Updated Champ amp heater circuit

[pdf64]

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?

[bob p]

if all you have is a resistor then everything looks like a nail, if you know what i mean.

[ric]

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.

[Steve A.]

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.

[Malcolm Irving]

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.

[Malcolm Irving]

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.

[Malcolm Irving]

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.

[Leo_Gnardo]

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.

[Chuck H]

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.

[Leo_Gnardo]

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.

[nickb]

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.

[Chuck H]

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.?.

[nickb]

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.

[Leo_Gnardo]

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.

[J M Fahey]

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.