100 ohm heater artifical center tap and lead dress question

[johndandry]

Is the consensus is that using the 2x 100 ohm resistors instead of the transformer heater center tap is quieter?

On lead dress, will it introduce any extra hum buy bundling the heater wires with the PT secondaries and 5v recto wires for a bit on its way out of the transformer? Thanks!!

[martin manning]

If you have a CT on the heater winding I'd use it. You would probably need to use a pot (instead of 2x 100R) to make any improvement. Re bundling the transformer leads, it's not a problem. I twist the heater wires together first.

[mark enger]

you would use the ct if pt has it, i use my cordless drill to twist my heater wires nice and tight but its not needed a few simple twists per inch is fine there's a vidio on you tube that show how to do it i beleave its under "tales from the tonegaradge" a few twists to the 5v is all thats needed its way away from anything that would pick it up mark

[Firestorm]

If you have a CT on the heater winding I'd use it. You would probably need to use a pot (instead of 2x 100R) to make any improvement. Re bundling the transformer leads, it's not a problem. I twist the heater wires together first.

Yes, use the center tap if you have one. 2X100Rs will not always be centered. A pot will let you dial out the hum, but is not stable; vibration can shift the wiper.

[Gaz]

If you have a CT on the heater winding I'd use it. You would probably need to use a pot (instead of 2x 100R) to make any improvement. Re bundling the transformer leads, it's not a problem. I twist the heater wires together first.

Yes, use the center tap if you have one. 2X100Rs will not always be centered. A pot will let you dial out the hum, but is not stable; vibration can shift the wiper.

Not if it's a locking pot But serisously, by the same logic, wouldn't a bias pot just move all over the place? If it's a concern the old nail polish trick works well.

[Milkmansound]

I like using 2x100R and not grounding them - instead, put them onto the positive side of the cathode ground for the output tubes. For a high power amp, this DC reference can really quiet things down. For a non cathode biased amp, you can divide from the main B+ to get that reference.

[renshen1957]

Is the consensus is that using the 2x 100 ohm resistors instead of the transformer heater center tap is quieter?

On lead dress, will it introduce any extra hum buy bundling the heater wires with the PT secondaries and 5v recto wires for a bit on its way out of the transformer? Thanks!!

Why not use DC standoff in addition quiet the hum? DC heaters work, too, but are more expensive to implement.

Best Regards,

Steve

[Structo]

I haven't tried elevating the heater voltage before but I think that using a 2W 100R pot known as a humdinger is probably the best way because of the adjustability.

In addition to that I would probably still use two matched 1/2w 100R resistors because if a power tube shorts heater to cathode, the resistors will burn and open saving the heater winding.

Here is some pertinent reading for it.

http://www.freewebs.com/valvewizard/heater.html

[Cliff Schecht]

I've never had the potentiometer-based humdinger circuit do much of anything to help with hum in an amp. I don't use them anymore, it's a waste of a good 250 Ohm pot and space in the amp.

What I do use is the elevated heater trick either on a CT or artificial centertap for the heaters, especially if the amp has cathode followers in it. In the 300W bass amp I finished recently, I elevated the heaters by about 40-45V and that amp was DEAD quiet, I couldn't even tell it was on with my noiseless pickup-equipped strat plugged in...Until I hit a chord!

[Gaz]

I've had the humdinger work on many amps to null out hum. I also elevate the wiper to a DC voltage around 50-80vdc. Best of both worlds.

I wouldn't imagine a typical bass amp would be very noisy since they are usually very low gain. With high gain, every little bit helps.

[pdf64]

if a power tube shorts heater to cathode, the resistors will burn and open saving the heater winding

Tom, if a power tube does short as above, then the heater circuit is best protected by a ground reference being maintained (the B+ / line fuse should blow pretty quickly).
Because if the 100 ohm artificial resistors blow out, the whole heater circuit can get pulled up (by the short) to a potential above the rating of the heater cathode insulation of every tube on that circuit.
I now use beefy >3W 100 ohm resistors for the artificial centre tap.
Pete

[rooster]

I will always chose the 100 ohm pair over the center tap, every time. I agree with Cliff, using a pot is a waste of money. Case in point, install the pot, adjust it for hum and then measure the resistance from the wiper to either lug: identical, if you've made the adjustment properly.

Fender used 1/2 watt values and they've typically survived for 50+ years, but I typically use 1 watt values.

[Gaz]

I will always chose the 100 ohm pair over the center tap, every time. I agree with Cliff, using a pot is a waste of money. Case in point, install the pot, adjust it for hum and then measure the resistance from the wiper to either lug: identical, if you've made the adjustment properly.

Fender used 1/2 watt values and they've typically survived for 50+ years, but I typically use 1 watt values.

That's true until you change tubes (preamp included) then the ideal position of the pot will change. And much respect to you two, but I think the "waste of a pot" thing is funny, as a lot of things we do could be considered a 'waste'.

[pdf64]

Fender used 1/2 watt values and they've typically survived for 50+ years

Well, everyone's experience will be different but I guess that of the amps I see that have them (mostly SF Fenders), maybe half have the humdinger pot or 100 ohm resistors blown, or they've previously been replaced.
And the only failure mode I can think of which could reasonably cause that is a (power) tube short (any other suggestions?).
Pete

[Cygnus X1]

In regards to elevating the filaments using the power tube cathode...I've found this injects an AC signal directly on to the chassis.
Makes it impossible to connect anything else to the input that has a conventional ground.
It injects the AC signal right back to the input making a nasty hum.

Any way to prevent this, or is it the price to pay?