Updated Champ amp heater circuit

[bob p]

"Fake Center Tap" is easier to say and takes up less interweb space...

Justin

As does saying that SF Fender amps are "ultralinear".

[bob p]

I guess it's time for an old curmudgeon rant. Feel free to skip this post if you like.

This person agrees with you.

I like Merlin's books. I think they're well written and they contain a wealth of useful information. But at the same time, I understand that their target audience is the homebrew type of amp builder who may not have formal electronics training and definitely does not have a Golden Era electronics background. They are written for the modern hobbyist who wants to learn how to build amps, they are not intended to be engineering reference material. As an example: On that page you linked, he uses the term "humdinger" right after he uses the term "artificial centre tap."

Humdinger. That's a scientific term if I've ever heard one.

That's just a tongue-in-cheek jab on my part, no disrespect intended, but the reference illustrates my point. His books commonly use imprecise slang conventions to get points across in a way that makes people understand with a minimal amount of writing. That's good. But slang can have it's downsides.

The "artificial centre tap" is an example. When imprecise slang becomes conventional there is a risk of a downward step in the level of comprehension. We know that the "artificial centre tap" isn't remotely close to being a real center tap, but we use the term anyway. We like to use terms like "fake center tap" to abbreviate the amount of writing that has to be done to express our ideas about heater elevation. In doing this there is a price that we have to pay, in the form of obfuscating the difference and perhaps diminishing some readers' understanding of what a center tap really is and what it actually does. There are people out there who read these books and read these threads who actually think that the resistors create out of phase waveforms. One problem that comes along with the belief that the resistors actually create a virtual center tap is that sooner or later somebody is going to come along and try to use them to wire up a full wave rectifier. Of course we know that the current flow through the windings just won't be the same as it would be with a real center tap.

My purpose isn't really to cause a debate on whether or not the terminology is correct; we all know that it isn't. What I'd really like to know is where the "virtual center tap" term came from originally. I know that Merlin uses it in his books, but I don't think he created the term, did he? And just because he uses the term "artificial centre tap" doesn't necessarily mean that he thinks it's technically equivalent to a real center tap, he's just using a slang convention.

To me this is a lot like saying that Fender amps are UL. They aren't. But someone began incorrectly applying that term and because most people who looked at the schematic didn't have the expertise to realize that the term was wrongly applied, the term has now become sticky. The result is that most people wrongly believe that SF Fender amps are something they are not.

Chuck, I don't know what the best name is for them. I don't think virtual center tap is a particularly good name, though it is in common use. Me? I just think of them as providing DC offset, so I call them DC offset resistors.

[Justin Thomas]

As does saying that SF Fender amps are "ultralinear".

"Ultralinear..." "Distributed Load..." But what <IS> the correct term?
Same number of syllables to me!

Jusrin***

***Maybe I'll use Jusrin as my goofy alter-ego... dang, Chuck, I'm gonna owe you a beer or seven if I ever get out west, as much of an "inspiration" you've been to me.

[Justin Thomas]

What do you call something that does the same job with the same end result as something else, even if done in a different way?
I don't know, I'm just asking. By my logic, a full wave bridge rectifier should be called a full wave fake rectifier... I just posit that anything called "artificial" should just be called "fake." And it's easier than "counterfeit," too...

Justin

[bob p]

I believe the term you are looking for is "Fugazi."

[g1]

Pseudo might raise fewer objections than fake.

[bob p]

ongoing curmudgeon rant:

Is pseudo really any better than fake? I think that both terms are inappropriate because they continue to use the "center tap" analogy, and perpetuate the premise which is founded on ignorance. While those resistors do provide a DC offset, they don't actually do anything in the way of emulating the functionality of a center tap. In that respect any kind of a reference to a center tap is technically incorrect.

This is a case where there is no real equivalence in function between the "virtual center tap" and a real center tap -- the only reason that anyone is using the term is because the picture just looks superficially similar to someone who either isn't paying attention or just doesn't understand the difference. I'm thinking that the origin of the term had to originate in the paint-by-numbers school of amp building... you know, that school where they don't teach you to read a schematic and you build an amp by copying someone else's build photos or maybe by using a layout diagram. In that school the picture looks close enough to a center tap, so the same words get applied to both circuits.

In the past I've made hair-splitting comments about things like "ultralinear." At least with UL the people who mis-apply the term are in the right ballpark when it comes to understanding the circuit's operation and the distinction amounts technically to hair splitting between operating modes. In the case of the virtual center tap, the misnomer isn't even close to accurately representing the operation of the circuit. It's totally wrong.

I know that ranting about this is a bit of a Qixotic quest, as the term isn't going to go away just because I ranted -- and I'm not calling for us to stop using the term. But it's persistence makes it hard for me to distinguish between the people who use the term because they don't know any better and those people who use the term just to appease the people who don't know any better. IMO if we use the term we all end up looking ignorant irregardless* of which group of people we belong to.

I guess my point is that this being the Theory section of the MEF, accuracy in nomenclature is more important than it is at a place like The Amp Garage.



* I use the emphatic form of regardless/irrespective in honor of Jusrin.

[Chuck H]

Ok, how about 'center tap substitute balance resistors'?

Say it ten times fast!

[Bloomfield]

I think it's just a fixed humdinger; nothing fake there..

[bob p]

Ok, how about 'center tap substitute balance resistors'?

Say it ten times fast!

I guess we could just call them dummy resistors, but that would be ambiguous too.

[Chuck H]

well, with all due recognition of the semantics and respect for my fellow posters... I think I'll just keep calling it a false/fake/artificial center tap in the interest of communicating modern parlance. But I promise to wince when I do. If that helps.

[g1]

One of the functions of the center tap is to reduce hum. If the resistors do that, how is that so absolutely not the same function? Do both methods not result in a ground reference that would otherwise be non-existant?
I'm certainly ready to be schooled on this, and hope some around here are willing to forgive my ignorance.

[The Dude]

Yep. I'm with you g1. The two terms "center tap" and "virtual center tap" are differentiated by the word "virtual". Of course it's different than an actual center tap, as it's description indicates. I don't see a problem with that- just hair splitting- potato, pototo- coke, pop, soda, etc. Virtual reality is not actual reality. If you want to start dissecting the English language as it relates to terminology, you're gonna be busy a while.

[Leo_Gnardo]

I think I'll just keep calling it a false/fake/artificial center tap in the interest of communicating modern parlance. But I promise to wince when I do. If that helps.

You left out ersatz.

However it's not bogus. Because it works.

[the fatch]

ongoing curmudgeon rant:

Is pseudo really any better than fake? I think that both terms are inappropriate because they continue to use the "center tap" analogy, and perpetuate the premise which is founded on ignorance. While those resistors do provide a DC offset, they don't actually do anything in the way of emulating the functionality of a center tap. In that respect any kind of a reference to a center tap is technically incorrect.

This is a case where there is no real equivalence in function between the "virtual center tap" and a real center tap -- the only reason that anyone is using the term is because the picture just looks superficially similar to someone who either isn't paying attention or just doesn't understand the difference. I'm thinking that the origin of the term had to originate in the paint-by-numbers school of amp building... you know, that school where they don't teach you to read a schematic and you build an amp by copying someone else's build photos or maybe by using a layout diagram. In that school the picture looks close enough to a center tap, so the same words get applied to both circuits.

In the past I've made hair-splitting comments about things like "ultralinear." At least with UL the people who mis-apply the term are in the right ballpark when it comes to understanding the circuit's operation and the distinction amounts technically to hair splitting between operating modes. In the case of the virtual center tap, the misnomer isn't even close to accurately representing the operation of the circuit. It's totally wrong.

I know that ranting about this is a bit of a Qixotic quest, as the term isn't going to go away just because I ranted -- and I'm not calling for us to stop using the term. But it's persistence makes it hard for me to distinguish between the people who use the term because they don't know any better and those people who use the term just to appease the people who don't know any better. IMO if we use the term we all end up looking ignorant irregardless* of which group of people we belong to.

I guess my point is that this being the Theory section of the MEF, accuracy in nomenclature is more important than it is at a place like The Amp Garage.



* I use the emphatic form of regardless/irrespective in honor of Jusrin.

The center tap to ground on a 6.3V AC heater winding results in currents of opposite polarity at 3.15V AC with respect to ground on the heater wires, as do the two resistors of the 'virtual center tap' connected to ground. This seems a lot like 'emulating the functionality of a center tap' to me.