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Thread: Updated Champ amp heater circuit

  1. #36
    Senior Member Malcolm Irving's Avatar
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    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.

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    Last edited by Malcolm Irving; 05-17-2018 at 09:51 AM.

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    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

    Quote Originally Posted by Bob M. View Post
    ...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.

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    Last edited by pdf64; 05-17-2018 at 07:55 PM.

  3. #38
    Better Tone thru Mathematics bob p's Avatar
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    I was only kidding with my previous Amp Garage comment. I had no idea there was such a thread out there.

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    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)?

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    Better Tone thru Mathematics bob p's Avatar
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    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.

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    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?

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    Better Tone thru Mathematics bob p's Avatar
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    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.

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    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.

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    Better Tone thru Mathematics bob p's Avatar
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    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.

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    Senior Member Malcolm Irving's Avatar
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    Quote Originally Posted by bob p View Post
    ... 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.

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    Last edited by Malcolm Irving; 05-18-2018 at 11:34 AM.

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    Whatever the dc reference point chosen, isn't the point of having the 2 resistors or CT tied to that node is that they force the heater circuit to be balanced with respect the ac 0V?

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    Better Tone thru Mathematics bob p's Avatar
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    if all you have is a resistor then everything looks like a nail, if you know what i mean.

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    Quote Originally Posted by bob p View Post
    if all you have is a resistor then everything looks like a nail, if you know what i mean.
    This place holds a real high average for intelligent worthwhile information.

    Keeping things scientifically accurate is important and commendable.

    Fifty years and more ago we always heard that "scientists now believe..." Now everything anybody says about anything is always just stated as fact. Even as we hear the laws of physics aren't what we used to think. Tomorrow who knows.

    Just my opinion....for now.

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    Member Emeritus Forever Steve A.'s Avatar
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    Quote Originally Posted by g1 View Post
    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.
    Agreed! So what is it called when one of the two filament leads is connected to ground as was often the case before the introduction of the maligned virtual center tap? I always assumed that wiring like that would (or could) be noisier... true or false?

    Steve A.

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    Senior Member Malcolm Irving's Avatar
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    As pdf mentioned, when the CT or artificial CT is ac grounded, the two wires of the heater supply are balanced. When one wire is instantaneously at some positive voltage the other wire is simultaneously at an equal negative voltage (with respect to ground). The electric field generated by the two wires cancels out, if the two wires are in the same place. It is therefore good to twist the wires together to put them as close as possible to being 'in the same place'. The current in the two wires is always the same, but travelling in opposite directions, so again we have good cancellation of the magnetic field, although the CT has no effect on the magnetic field.

    You should therefore get less hum with a CT, or artificial CT, than you would with just grounding one side.

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    Last edited by Malcolm Irving; 05-19-2018 at 12:52 PM.

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    Senior Member Malcolm Irving's Avatar
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    A 'humdinger' can give even lower hum by allowing the two resistances (in an artificial CT) to be adjusted away from being equal. I think this works because the most sensitive point or points in the signal circuit, where the hum is being picked up, will be closer to one of the heater wires than the other.

    Another possibility is that hum from the unbalanced heater wiring is out of phase with some other hum source.

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    Last edited by Malcolm Irving; 05-19-2018 at 05:23 PM.

  17. #52
    Senior Member Malcolm Irving's Avatar
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    Possibly one of the reasons that humdingers are not seen more often is the power dissipation. 6.3Vac across 200 ohms gives 0.2 watts - maybe a bit hot for a typical preset pot.

    For those of us who like to fiddle about with such things, how about a chain of four 50 ohm resistors - giving a choice of 5 possible points to ground (or DC elevate) - find the point with least hum by trial and error.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Steve A. View Post
    Agreed! So what is it called when one of the two filament leads is connected to ground as was often the case before the introduction of the maligned virtual center tap? I always assumed that wiring like that would (or could) be noisier... true or false?

    Steve A.
    Disclaimer: I don't know what it's called, Steve. But I'll flap my fingers across the keyboard nonetheless, as I do.

    A single sided filament line, with the chassis used as the current return, doesn't make much difference in a simple low gain amp like a Champ. I used to think it did, and was disappointed when altering the scheme to the balanced style didn't reduce hum. In Champs and Champ-like amps now I concentrate on filtering the high voltage better, and that works a treat. Hi gain amps with multiple cascaded preamp stages, there's where you need a balanced filament supply, or even better go DC for the gain stages.

    Malcolm, nice idea for the 4 x 50 ohm resistor and switch. Problem is, the sweet spot for hum reduction is more often found with a smaller difference between balancing resistors than can be achieved with your suggestion. In some cases I've put in an 82 and 120 ohm fil balance pair, or 82 and 100, or 100 and 120. By the time I've finished faffing, I could have installed a humdinger. Best pot for this I can easily find is a wire wound 100 ohm from Antique/CE. It's not ideal, with a sort of flimsy splined "stick." Best would be a pot like Fender used on their 70's large amps, with an inset screwdriver adjust, leaving nothing sticking out that could be accidentally broken off or twisted off its mark.

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    Bent Member Chuck H's Avatar
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    Mouser sells a better looking CTS pot for a couple more bucks. Rated at 5W and 500V!

    https://www.mouser.com/ProductDetail...aBSSW89g%3d%3d

    I've never played with the humdinger design or incorporated one in a build. I've been lucky (?) than any hum in my builds so far was able to be eliminated with ground scheme refinement. But with modern tubes (where balancing on power tubes is just "ok" and preamp tubes sometimes hum like they don't know the words) it'll probably come up at some point.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Chuck H View Post
    Mouser sells a better looking CTS pot for a couple more bucks. Rated at 5W and 500V!

    https://www.mouser.com/ProductDetail...aBSSW89g%3d%3d
    Thanks for that lead Chuck! I'll have to include a batch on my next Mouser order.

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  21. #56
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Malcolm Irving View Post
    Possibly one of the reasons that humdingers are not seen more often is the power dissipation. 6.3Vac across 200 ohms gives 0.2 watts - maybe a bit hot for a typical preset pot.
    It's worth noting that the actual current required to make the humdinger work is small and therefore a higher value pot say 1k to 10k 0.5W (which are readily and cheaply available) will do the job. You have have to provide an alternative path to ground for fault currents. This can be done using a TVS diode. These 5KP12A ones give a good voltage margin and will take 400 amps for long enough for the fuse to blow. Less expensive than a 200 ohm 5W (to handle the fault current) pot, I should think. Just connect it with the cathode (stripe) to the heater supply and anode to ground.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by nickb View Post
    It's worth noting that the actual current required to make the humdinger work is small and therefore a higher value pot say 1k to 10k 0.5W (which are readily and cheaply available) will do the job. You have have to provide an alternative path to ground for fault currents. This can be done using a TVS diode. These 5KP12A ones give a good voltage margin and will take 400 amps for long enough for the fuse to blow. Less expensive than a 200 ohm 5W (to handle the fault current) pot, I should think. Just connect it with the cathode (stripe) to the heater supply and anode to ground.
    Well... Not an engineer so it goes beyond my wee mind, but I always just assumed that the 100R value for the balance resistors was relative to circuit impedance. If a pair of 5k resistors "will do the job" then, are there any disadvantages to the higher value? I mean, the 100R value WAS chosen at some point by guys that knew what they were doing, right? Also, if the 1k to 5k value is suitable I would think it could mitigate those components blowing in the event of a plate to heater short, which Pete indicated as undesirable. Seems like a win/win if there's no downside.?.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Chuck H View Post
    Well... Not an engineer so it goes beyond my wee mind, but I always just assumed that the 100R value for the balance resistors was relative to circuit impedance. If a pair of 5k resistors "will do the job" then, are there any disadvantages to the higher value? I mean, the 100R value WAS chosen at some point by guys that knew what they were doing, right? Also, if the 1k to 5k value is suitable I would think it could mitigate those components blowing in the event of a plate to heater short, which Pete indicated as undesirable. Seems like a win/win if there's no downside.?.
    The low value and high power rating was chosen to handle fault currents, not for the humdinger current which is in the microamps. And don't forget they didn't have TVS diodes back then either.

    The high value pot on its own is no use as the voltage under fault conditions would be too high so you could damage the pot and all the tubes.

    BTW, I recently applied 600VDC to a Sovtek 12AXLPS both ways from cathode to heater. The insulation did not break down. No leakage at all.

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    Last edited by nickb; 05-19-2018 at 07:37 PM.
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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Chuck H View Post
    Well... Not an engineer so it goes beyond my wee mind, but I always just assumed that the 100R value for the balance resistors was relative to circuit impedance. If a pair of 5k resistors "will do the job" then, are there any disadvantages to the higher value? I mean, the 100R value WAS chosen at some point by guys that knew what they were doing, right? Also, if the 1k to 5k value is suitable I would think it could mitigate those components blowing in the event of a plate to heater short, which Pete indicated as undesirable. Seems like a win/win if there's no downside.?.
    Why 100 ohms? Why indeed? I've seen some Fenders with 47 ohm, and once or twice factory installed 82 ohm. If they ran out of 100's, gotta punt to fill those orders. For humdingers, I've used up to 500 ohm with no ill effects. I got a batch of Ohmite 250 and 500 ohm mil spec pots for dead cheap, but they're long since used up. Will have to experiment with higher values.

    As far as mitigating damage from high voltage shorts, that's not going to work. Any resistors or pots with values in this range would be vaporized by a short from a tube amp's B+. One possible solution: remember those mid 60's Ampegs that used a capacitor between their filament center tap and ground? Sometimes that's the best way to go and certainly worth a try if you're flailing around trying to minimize filament induced hum/buzz. So - try a good quality film cap from your humdinger pot wiper, or resistor junction, to ground. A cap with 600-630V rating will safely keep the pot/resistors from passing fault current to ground and wrecking them. Caps 0.022 to 0.1 uF are usually used in this application, but you can try other values and maybe find one that crushes that awful hum. Granted this isn't the solution in every case. Different amps require different solutions, and eliminating hum/buzz - despite application of standard electrical truths - still seems to be a "black art." That's not to say it's magic. But often the best solutions require experimentation, and what works in one amp may not work well at all in another.

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    Old Timer J M Fahey's Avatar
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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.
    Sorry but you are quite wrong
    And to make it clear itīs just not a typo, you repeat same wrong concept in different ways

    The center tap, whether real (copper wire tap in the middle of the winding) or a halfway point between 6.3V wires created with two same value resistors is NOt just a "DC reference" (even that is wrong, itīs actually an AC reference to chassis, no DC there) BUT a way to generate 2 x 3.15V **balanced** voltages, which to boot are preferrably carried on *twisted* wires (ever thought about the reason for that?) so they cancel each other and donīt induce hum on nearby wiring.

    In fact, some tubes have spiral wound filaments for the same reason, but of course they again need balanced filament voltage to achieve that.

    And since balancing may not be perfect, some amps have a wirewound "Hum Balance" rheostat to fine tune Hum reduction.

    But said balanced voltage can be created either by a center tap or two resistors, same thing.

    In fact, on some old amps which actually had a copper wire center tap itīs often suggested to remove and tape it out and instead use a couple resistors, only for the very good reason that resistors can easily be found with 5% tolerance (or better, but itīs not necessary) , while a winding "center tap" may actually not be in the exact center.
    Since those are low turns count windings, maybe one turn is "too low" while the next one is "too high", you have definite "voltage steps" from turn to turn, and if, say, you have an odd number of turns there is actually NO exact center tap.
    Suppose you have 13 turns (0.5V per turn which gives a 6.5V secondary) , you either pick the turn #6 or the #7 one, flip a coin, both are "wrong".
    While 2 resistors may be much closer.

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    Valvulados.com jmaf's Avatar
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    Quote Originally Posted by bob p View Post

    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.
    It originated from the fact that the 2x100 ohm resistor trick is simply a poor man's center tap for amplifiers where you didn't have 3.15 + 3.15 VAC. Like almost everything else in guitar amplifiers, the terminology is street based. A lot of the amp modders and builders are self taught and don't have a fancy EE degree from MIT and last I checked they don't deploy tube amps at NASA, so folks use the terms they see which is fine IMO, everyone knows what is being referred to here. I don't understand what the issue is, virtual center tap has a nice ring to it.

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    Better Tone thru Mathematics bob p's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Sorry but you are quite wrong
    And to make it clear itīs just not a typo, you repeat same wrong concept in different ways
    Maybe you're not following parts of the conversation that haven't been explicitly fleshed out with schematics. Maybe the problem is that you jumped to a conclusion when you read post 22 on page 1 and you didn't bother to read posts 39-44 on page 2, which clarified those things that you falsely accused me of not understanding. As if I can't tell the difference between AC and DC! Come on!

    It seems that there are a lot of people reading this thread who are wearing blinders that cause them only to think about hooking up resistors to the secondary and wiring the junction to ground, which is actually not such a great way to build your amp. The fact that the two types of secondary windings behave similarly in the case where resistors are attached to them has made some people ask if the windings will behave the same way when something else -- like diodes -- are attached to them. Of course that won't be the case. Current will flow through the center tap. There is no equivalent circuit in that case with non-center tapped secondary, so the simplified banter that the two secondary designs are equivalent in the case of hanging resistors on them is based upon flawed logic as it only considers a narrow subset of transformer behavior. This is precisely why center-tapped and non-center-tapped secondaries have to use different rectifier topologies.

    The center tap, whether real (copper wire tap in the middle of the winding) or a halfway point between 6.3V wires created with two same value resistors is NOt just a "DC reference" (even that is wrong, itīs actually an AC reference to chassis, no DC there)
    We've already clarified that I never said it was a DC reference in that case -- what I said is that I call them DC offset resistors when I use them, because I provide a DC offset in my builds. Again, it seems that you've made the misteak of imagining a schematic that's different than what I'm talking about. In the posts on page 2 I clarified this -- the resistors provide an AC reference + balancing when you connect the resistor junction to ground, and provide a DC reference + balancing when you connect the resistor junction to an elevated DC source, as the OP asked about the original post, which everyone seems to have ignored. For some reason everyone seems to be ignoring that part of the discussion, and limiting their thoughts to the two resistors to ground model. It's time to take off the blinders.

    It seems evident that some people keep reading this thread with one specific schematic in their mind, and are ignoring other topologies that have been discussed. I didn't think I'd have to draw pictures to make it clear, but I guess I was wrong on that. My bad.

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    Quote Originally Posted by bob p View Post
    It seems that there are a lot of people reading this thread who are wearing blinders that cause them only to think about hooking up resistors to the secondary and wiring the junction to ground, which is actually not such a great way to build your amp. The fact that the two types of secondary windings behave similarly in the case where resistors are attached to them has made some people ask if the windings will behave the same way when something else -- like diodes -- are attached to them. Of course that won't be the case. Current will flow through the center tap. There is no equivalent circuit in that case with non-center tapped secondary, so the simplified banter that the two secondary designs are equivalent in the case of hanging resistors on them is based upon flawed logic as it only considers a narrow subset of transformer behavior. This is precisely why center-tapped and non-center-tapped secondaries have to use different rectifier topologies.
    I've read this 4X (for real) and I have no idea what he's trying to say here. CT and non CT secondaries have to use different topologies due to the limitations of the era, back in the day rectifiers were 2 plate tubes, then we got flooded with cheap silicone diodes. They're completely different circuits but as with any circuit, they obey the same laws.

    Dangling two 100 ohm resistors across any power supply is not rocket science and it quiets most hum. What's the big deal here? I couldn't imagine a 2 page thread about something so simple TBH. Are we really discussing OHMs law and 2 100 ohm resistors here?

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    I'm glad that Malcolm pointed out that there's virtue in using 'virtual' in electronics.

    I'm glad to see that G1 brought this argument full circle.

    Obviously, some people are in the DC offset bias camp; others are just grounding their resistors or pot. I decided to try the resistor grounding idea first and my amp is as quiet as quiet can be (that's an unscientific term) so there's no reason to try anything else. Of course, part of my good luck has to do with shielded grid wires and a very good grounding scheme.

    Now, I'm trying to decide if I like the 5V4 rectifier better than the stock 5Y3 and I'm experimenting with a couple of single ended replacement output transformers that I have in stock. For those who have replaced the output transformer on their Champ, VC and/or Bronco amps, what are some successful good choices for an upgraded improvement of this amp's sound?

    Thanks all for your input and knowledge,

    Bob M.

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