When to use heater center tap

[es345]

Might be that I was not clear enough, sorry for that. Let me try to clarfy.

My starting point was to the beginning.

Is it better to use the center tap of a heater winding, or leave it disconnected and use 100 ohm resistors to ground from each leg of the heater winding?

So here is my extended answer.

In case of two heater windings:
- center tap for power tubes.
- hum balance pot for preamp.

In case you have one heater winding for all only, I personally from my experience prefer the hum balance pot.
So please take it just as as a comment.

Hans Georg

[pdf64]

There is a third option which is to use the CT but connect it to ground through a small (e.g. 47R) fusible resistor. Thus you get the same fusing benefit of the artificial centre tap, but with a tiny bit less wiring.

What's the benefit (of the heater reference fusing)?
My thinking is that's a drawback (see post #4 for the rationale).

[Chuck H]

What's the benefit (of the heater reference fusing)?
My thinking is that's a drawback (see post #4 for the rationale).

I'm sure Merlin was speaking practically. Most amps DON'T have an HV rail fuse. One resistor is cheaper than two and the circuit he proposed is simpler as well.

Re reading post #4 (I must have moved through the posts too fast first time through) I'm inclined to agree with you. But sans an HV fuse, isn't there still some benefit to fusing the filament CT? In other words...
No HV fuse and filament wind CT=bad.
No HV fuse, but filament CT fused=better.
HV fuse with robust filament CT=best.

[pdf64]

Even without a B+ fuse, a heavy short on the B+ should pop a suitable line fuse fairly quickly, hopefully before the transformer windings etc have been damaged.
Whereas a blown reference resistor still requires a visit to a tech, along with potential for multiple tube replacement (ok the bad one that started it all off would need replacing anyway, probably along with its p-p partner).
And even when they blow the heater reference resistors, shorted power tubes usually seem to pop the line fuse anyway, eventually.

[trobbins]

Line fusing is not that helpful with many valve amplifier faults, especially if there is no NTC or soft start to allow a lower fuse rating. Even PT secondary side fusing is not easy when there is OT winding resistance to go through, and even less easy if going through output valve A-K path, or a valve rectifier diode power supply. Many OT's seem to sacrifice themselves - RIP.

Anode to heater faults (pin 2-3 flashover) really should be addressed by other protection means than trying to focus on humdinger or CT current paths.

[g1]

Anode to heater faults (pin 2-3 flashover) really should be addressed by other protection means than trying to focus on humdinger or CT current paths.

I've assumed that's what this Crate circuit (TR1 Sidac) is solving for?

[Enzo]

Yep.

[pdf64]

That's nice! Thanks for pointing it out.
Assume that the B+ is fused somehow too?

[diydidi]

My logic is that without the CT, you can't do a Heater elevation mod. I'm talking about the 2 resistor potential divider circuit from the HV rail, and a cap to ground from the junction formed where the CT and the 2 resistors meet.

[Tom Phillips]

My logic is that without the CT, you can't do a Heater elevation mod...

No. A transformer center tap is not required. If there is no CT then the elevation voltage can be applied to a virtual center tap created with resistors. There is a good article at The Valve Wizard that includes several examples of heater hum reduction circuits. See the Heater Elevation section for an explanation of that. The example shows the power tube cathode voltage being used for the elevation voltage but the approach also works with elevation voltage derived from the B+ as you described.

[g1]

Assume that the B+ is fused somehow too?

Erm, nope

[diydidi]

No. A transformer center tap is not required. If there is no CT then the elevation voltage can be applied to a virtual center tap created with resistors. There is a good article at The Valve Wizard that includes several examples of heater hum reduction circuits. See the Heater Elevation section for an explanation of that. The example shows the power tube cathode voltage being used for the elevation voltage but the approach also works with elevation voltage derived from the B+ as you described.

Thank you Tom. I stand corrected.

[Chuck H]

No. A transformer center tap is not required. If there is no CT then the elevation voltage can be applied to a virtual center tap created with resistors. There is a good article at The Valve Wizard that includes several examples of heater hum reduction circuits. See the Heater Elevation section for an explanation of that. The example shows the power tube cathode voltage being used for the elevation voltage but the approach also works with elevation voltage derived from the B+ as you described.

Somehow I thought that diydidy was including virtual center taps with winding center taps in that post. I thought he was asking after simple, non center tapped filament circuits like those found in most vintage single ended amps.

[pdf64]

Line fusing is not that helpful with many valve amplifier faults, especially if there is no NTC or soft start to allow a lower fuse rating...

Yes, as RG occasionally reminds, the line / primary side fuse is there to mitigate equipment failure resulting in the building catching fire.

..Even PT secondary side fusing is not easy when there is OT winding resistance to go through, and even less easy if going through output valve A-K path, or a valve rectifier diode power supply. Many OT's seem to sacrifice themselves - RIP...

Especially the case on amps with >2 power tubes.

Anode to heater faults (pin 2-3 flashover) really should be addressed by other protection means than trying to focus on humdinger or CT current paths.

But even fusing each power tube cathode doesn't mitigate plate-heater or screen-heater shorts.
If trying to do it 'properly', fusing can get very convoluted!

It seems strange that the Crate BV120 doesn't have B+ fusing but maybe sidac pop's the line fuse when B+ ends up down the heater line? It has a slow blow primary fuse and a thermistor though?

[Enzo]

To be very honest, in my shop experience, the largest cause of B+ to heater shorts is a tube with the center locating peg broken off, and the tube stuck in the socket with the wrong pin orientation.