Heater CT with balance resistors

[Guy77]

I thought I would show everyone how one boutique builder is sending their heater center taps to ground.
This was a 2rock Bloomfield drive amp.
Looks like these 2 large (grey) 100 ohm balancing resistors ( red arrow) are 2 or 3 watts. Not like the 1/2 watt that Fender used back in the day.

heaters.jpg

Cheers
G

[pdf64]

I think it’s a good idea for output valve short not to blow the heater circuit reference. Rather the weak point should be a fast acting HT fuse.
Vintage production valves can be the most expensive part in an amp, and it seems a good thing to protect them

[Guy77]

I think it’s a good idea for output valve short not to blow the heater circuit reference. Rather the weak point should be a fast acting HT fuse.
Vintage production valves can be the most expensive part in an amp, and it seems a good thing to protect them

Yes replacing tubes these days can be very pricey. What I found interesting is that these 2 Rock amps only use a single fuse on the MAINS.

G.

[FUCHSAUDIO]

It also appears they may have a heater CT on that same terminal strip (?).

Either method (balancing resistors or ct) is usually fine, but both at the same time makes no sense.

If you do the math, the values are not critical, nor is there much current through them either. In production, I use both methods depending on the model, but not together.
In some mods and some repairs, adding an actual hum balance pot does let you tweak the noise floor down quite a bit sometimes.
The advantage of a heater CT is usually when a power tube dies it just blows the fuse. With balance resistors, the resistors can burn sometimes.

Their earth ground is the wrong color code and I see no ground identification sticker either...

[Guy77]

It also appears they may have a heater CT on that same terminal strip (?).

Either method (balancing resistors or ct) is usually fine, but both at the same time makes no sense.

If you do the math, the values are not critical, nor is there much current through them either. In production, I use both methods depending on the model, but not together.
In some mods and some repairs, adding an actual hum balance pot does let you tweak the noise floor down quite a bit sometimes.
The advantage of a heater CT is usually when a power tube dies it just blows the fuse. With balance resistors, the resistors can burn sometimes.

Their earth ground is the wrong color code and I see no ground identification sticker either...

Yes I believe that is why Fender used the 1/2 watt resistors so that they can act like a fuse.
In this one they just used the balance resistors and shrink wrapped the green/yellow heater center tap wire. So even though their transformers have the Green/Yellow heater CT wire they prefer not using it and use the 2 watt 100ohm balance resistors. I have seen this same behavior in several of their amps.
It seems their main concern here is too just keep the amp quiet since if they wanted this to act like a fuse they would use a smaller resistor.
I guess this is there personnel preference. Or maybe they are thinking the large resistors will survive and the MAINS fuse will blow first?

Cheers
G.

[R.G.]

You cannot be sure that a heater short will blow a mains fuse, full stop. A worst case soft short on a heater can just sit there and roast the winding.
Using a resistor as a fuse is somewhere between wishful thinking and naivety. The opening time is not predictable.

[sluckey]

I don't think Leo ever intended these resistors to act like fuses. But history has shown that they have probably saved a lot of more expensive stuff.

[martin manning]

A fuse in the heater CT might not be a bad idea, then?

[R.G.]

Sorry - eat before reading.

A CT fuse will provide no protection for a short from heater to heater.

[Guy77]

Thanks for everyone's input here. The more I learn I realize there is more " to learn " and the input here has been great!

One thing I wanted to mention is that the more common tube failure I have read and seen is the internal short that connects HT current pin to the power tubes heater pin (circuit). In this case scenario I believe if there is a path to ground through the CT connection or balancing resistors connection to ground, than it will carry that current to ground and if the resistors are beefy enough then the MAINS fuse will pop first? This would explain why 2 Rock is always using the beefy CT resistors.

G.

[R.G.]

Good reasoning and insight on your part.

There is some subtlety in fuse use. The mains fuse is not there to protect anything in the amp. It's there to prevent a fault in the amp AC wiring or PT itself from starting a house fire.

A fault on the high voltage supply will nearly always blow the primary fuse, but to actually protect the high voltage windings, you have to fuse the high voltage supply itself. This is why the HT fuse is there in many amps. This fuse, if well selected, can stop a tube plate-to-heater short from killing the high voltage secondary winding - as well as probably protecting the rectifiers and main filter caps from a tube short.

To protect the PT from a rectifier or filter capacitor fault, as well as a tube shorting after the rectifier/filter, you need to fuse both high voltage half-secondaries separately. About here, amp makers think or say "Hey! That's way too many fuses! Fender (or Marshall, or insert your favorite brand here) only used a mains fuse. Why should I use more?" The answer is that you don't have to. You just have to pick the level of risk you're willing to bear.

A useful exercise in the protection racket is to take a parts list of the amp and sort it by price, most expensive to replace first. Then think of what level of money and time you're willing to expend for a given part to keep a lower-cost part failure from killing things above it. In a guitar amp, the parts list usually has the PT and OT, then tubes and/or choke right up at the top.

[Guy77]

Good reasoning and insight on your part.

There is some subtlety in fuse use. The mains fuse is not there to protect anything in the amp. It's there to prevent a fault in the amp AC wiring or PT itself from starting a house fire.

A fault on the high voltage supply will nearly always blow the primary fuse, but to actually protect the high voltage windings, you have to fuse the high voltage supply itself. This is why the HT fuse is there in many amps. This fuse, if well selected, can stop a tube plate-to-heater short from killing the high voltage secondary winding - as well as probably protecting the rectifiers and main filter caps from a tube short.

To protect the PT from a rectifier or filter capacitor fault, as well as a tube shorting after the rectifier/filter, you need to fuse both high voltage half-secondaries separately. About here, amp makers think or say "Hey! That's way too many fuses! Fender (or Marshall, or insert your favorite brand here) only used a mains fuse. Why should I use more?" The answer is that you don't have to. You just have to pick the level of risk you're willing to bear.

A useful exercise in the protection racket is to take a parts list of the amp and sort it by price, most expensive to replace first. Then think of what level of money and time you're willing to expend for a given part to keep a lower-cost part failure from killing things above it. In a guitar amp, the parts list usually has the PT and OT, then tubes and/or choke right up at the top.

Thanks R.G. for listing out all those fusing options in detail. Great info on this post, Thanks everyone!

Cheers
Guy