Updated Champ amp heater circuit

[pdf64]

The usual action of the heater winding CT is to force the heater circuit to become 'balanced', with respect to 0V / circuit common.
As (in normal operation) there's no current flow via the CT, a suitable pair of resistors can do the exact same thing when there's no CT, nothing artificial / false / fake about it.
Hence I suggest they may be referred to as 'heater circuit balancing resistors' or just 'heater balancing resistors'

[Chuck H]

The usual action of the heater winding CT is to force the heater circuit to become 'balanced', with respect to 0V / circuit common.
As (in normal operation) there's no current flow via the CT, a suitable pair of resistors can do the exact same thing when there's no CT, nothing artificial / false / fake about it.
Hence I suggest they may be referred to as 'heater circuit balancing resistors' or just 'heater balancing resistors'

Ok. Good call. And I think Bob would be better with that I don't think anyone would become confused WRT a filament winding if any of us were to write "balance resistors" or "balancing resistors" (since we actually HAVE done this) instead of "virtual center tap". I may try to commit to this.

[eschertron]

The usual action of the heater winding CT is to force the heater circuit to become 'balanced', with respect to 0V / circuit common.
As (in normal operation) there's no current flow via the CT, a suitable pair of resistors can do the exact same thing when there's no CT, nothing artificial / false / fake about it.
Hence I suggest they may be referred to as 'heater circuit balancing resistors' or just 'heater balancing resistors'

I agree too. However, we will need to begin calling the heater "center tap" a more functionally-correct name: the heater "balance tap" in order to keep the nomenclature proper.

[bob p]

The usual action of the heater winding CT is to force the heater circuit to become 'balanced', with respect to 0V / circuit common.
As (in normal operation) there's no current flow via the CT, a suitable pair of resistors can do the exact same thing when there's no CT, nothing artificial / false / fake about it.
Hence I suggest they may be referred to as 'heater circuit balancing resistors' or just 'heater balancing resistors'

Just to be clear, I'm not intent on forcing terminology on anyone, and I'm not trying to force a change in what the resistors are called. I started off asking where the "virtual center tap" term originated and then I complained that that terminology wasn't a particularly good description because although we observe similar behavior in one circumstance, those similarities can't be extrapolated to all center-tapped secondary behavior.

Anyone can use any term they like that gets their idea across. (Chuck, I'd prefer if you didn't even pause to think about it.) I'm just glad that some people think enough of the this topic to humor me by suggesting other names.

I do like a lot of the ideas that have been suggested. Being a curmudgeon, I just prefer terms that don't inject confusion by oversimplifying and potentially implying that a pair of resistors can transform a non-center-tapped secondary into the equivalent of a center-tapped secondary. Of course we know better.

What to call the resistors? Any term that gets the point across is a term that works. Me? As far as slang goes, I'm biased toward descriptions that describe the desired result (DC voltage offset) or effect (hum reduction), rather than descriptions that are based on the superficial appearance of a schematic rendering that doesn't accurately represent an equivalent circuit. Because the resistors' effects on hum is their intended purpose, I guess it makes sense to refer to them that way, with a name that describes their function. In that respect I like the suggestion for "hum balancing" or even "humdinger" though I personally like "DC offset." Those hum-based terms seem to describe their audible effect, which seems to be what some people are after. And there's the added benefit that "hum balancing" and "humdinger" don't have very many syllables, for those who care about that sort of thing.

All of the terms that have suggested so far sound a lot better to me than "virtual center tap". For some reason that term has always bugged me (as if you haven't noticed).

[Bob M.]

Well, I had no idea what a semantic can of worms I was opening.

My comments were about my Fender Champ amp, not the VibroChamp although they are 'kissin' cousins' and pretty much the same thing, minus the tremolo. My Champ is somewhat stock but over the years I've developed some things that improve these little workhorses and I decided to dive in and do that to this one while at the same time trying out a new preamp design that I like alot. The amp wasn't broken but I just had a free day and it was raining hard outside, a perfect shop day. I've got about six of these Champs and VCs lying about: I've got a Rem-Champ (Remaking of a Champ - Glass Audio), a Swamp Champ (really a one channel Bandmaster in a VC box), a super stock VC (a tricked out, blueprinted but still pretty stock VC - great amp) and a couple of beaters as well.

To set the record straight, I don't I've ever said (or used) virtual center tap. I've normally said ground reference resistors. I think (but I'm not sure) that I picked it up from Kevin O'Conner and my rationale being if it was good enough for him, it's good enough for me. I think everyone knows what I mean.

You know, the music (and amp) biz is littered with expressions we've all coined over the years to describe our gear, some really descriptive, some not so and very few are actually scientific: hardtail, sag, dive-bombing, speaker farting come to mind immediately. A guy came over the other day with his Strat and was talking about about his affinity for the #2 and #4 positions on his 5-way switch calling them the out of phase sounds. I said, you know, they are really make-before-break sounds, not out of phase sounds. He said, I know but everyone calls them that. And I said, you're right. Leo called it vibrato (the Vibro-this and a Vibra-that) but we all know that its tremolo and we know what he meant.

Loved Mozz's comment about measuring AC millivolts at the speaker or dummy load.

If you had 20 Fender (tube) amps, ten of which had a heater winding center tap and ten which used 2x 100 ohm resistors (I'll refrain from calling these resistor by any name as to not offend any hard-line semanticists) and then you measured the heater voltage at both sides of the heater winding, I think you'd find that generally, amps using the resistors gave closer (to each other) winding readings and also made for generally quieter amps. But not in all cases: The amp I just finished with, a 1965 BF Pro Reverb had very close to identical voltages on each side of the heater winding and used the stock center tap and the amp was dead-quiet (I followed Justin's sage advice: If it ain't broke...). The great side benefit of using the 2x resistors instead of the stock center tap is when the dreaded plate-to-heater short rears its ugly head, with the resistors in place you might only have to replace these resistors (provided they are sized for this important side function) rather than a tube socket or a transformer, both of which would be a bigger deal. That's the kind of forethought I like.

Steel guitarists have a similar saying to Justin's "if you build it, it will hum". They say once you put your bar on the strings, you're out of tune.

Thanks for the lively give and take,

Bob M.

[Malcolm Irving]

One other use of the term 'virtual' in electronics, which is widely accepted, is 'virtual ground' in an operational amplifier (op-amp) circuit. In that case the 'virtual ground' has very close to the same voltage as the signal ground, even though they are not directly connected.

If we connect two equal resistors in series across a (centre-tapped) transformer winding, the voltage at the junction of the two resistors will be very close to the voltage at the centre-tap.

So the term 'virtual centre-tap' seems OK to me.

EDIT: Of course, we only use a 'virtual centre-tap' when the winding has no centre-tap, but in that case the voltage at the 'virtual centre-tap' is the same as the voltage would be at a real centre-tap (if there was one). I think I'll stop there before I get even more confusing.

[pdf64]

The problem with the terminology focusing on the centre tap, real or virtual, is that folks then get thinking it's the centre tap that's the important thing here, which seems to be putting 'the cart before the horse'. As I see it, what's important is that the heater circuit gets balanced and referenced.
Here's an example of confusion that may have been caused by the 'centre tap' terminology Is a PT secondary center tap a must? - The Amp Garage

...The great side benefit of using the 2x resistors instead of the stock center tap is when the dreaded plate-to-heater short rears its ugly head, with the resistors in place you might only have to replace these resistors (provided they are sized for this important side function) rather than a tube socket or a transformer, both of which would be a bigger deal...

I think that is a downside of the resistor balancing method, rather than a benefit. Electrically robust resistors might best be used to mitigate the risk of it happening, eg >3 watt vitreous wirewound.
The issue being that if the heater circuit loses its 0V reference and a tube short pulls it up towards HT, then the heater-cathode insulation of every tube in that circuit may be damaged.
To avoid the risk of transformer etc damage, the correct value and type of fusing must be used of course, but that's a fundamental (if often ignored) aspect of amp ownership anyway.

[bob p]

I was only kidding with my previous Amp Garage comment. I had no idea there was such a thread out there.

[g1]

bob p, I know you are being very precise with your terminology, so your preference for "DC voltage offset" is throwing me off. What am I missing (where does the DC voltage come in)?

[bob p]

Where does the DC voltage come in? It came in in BobM's original post where he talked about elevating the heaters with a bias supply reference.

[g1]

Ok. So that term would only be applicable where there is DC elevation, and not in the most common or 'traditional' arrangement (non-DC) that is referred to as 'virtual center tap'. (fender for example)
What would be your preference there?

[bob p]

I don't know if grounding the resistors is "most common" or "traditional." If that's the most common method then I'd expect that there are a lot of unnecessarily noisy amps out there. Chances are that the "most common" or "traditional" methods of wiring the heater circuits don't even have any balancing resistors.

My preference is for providing DC offset to the heaters, which is why I refer to them as DC offset resistors. Granted, if you just ground them then you don't get the benefit of the DC offset, and you don't get to call them DC offset resistors either. If you go grounding them then calling them something like balancers or humdingers is probably a better term.

Getting back to the OP's question, I think that DC elevation sounds better than grounding, so I wouldn't even consider grounding the resistors.

[g1]

Well that's drifting a fair bit from your original objection to the terminology I think.
I'm pretty sure the bulk of folks here consider 'virtual center tap' to mean resistors to ground like the standard (CBS) Fender arrangement.
I'd even go as far as to wager most here who commented were discussing that exact scenario.

If your objection to the 'virtual center tap' term is only where there is DC elevation, then I guess I missed the plot.

[bob p]

I don't get your idea of drift. That I personally wire them up to a DC source and refer to them as offsetters (or elevators, but never alligators) is separate from the objection that the VCT misnomer is misguided. My objection to the VCT has always been that a VCT cannot behave in all ways like a true CT, which renders the CT analogy invalid. Fussing about offset vs. balancing resistors or agreeing to call them this or that, or pretending that voltages are virtually the same, ignores the central issue that the current flow through the secondary windings can never be the same in center-tapped and non-center-tapped windings.

[Malcolm Irving]

... the central issue that the current flow through the secondary windings can never be the same in center-tapped and non-center-tapped windings.

Sorry, I don't get that. In a heater winding, the current is exactly the same whether there is a centre-tap or not and whether or not that CT is tied to a ground reference or to an elevated DC reference.

The current could only be different if some current is coming in or out via the CT, which is not the case here. The connection to the CT is just providing a voltage reference for the winding, which would otherwise be floating.