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Grounded CT or 100Ohm Reference for Heaters?

5/4/2005 12:58 PM
BK Grounded CT or 100Ohm Reference for Heaters?
Looking to maximize quietness on an AC filament supply, I wonder which is the best approach?  
1) Just ground the CT and be done with it.  
2) Lift the CT and use 2 100-Ohm resistors as reference.  
3) Lift CT and use a 100Ohm Balance pot with 2 47Ohm resistors on each leg to prevent shorting one leg.  
5/4/2005 2:50 PM
I'll take window #3.  
You'll get maximum hum reduction and you'll still have PT protection. If a tube shorts the resistors act like a fuse and blow.  
If your using cathode bias or you don't mind building a V+ rail you can connect the two resistors to either the top of the cathode resistor or the new V+ rail and get really low hum.  
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5/4/2005 3:17 PM
Greg Simon

You can also use a voltage divider and generate any voltage you want between 20v and 80v and put the CT, or the junction of your 100 ohm resistors at that voltage. You don't need the amp to be cathode biased to use this method either. It will reduce hum quite a bit, and even more if you put a cap from the junction to ground. I used a 10uf 160v cap in a Silvertone rebuild.  
5/5/2005 3:31 AM

Would you mind explaining exactely how I make that voltage divider?  
The point of doing this, is to lift the 6,3V.... sort of, still having 6.3V AC, but with a 20-80 V DC with it?  
5/5/2005 4:02 AM
Greg Simon

Hi Tommy. You have the idea right. The reason it is a range of 20v to 80v DC and not some specific voltage is because different voltages will work better for different tubes and circuits. It requires some experimentation to see what gives you the least amount of hum, but doing anything in that range will give you a bigger hum drop than if you just leave it 6.3v AC stock. The 6.3V AC rides on top of whatever DC voltage you put on the filament center tap, BUT, there are heater to cathode votlage minimums and maximums that you have to be aware of for each tube type, so that is why you don't want to put say 200v on it. Raising the filaments like this is commonly referred to biasing the filament string upwards, and the idea is that when at ground with the center tap, the 6.3V AC is very close to ground, which is noisy. By moving it up 50v or something, that is quieter up there, so it won't pick up so much hum from ground.  
You make a voltage divider with two resistors, one end of the first resistor goes to your high voltage, and you can use B+ if you want. The other end of that resistor goes to a junction with the other resistor and a cap. The other end of the second resistor and the other end of the cap go to ground. The cap value can be something like 10uf at 160v. For the resistor values, you want to use a large resistor on the B+ side of the junction, or your plate voltage can drop. Something like a 1M usually works well. On the other end, you try different resistors out until you get the voltage you are looking for. In amps in the past, I've used a 1M/270k combination, and a 1M/470k combination to get the voltages I was after. The first one gave me 40v, and the second gave me 63v, which was close enough for my purposes. Both amps had different plate voltages that I was starting with, so you can use those as starting points, but there is no substitue to actually dialing in your votlage with various resistor values. I would suggest to use 2W metal film resistors in this application, and an electrolytic cap is just fine for the cap. If you want to know more about it, go to Aiken's site, click on the introductory section, and then the voltage divider rule section and read all about the math involved with it.  
Hope that helps?  
5/5/2005 5:26 AM

yes, that helps, thanks for a great explanation!  
I don't know if my hum is caused by the heater.. But the sound is much like the sound you can hear outside of a big transformer around a power station... Not a sharp sparking sound, but a muffled, round hummmmmm. Is that sound generally caused by the heater?  
5/5/2005 8:28 AM
I don’t think Greg's explanation is entirely correct - but it's close ...:)  
If the hum is caused by the heater supply (critical that you sort this out), it will be 60 hz and it will be easy to see on a scope. If it’s 120 hz, it’s probably coming from the plate supply – a different problem.  
This hum gets impressed on the Grid(s) – somewhere – the louder it is, the closer to the front end its source is.  
In a cathode biased tube circuit, remember that the grids are positive with respect to ground, because of the voltage dropped across the cathode resistor. This is typically just a few volts in the preamp stages and more - perhaps 30+ volts, at the power tubes.  
So, if you take that 30 volts at the PT cathode and use it to elevate the heater voltage, the only thing the preamp tube grids can see from the power supply – is dc (theoretically not very noisy).  
So as long as your PT cathode dc is clean, the hum will go away.  
Of course, this is not always the case.  
I suppose that’s why some folks recommend the voltage divider approach whereby you gather your dc source (20v-80v) from the power supply string somewhere after a few caps and resistors have smoothed out the ripple.  
I have had mixed results from either of these approaches - experience of others will/may differ.  
That’s why I am struggling with a dc heater supply – see the “12 heater DC” thread.  
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