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Thread: Updated Champ amp heater circuit

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    Updated Champ amp heater circuit

    Ok, guys, here's a real low-tech question:

    I'm updating my workhorse and very reliable, around the house SF 70's Fender Champ amp. I"m gonna put in a twisted pair heater circuit w/ 2x 100 ohm ground reference resistors. Do I want to attach these resistors to the bias supply (approx 28 Vdc)? Sometimes, I've just grounded the ground reference resistors and other times, I've 'elevated' them via the bias supply. I've never really made a study of what yields the quietest amp but I've have had very good luck, noise-wise (or lack thereof) with just grounding these resistors in the past.

    Any and all on-point comments are welcome.

    Bob M.

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    Bent Member Chuck H's Avatar
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    If you REALLY want to know, try it both ways. It's one wire. If there doesn't seem to be any advantage to elevating the false CT at the cathode resistor just ground it. The last two I did were elevated, but only because I didn't try it both ways so I can't say what difference it may have made.
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    And my '79 was dead quiet with just the standard heater wiring as it left the factory...
    So, they all can work. But if it ain't broke...

    Jusrin
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    Bent Member Chuck H's Avatar
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    Ok... Let's take it next level. Since the twisted pair is only effectively reducing hum in the single preamp tube (the tremolo tube isn't in the signal path and the power tube has one filament) you could consider a DC circuit. The winding would probably handle it (though I haven't researched it for certain) since that PT was also used in the Princeton Reverb which draws A LOT more filament current.

    EDIT: Rethinking this... Short of DC filaments, a long as the cathodes of the first preamp tube are fully bypassed (the first triode is, the second virtually), as it IS in the stock circuit then the stock circuit should work fine. Now, if you've done something like a partial bypass of the first triode then you may benefit from a different filament winding.
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    Better Tone thru Mathematics bob p's Avatar
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    > Any and all on-point comments are welcome.

    oops.

    Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on. No offense intended, Bob and Chuck. I know that you know that it's not really a center tap and that the purpose of the resistors is just to provide a DC offset to the heater supply -- I'm just wondering where that "virtual center tap" terminology came from, because even though it's wrong it sure seems to have become sticky.
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    Even if you can't hear any difference, put your true rms meter on the speaker leads(or dummy load), you will measure millivolts on the AC scale. Rewire with twised pair, remeasure, elevate, remeasure again.

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    Better Tone thru Mathematics bob p's Avatar
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    Quote Originally Posted by Justin Thomas View Post
    And my '79 was dead quiet with just the standard heater wiring as it left the factory...
    So, they all can work. But if it ain't broke...

    Jusrin
    My SFVC is dead quiet too. If it got noisy all of a sudden, I'd be thinking about a bad cap or a bad tube.
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    Bent Member Chuck H's Avatar
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    Jusrin () and Bob,

    Do your VC's have the first triode fully bypassed like the stock circuit? Because I REALLY think that's the key to quiet operation with the stock filament wiring.
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    Yeah, mine does. Cuz I'm not clown shit crazy.

    Jusrin
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    Better Tone thru Mathematics bob p's Avatar
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    absolutely stock.
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    Quote Originally Posted by bob p View Post
    > Any and all on-point comments are welcome.

    oops.

    Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on. No offense intended, Bob and Chuck. I know that you know that it's not really a center tap and that the purpose of the resistors is just to provide a DC offset to the heater supply -- I'm just wondering where that "virtual center tap" terminology came from, because even though it's wrong it sure seems to have become sticky.
    Bob, it's a good thing your here to enforce nomenclature compliance. If it weren't for you, I'd still be calling it "dampening".
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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by bob p View Post
    > Any and all on-point comments are welcome.

    oops.

    Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on.
    Well... Both an actual CT and the 100R balance resistors act as a 0V reference with very little current aspect. What would YOU call it? Not being snarky, it just seems like a fair question. I didn't make up "virtual center tap" but I'll use the term for lack of a better one. With 0V reference grid bias resistors we call them "grid load". Maybe "filament load"? Except it's not really a load WRT the filament circuit impedance where a grid load IS a load.
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    Quote Originally Posted by bob p View Post
    > Any and all on-point comments are welcome.

    oops.

    Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on. No offense intended, Bob and Chuck. I know that you know that it's not really a center tap and that the purpose of the resistors is just to provide a DC offset to the heater supply -- I'm just wondering where that "virtual center tap" terminology came from, because even though it's wrong it sure seems to have become sticky.
    An actual center tap creates out of phase currents at 3.15V with respect to ground (or to an elevation voltage), so that with tightly twisted wires the electrostatic field due to one wire is cancelled by the field due to the other wire. The two 'virtual center tap' resistors perform the same function. Perhaps 'artificial center tap' would be a more accurate description.
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    Tqi
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    Quote Originally Posted by the fatch View Post
    Perhaps 'artificial center tap' would be a more accurate description.
    The Valve Wizard

    This person agrees with you.

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

    Justin
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    Better Tone thru Mathematics bob p's Avatar
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    Quote Originally Posted by Justin Thomas View Post
    "Fake Center Tap" is easier to say and takes up less interweb space...

    Justin
    As does saying that SF Fender amps are "ultralinear".
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    Better Tone thru Mathematics bob p's Avatar
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    I guess it's time for an old curmudgeon rant. Feel free to skip this post if you like.

    Quote Originally Posted by Tqi View Post
    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.
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    Quote Originally Posted by bob p View Post
    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.
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    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
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    Better Tone thru Mathematics bob p's Avatar
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    I believe the term you are looking for is "Fugazi."
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    Pseudo might raise fewer objections than fake.
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    Better Tone thru Mathematics bob p's Avatar
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    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.
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    Bent Member Chuck H's Avatar
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    Ok, how about 'center tap substitute balance resistors'?

    Say it ten times fast!
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    I think it's just a fixed humdinger; nothing fake there..
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    Better Tone thru Mathematics bob p's Avatar
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    Quote Originally Posted by Chuck H View Post
    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.
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    Bent Member Chuck H's Avatar
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    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.
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    g1
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    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.
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    Supporting Member The Dude's Avatar
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    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.
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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Chuck H View Post
    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.
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    Quote Originally Posted by bob p View Post
    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.
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    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'

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    Quote Originally Posted by pdf64 View Post
    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.
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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by pdf64 View Post
    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.
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    Distortion is a form of signal degradation. It is pretty easy to define what a good "clean" signal is, but trying to achieve just the right kind of "broken" can be pretty maddening.

  34. #34
    Better Tone thru Mathematics bob p's Avatar
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    Quote Originally Posted by pdf64 View Post
    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).
    Last edited by bob p; 05-16-2018 at 09:06 PM.
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    "Stand back, I'm holding a calculator." - chinrest

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    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.
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