current?



Merlinb 18th July 2008 10:19 AM


Originally posted by Rob11966
Thanks DigitalJunkie,
do I have to run separate wires at all - does it make a difference to this amp?

The method adopted by Fender is definately for cost cutting, and would certainly result in more hum that that obtainable with a twisted pair and grounded centre tap.

Rob11966 18th July 2008 10:47 AM

Thanks Merlin,

This is as I suspected. Problem is, I don't have a center tap on my heater supply. Can I just run an isolated loop, or should I ground one limb via something like a 0.02uf green cap?

Thanks again,


DigitalJunkie 18th July 2008 10:48 AM

Ohh,in that case,I agree with Merlinb -Go with the twisted wires!

Rob11966 18th July 2008 11:03 AM

I agree DigitalJunkie,

That's what I want to do but what about the lack of a center tap on the heater supply....

zigzagflux 18th July 2008 11:20 AM

Two 100 ohm resistors from each leg, then tied to each other, will form a virtual center tap.

Tom Bavis 18th July 2008 12:33 PM

DON'T run heater current through the chassis... I doubt Fender did - a lazy draftsman probably just used the ground symbol instead of showing the actual connections. If heater circuit is connected to chassis, it should only be in ONE place. If there's one rule for making a quiet amplifier, it's to provide a return circuit for every current - don't allow high currents to share a path with signals.

Rob11966 18th July 2008 09:57 PM

Thanks Zigzagflux, that's what I will do. Do you think that I should ground my virtual center tap through a capacitor or just directly?

Hi Tom, this is what I initially thought but everything that I can see suggests otherwise. If you look at that particular drawing which is supposed to be an accurate layout wiring layout, they have grounded one heater wire (green) with the B+ center tap. Perhaps these guitar amp people don't worry so much about hum! I agree with you though, I am not going to do it this way.


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