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DC Heater Wiring

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9/22/2004 11:29 PM

DC Heater Wiring

I was having a problem with 60hz & 120hz hum.
Have greatly reduced the problem.
However, some 120hz hum remains.
Neither the PT 6.3v center tap or a 100 ohm artificial center tap seemed to have helped much.
Nor did referencing the heater voltage through the cathode resistor.
I have installed a bridge recitifier & 400uF cap to convert the 6.3 heaters to DC.
The PT center tap is now taped off & the negative of the bridge is grounded through a .1uF cap.
With the DC heaters, what is the best way to ground the circuit?
Can I still use the PT center tap (& disconnect the ground from the bridge?)?
And can I still use the cathode to reference the heater voltage?
How would I wire that?
Thanks.
-Ed

9/22/2004 10:28 PM
Bruce /Mission Amps	Changing the rectifier circuit to DC will NOT fix this if you are hearing 120Hz hum. You have B+ power supply hum if it's 120Hz. And no, don't use the cathode resistor of the 6V6s to apply DC to the filament supply, just ground the anodes of your FWB at the same point at the HV center tap of the PT. Bruce

9/22/2004 11:42 PM

Bruce-
OK.
That's what I thought.
The DC heaters helped eliminate the 60 cycle hum I had.
I know the DC heaters are working correctly, but I've seen a lot of different ways to do this & I want to make sure I haven't overlooked anything.
So, you're saying that I can't reference the 6.3 volt DC through the cathode resistor?
I had read that referencing the heaters above ground would reduce noise from the power section & that it could be done with DC voltage also.
Is there a different way to do it?
Also, right now I have the - side of the bridge grounded through the .1uF cap.
I'm not following you about grounding the anodes at the HV center tap.
Thanks.
-Ed

9/23/2004 3:01 AM

Bruce /Mission Amps

The negative side of the FWB is two anodes of two of the four diodes in the FWB.
You are taking them to ground through the .1uf cap.
You can build a voltage divider after the last filter cap.
I have never "biased" a DC filament supply with more DC so I can't tell you what to expect.
A 270K resistor in series with a 27K resistor right to ground will give you about nice divider to work with.
Using a 22uF@100v cap to ground at the 27k resistor will net you an extremely well filterd DC suppy of around 1/10th the B+ rail...
ex: 300vdc at the 270K and 30vdc at the 27K.

Bruce

9/23/2004 3:07 AM
Bruce /Mission Amps	I got kicked off before I re read that... sorry Actually you are not connecting the anodes to chassis ground since you are using the .1uf cap so you migh tbe able to inject a +DC voltage there anyhow. The anodes are the most negative part of the DC filament supply. Bruce

10/3/2004 4:23 PM

Just a word of caution. If all you did was convert the fil voltage to DC and add a capacitive input filter then you just jacked up your filament voltage 30-40%. A good way to burn out your tubes prematurely.

DC on fils is a great idea and certrainly is one of the best ways to reduce 60hz hum. However it is rarely used because of the voltage problem described. Some people blow off the extra voltage by using a series resistor, others through a small variac, others by using a lower voltage external fil xfmr. If you did none of these things then you are probably over heating your filaments.

Brutus

10/3/2004 5:34 PM

Bruce /Mission Amps

Using a DC rectifier in the 6.3vac line to build a DC filament supply will lower the actual voltage down by at least .6v to .7v, but, the actual filtered DC is still close to 7.8vdc when doing this.
The resistor trick works but is best used in the AC legs to the rectifier.
And, if you didn't want to build the DC filament supply with a +6VDC regulator at the end, I guess you could also use a couple more 6a silicone diodes in series with the DC lines to lower it a bit more.
Comments?
Bruce

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