Elevating heater via 100K resistors...

[Bob Simpson]

Better for noise reduction than grounding heater center tap?

Or is this a must do?

Bob

[Structo]

Better for noise reduction than grounding heater center tap?

Or is this a must do?

Bob

I believe the correct value is 100R.

It is debatable if there is any added noise reduction with the resistors vs the center tap.
I suppose it depends on the transformer.

Another trick is to use the + side of the cathode resistor on a cathode biased amp to elevate the voltage, I haven't tried this myself so do some research before trying it.

[ampedup]

I like using the elevated heater, there's a schematic for center tapped or non center tapped here:http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/

[talbany]

You can also try the 12v DC relay supply if you don't have a cathode biased amp...If your layout is clean and everything is grounded correctly the 100R should work just fine.. If you follow the mans grounding scheme like the layout I did on 124 in the sticky in the files section...Should be Dead Quiet!!!


Tony VVT

[heisthl]

Better for noise reduction than grounding heater center tap?

Or is this a must do?

Bob

IMHO if your transformer has the center tap it's best to use it instead of the resistors. This style of amp build is not noisy with standard AC heaters and you have the added benefit of not burning up the 100 ohm resistors when an output tube shorts. I don't know how many amps I've repaired with burned up heater resistors, they seem to burn way before the fuse blows. I could be wrong but I think the only reason HAD used the resistors is he was using power transformers that did not have a heater center tap.

[bluesfendermanblues]

I use a 470 Ohm trim pot. The outer legs are conneted to the hear voltage and the center log is connected to ground. Its an old trick, but it allows med fine tune for no hum.

[talbany]

Double
Tony VVT

[talbany]

IMHO if your transformer has the center tap it's best to use it instead of the resistors. This style of amp build is not noisy with standard AC heaters and you have the added benefit of not burning up the 100 ohm resistors when an output tube shorts. I don't know how many amps I've repaired with burned up heater resistors, they seem to burn way before the fuse blows. I could be wrong but I think the only reason HAD used the resistors is he was using power transformers that did not have a heater center tap.

Henry
I think I remember reading a while back Fender went to the 100 ohms to help protect the power transformer from a heater cathode short...Takes out the resistors instead of straining the tranny.... just some added protection there...




Tony VVT

[jelle]

Cheap insurance.

[talbany]

I know this is a bit off topic here but kind of interesting.. I have a 1950 5C5 TV front pro...It had been modded to 12 AX's when I got it.. I rebuilt it back stock w/6SC7's Check out how they did the filaments.. with this method if you get a heater cathode short here it will more than likely take out the power transformer(Dead chassis short) and do it quickly...The amp has original transformers so I try not to leave tubes in there for any real length of time and test all the tubes I install.. Also put in a fast blow fuse...These amps use the grid leak bias method and with a nice Eminence Legend sounds pretty good ..If anyone has one of these I recommend installing a choke really helps smooth this amp out... Also the guy who sold me the amp said it originally was covered in white vinyl...He recovered it in tweed cause he said he couldn't stand the ugly white look... Come to find out Fender made only a hand full of Pros in White Vinyl... DOH!!!!


Tony VVT

[heisthl]

Henry
I think I remember reading a while back Fender went to the 100 ohms to help protect the power transformer from a heater cathode short...Takes out the resistors instead of straining the tranny.... just some added protection there...

Tony VVT

I have to think about this for awhile but off the top of my head this makes no sense. The heater winding is the most robust winding in the transformer and in the case of a grounded center tap (or like your old PRO amp one leg at ground already) you would have to have a very large fuse to allow damage to the heater winding because whatever shorts to it has a short path to ground and probably couldn't exceed its amperage rating before the fuse goes. I don't think arcing within the PT secondary would be an issue either. If it's a worry point just install a small amperage B+ fuse. I could bave CRS (cant remember $$it) but I don't remember ever replacing a center tapped Fender PT because of a ruined heater winding.

[talbany]

Obviously heater cathode shorts are not a common problem.. but do on occasion happen if the amp is susceptible to heavy vibration..If you have changed them due to a tube going out they did there job... Actually 1/4 watt resistors will give you a little added protection than the 1 watters will...One other thing is the resistor method should provide a little better balancing of 2 halves of the winding helping with the Humm!! Or so they say!!This really is nit picking here!!


Tony VVT

[cornel moldovan]

BEST SOLUTION IS TO HAVE READY WIRED SIMETRICAL 6.3 V AC SECONDARIES,MIDDLE GROUNDED , SEPARATE FOR PREAMP AND SEPARATE FOR 6L6 . OTHERWISE , TWO 100 OHM 1W RESISTORS FOR 12AX7 , AND TWO 150 OHM 5W FOR 6L6 END GROUNDED, AS USUAL , BUT THIS CLASSIC SOLUTION MEANS ENERGY WASTED , TRANSFORMED IN HEAT.FOR THIS YOU ONLY HAVE TO ORDER POWER TRANSFORMERS WIRED WITH SIMETRICAL SECONDARY HEATERS MIDDLE LUG GROUNDED.

[tubeswell]

100R 'artificial centre tap' heater circuits are not elevated per se, they are just another way of getting a hum-balanced ground reference. A humdinger pot is another way. A heater winding CT is another way.

Sometimes 100R 1% resistors are preferred over a heater winding CT because they provide an identical resistance from either side of the winding to ground to ensure that both ends of the winding are putting out balanced AC that cancels out heater noise affecting the cathodes. But one (minor) drawback of 100R resistors is that they use up a wee bit of current (if that is an issue), whereas a heater winding CT doesn't use up any current.

However w.r.t. heater winding Cts, one (again minor) drawback can be that they may not be perfectly in the middle of the heater winding, therefore they might not balance the AC on opposite sides of the winding as well as they otherwise might if they were perfectly in the middle of the winding. However I prefer to use this method myself.

On cathode biased amps, you can (however) elevate the heater circuit by putting the Heater winding CT (or the 100R resistors) at the cathode of the output tubes. This increases the heater-to-cathode voltage in the pre-amp tubes, thereby eliminating a potential source of hum in this sensitive part of the amp. (You can also elevate the heater in the same way by using a plain old voltage divider off your HT secondary, so that the heaters are at a voltage that is different from all the cathodes in the amp, as long as you don't exceed the maximum recommended heater-to-cathode voltage for the tubes).

If you have cathode follower stages or any stages which has a high cathode voltage (90V or so), it is prudent to elevate the heaters to minimise the otherwise high heater-to-cathode voltage.

Sorry about the boring ramble if it is stuff you already know (but your OP appeared to be a bit confused about the distinction between elevated heaters and heater winding balancing)

[amplifiednation]

Good info in this thread on artificial ct's for heaters, I need to do this on a current mini wreck build and understand it a wee bit more.