Simple question about heater wiring

[rutledj]

If the 6.3 VAC winding has a center tap hooked to ground is there still a need for two 100 ohm resisters to ground?

Thx,
Rut

[pompeiisneaks]

No in fact I think it can be harmful to do so. Also I've had it explained to me that even if the transformer has a center tap, it's safer for the heater winding and the tubes to not use it, shrink tube it up and roll away. The 100R resistors to ground perform a great optional feature of being a very easily burned up fuse if something shorts to the heaters, and then the amp stops working, you find the problem and replace a few pennies worth of resistors

~Phil

[pdf64]

My view is that if the heater circuit is to be referenced to 0V, it is better if that reference is robust. Hence if there's one available, the winding CT is preferred to balancing resistors.
There's no benefit of using both.
A robust 0V reference is beneficial due to a common tube failure mode of an internal short that connects HT current to the heater circuit.
That results in HT fault current flowing through the heater circuit to its 0V connection.
If resistors are used for that 0V connection they will invariably burn out.
That then allows the short to pull the entire heater circuit up to HT potential.
That condition could result in damage the heater to cathode insulation of every other tube in the amp, as the limiting value would be massively exceeded.

A CT 0V connection will carry the fault current and pop the HT or mains fuse.

If there's no CT, I like to use beefy 5W balancing resistors that will hopefully maintain the 0V reference in the face of such a short, long enough for a fuse to blow.

[pompeiisneaks]

My view is that if the heater circuit is to be referenced to 0V, it is better if that reference is robust. Hence if there's one available, the winding CT is preferred to balancing resistors.
There's no benefit of using both.
A robust 0V reference is beneficial due to a common tube failure mode of an internal short that connects HT current to the heater circuit.
That results in HT fault current flowing through the heater circuit to its 0V connection.
If resistors are used for that 0V connection they will invariably burn out.
That then allows the short to pull the entire heater circuit up to HT potential.
That condition could result in damage the heater to cathode insulation of every other tube in the amp, as the limiting value would be massively exceeded.

A CT 0V connection will carry the fault current and pop the HT or mains fuse.

If there's no CT, I like to use beefy 5W balancing resistors that will hopefully maintain the 0V reference in the face of such a short, long enough for a fuse to blow.

That sounds like the opposite of how it was explained to me. If there's no center tap, and only the two 100 ohm resistors giving reference to ground and they burn up, then the heater winding is floating and has no 'route' for the HT to burn up the winding or other things. Since 2 100 ohm 1/2w resistors will burn up very fast when HT hits them, it effectively works like a fuse. The problem with having a reference to chassis is that if HT shorts to the heaters, it can continue to run the full HT through the heater winding's to ground until they burn up and they become the fuse.

Maybe I'm completely confused though. I often am.

~Phil

[pdf64]

The heater to cathode insulation of tubes has a voltage limit, 100 to 200V.
If the heater balancing resistors blow, the heater circuit will get pulled up to HT potential, eg >300V.
That's bad, all the tubes may get damaged and become buzzy, just because 1 tube shorted.
Hence use a CT or beefy resistors to keep the heater circuit tied to 0V even if a tube shorts.
The sustained fault current should pop a HT or mains fuse pretty quickly.

My rationale is that it is better for the stress of the fault to be carried by the PT (as that can be easily protected from the stress by fusing) rather than the heater to cathode insulation of all the tubes in the amp (as they are tricky to protect).
Hope that makes sense?

[pompeiisneaks]

The heater to cathode insulation of tubes has a voltage limit, 100 to 200V.
If the heater balancing resistors blow, the heater circuit will get pulled up to HT potential, eg >300V.
That's bad, all the tubes may get damaged and become buzzy, just because 1 tube shorted.
Hence use a CT or beefy resistors to keep the heater circuit tied to 0V even if a tube shorts.
The sustained fault current should pop a HT or mains fuse pretty quickly.

My rationale is that it is better for the stress of the fault to be carried by the PT (as that can be easily protected from the stress by fusing) rather than the heater to cathode insulation of all the tubes in the amp (as they are tricky to protect).
Hope that makes sense?

It doesn't, not yet. The voltage on a circuit that is carrying 0 current is only potential voltage right? so there's 300VDC sitting 'wanting' to go across the heaters, and it will with a tube short, so long as the heaters can conduct the current. if center tapped that means it will go the full current of it, with nothing but mains fuse to stop it. (or b+ as well if there is one, but there isn't always one). but if the fuse like 100 ohm resistors burn up, they leave nowhere for the current to go, there's two ends of a heater circuit with no ground reference. They woudl blow WHEN the heaters reached the 300VDC potential or somewhere en route to it. The heaters suddenly went from 6v to 300 v the first thing to burn in that case is the 100 ohm resistors, that's a helluva lot more than 1/2 w hitting 100R at 300V. (I've seen it happen w/ an amp that had a bad PT that was sending B+ to the heaters.)

I was given this explanation by Jelle I think as well as several other regular amp techs on FB, but agian, I'm passing off potentially misinformed information. Thus why I admit I might be wrong.

Your explanation makes me think you're hopgin a 2 or 3 amp fuse on 120v to blow before a 6.3v heater winding at say 6A
The B+ is going to burn up the mains at 240 Watts (120V * 2 amps)
those same 2A in the heaters is going to be transformed down to power but up in amps no (If I recall how the transformers work, if they up voltage, they can do less current, and vice versa)

that's a 19:1 ratio from 120 to 6 ish. doesn't that mean that 6V at 5A is equal to .26 amps at 120V across the primary side. nowhere near the fuse level?

maybe, again, I'm just confusing the transformer math. I got the impression that B+ on heaters w/ a center tap is very likely to burn up the heater winding before the fuse blows, as well as the tubes heaters in the process
whereas the resistors, if they blow, stop all current in the heaters.

No?

Someone help me

~Phil

[statorvane]

I'm with sneaks here.
The only addition I do with heater wiring is reference the artificial center tap resistors to the power tube cathode; raise the voltage a little bit.
I have had the resistors (1/2 watt 100R) blow on occasion when checking voltages and accidentally shorting the winding. Poof - there's no heater voltage. There's also no poof from the tubes or transformer. Pain to solder in the new resistors, but worth it in my opinion.

[pdf64]

Please note that heater balancing resistors don’t act as fuses that protect the PT’s heater winding from excess current. Consider that even if there was a direct short across the winding drawing fault current, that short would reduce the Vac across the resistors (and hence their dissipation) to zero.
They offer no protective benefit over connecting a winding CT to 0V.
If the heater balancing resistors blow, it doesn’t disable the heater supply to the tubes, they continue to draw heater current and reach normal emission;
The heater circuit just loses its reference to, and balance around, 0V.
The only obvious symptom of that (without there being any other issues) is that the amp would be rather buzzy.

[pompeiisneaks]

Please note that heater balancing resistors don’t act as fuses that protect the PT’s heater winding from excess current. Consider that even if there was a direct short across the winding drawing fault current, that short would reduce the Vac across the resistors (and hence their dissipation) to zero.
They offer no protective benefit over connecting a winding CT to 0V.
If the heater balancing resistors blow, it doesn’t disable the heater supply to the tubes, they continue to draw heater current and reach normal emission;
The heater circuit just loses its reference to, and balance around, 0V.
The only obvious symptom of that (without there being any other issues) is that the amp would be rather buzzy.

Again, this is where I'm confused. how can a winding conduct if it has no closed circuit? Either the center tap, or the resistors, close the circuit don't they? Or am I missing something obvious?

~Phil

[xtian]

Again, this is where I'm confused. how can a winding conduct if it has no closed circuit? Either the center tap, or the resistors, close the circuit don't they? Or am I missing something obvious?

Imagine you simply connect your 6vAC winding (no center tap at all) to pins 2 and 7 of a power tube and turn it on.

-Captain Obvious

[pompeiisneaks]

Again, this is where I'm confused. how can a winding conduct if it has no closed circuit? Either the center tap, or the resistors, close the circuit don't they? Or am I missing something obvious?

Imagine you simply connect your 6vAC winding (no center tap at all) to pins 2 and 7 of a power tube and turn it on.

-Captain Obvious

OH, duh, the TUBES close the circuit. Then why is it, if you don't put a reference to ground they work so poorly? Or not at all.

At least I learned something today.

So if I'm understanding that, the center tap is no more/less dangerous than a virtual center tap?

Code: Select all

 What does the center tap gain the amp then?  Just reduces hum from the heater lines?

I just decided to look that up again in merlin's site and yet again, he made it make perfect sense, it IS to balance the hum only:

http://www.valvewizard.co.uk/heater.html

duh.

Thanks captain obvious

~Phil