The capacitor in series with filaments should not get hot. Do not use a polarized cap.
A cap 8mf @ 100VAC should have been ok. Even though voltage rating would seem low to some it should do the job.
AES, www.tubesandmore.com sells caps that will work.
You could also use a diode, 1N4004 - 1N4007, along with some resistance in series with filaments.
Norm
:I tried to replace the resistance cord on this Emerson model 108 the way it says in the article but the capacitor keeps getting hot and smokes after a few minutes. Has anyone ever replaced one of those resistance cords? I monitored the current through the cap and it is .3 amps so about right. I have used an 8ufd at 100vac nonpolarized and also tried an 8ufd at 450vdc plarized and both get hot. If I put 200ohm resistor 50 watt in there it works but don't want to use the resistor as the radio has a plastic case and 'am afraid it's too much heat for the plastic as the chassis barely fits into the case.
:Hi John
:
: The capacitor in series with filaments should not get hot. Do not use a polarized cap.
:
: A cap 8mf @ 100VAC should have been ok. Even though voltage rating would seem low to some it should do the job.
:
: AES, www.tubesandmore.com sells caps that will work.
:
: You could also use a diode, 1N4004 - 1N4007, along with some resistance in series with filaments.
:
:Norm
:
::I tried to replace the resistance cord on this Emerson model 108 the way it says in the article but the capacitor keeps getting hot and smokes after a few minutes. Has anyone ever replaced one of those resistance cords? I monitored the current through the cap and it is .3 amps so about right. I have used an 8ufd at 100vac nonpolarized and also tried an 8ufd at 450vdc plarized and both get hot. If I put 200ohm resistor 50 watt in there it works but don't want to use the resistor as the radio has a plastic case and 'am afraid it's too much heat for the plastic as the chassis barely fits into the case.
You are doing things right. Cap must have leakage? Certain types of caps can't take the switching rate. Two electrolytics back to back may have this problem but your cap was made for AC.
Maybe the cap was an old paper type which had leakage. Can you measure leakage across this cap?
A new 8 mf cap as sold by AES and others won't have this problem.
Norm
:Thanks Norm,
:I had the 8ufd at 100vac in the radio and the radio ran fine for 20 minutes but now when I try it the cap gets hot and smokes after just a few minutes. Like I said I monitored the current through it and it was .3amps so I don't get it. Don't see why the cap gets hot. Voltage across the cap is about 58vac.
:John
:
:
::Hi John
::
:: The capacitor in series with filaments should not get hot. Do not use a polarized cap.
::
:: A cap 8mf @ 100VAC should have been ok. Even though voltage rating would seem low to some it should do the job.
::
:: AES, www.tubesandmore.com sells caps that will work.
::
:: You could also use a diode, 1N4004 - 1N4007, along with some resistance in series with filaments.
::
::Norm
::
:::I tried to replace the resistance cord on this Emerson model 108 the way it says in the article but the capacitor keeps getting hot and smokes after a few minutes. Has anyone ever replaced one of those resistance cords? I monitored the current through the cap and it is .3 amps so about right. I have used an 8ufd at 100vac nonpolarized and also tried an 8ufd at 450vdc plarized and both get hot. If I put 200ohm resistor 50 watt in there it works but don't want to use the resistor as the radio has a plastic case and 'am afraid it's too much heat for the plastic as the chassis barely fits into the case.
Here's my thoughts from what I learned lately:
I think that 100vac CAP may not be rated high enough.
The proper way to determine the EXACT voltage that the dropper cap will be exposed to is this:
It is a Pythagorean function.
-----------------------------reference ---------
http://www.vintage-radio.com/repair-restore-information/valve_dropper-calcs.html
EXCERPT:
DROPPER CAPACITOR (Actual Voltage Drop) calculation
If you connect a resistor and a capacitor in series and apply an AC voltage to the combination, then measure the voltage across the capacitor and resistor and add them together, the result will not be the applied voltage. It will be higher.
Do you remember all that stuff from school about right-angle triangles, and Pythagoras? See this page for a reminder. We now have a use for it!
Look at the right-angle triangle below. The hypotenuse (the long side that doesn’t connect to the 90 degree corner) represents the supply voltage (Vin). The other two sides represent the voltage across the capacitor (Vc) and the voltage across the heaters (Vh), which are at 90 degrees to each other.
Pythagorean Theorem states: "The square of the hypotenuse is equal to the sum of the squares on the other two sides". Taking our values, the formula is:
Where: VC = Voltage across capacitor (Volts)
Vin = Supply voltage (Volts)
Vh = Heater voltage (Volts)
We know the supply voltage and the heater voltage, so by rearranging the formula we can calculate the voltage across the capacitor:
-----------------------------
Now your radio requires 68 volts for the filament string and if you add a #44 6v lamp the total is 74 volts @ 300ma
1.) take the filament voltage (Vh) and square it =74sq= 5476
2.) Take the input voltage (Vin) = 125v and square that too. = 125sq =15625
3.) subtract (Vh-sq from Vin-sq) step #1 from Step#2 = 15625-5476=10149
4.) Take the square root of that answer = 10149 SQRT= 100.75 volts AC RMS across that dropper cap (Vc)
..but.... BIG BUT HERE....
5.) NOW ( 100.75v = Vc) that's just the RMS voltage so you now have to get the PEAK voltage = 100.75 x 1.414 = 142 volts PEAK AC
So the dropper cap will have to deal with as much as 142 volts AC peak.
If the cap that you have that is only rated for 100vac ....is it actually able to handle 142v Peak is the question.
I would suggest that you you buy an 8uf non-polar cap rated for 200vAC or better.
ALTERNATE idea:
NOW it is also possible that you CAN safely also use 2 back to back electrolytic caps of the same value and rating to create a NON-POLAR cap... providing that you AGAIN observe the voltage exposure calculations above.
----------------- reference ----
http://electrochem.cwru.edu/ed/encycl/misc/c04-appguide.pdf
CORNELL DUBILIER
Excerpt:
Application Guide, Aluminum Electrolytic Capacitors
If two, same-value, aluminum electrolytic capacitors are connected in series, back-to-back with the positive terminals or the negative terminals connected, the resulting single capacitor is a non-polar capacitor with half the capacitance. The two capacitors rectify the applied voltage and act as if they had been bypassed by diodes. When voltage is applied, the correct-polarity capacitor gets the full voltage. In non-polar aluminum electrolytic capacitors and motor-start aluminum electrolytic capacitors a second anode foil substitutes for the cathode foil to achieve a non-polar capacitor in a single case.
-------------------------------------------
So if you use two 16uf 450vDC electrolytics back to back ( plus to plus or neg to neg)
The total capacitance will be 8uf.
HOWEVER when used back to back the voltage rating DOES NOT double like it would if used in series (plus to minus) normally.
"Each cap will see the full voltage" as noted in the excerpt above.
Now ALL DC rated caps MUST be de-rated down to 60% of their DC rating for use in AC non-polar applications like this.
So a 450v DC rating can be used in non-polar AC applications at 60% of the DC rating...or only up to 270 volts AC
In your case the PEAK AC is 142 volts so de-rating a 450v DC cap down 60% to 270V for AC should be safe.
He doesn't need to consider peak voltage in this case. The capacitor is already rated 100VAC. If it was a DC cap then allowing for peak would be important.
Norm
:Hi Norm:
:Did you consider the PEAK AC exposure?
:
:
:Here's my thoughts from what I learned lately:
:
:I think that 100vac CAP may not be rated high enough.
:
:The proper way to determine the EXACT voltage that the dropper cap will be exposed to is this:
:
:It is a Pythagorean function.
:
:-----------------------------reference ---------
:http://www.vintage-radio.com/repair-restore-information/valve_dropper-calcs.html
:
:EXCERPT:
:
:DROPPER CAPACITOR (Actual Voltage Drop) calculation
:
:If you connect a resistor and a capacitor in series and apply an AC voltage to the combination, then measure the voltage across the capacitor and resistor and add them together, the result will not be the applied voltage. It will be higher.
:Do you remember all that stuff from school about right-angle triangles, and Pythagoras? See this page for a reminder. We now have a use for it!
:Look at the right-angle triangle below. The hypotenuse (the long side that doesn’t connect to the 90 degree corner) represents the supply voltage (Vin). The other two sides represent the voltage across the capacitor (Vc) and the voltage across the heaters (Vh), which are at 90 degrees to each other.
:
:Pythagorean Theorem states: "The square of the hypotenuse is equal to the sum of the squares on the other two sides". Taking our values, the formula is:
:
:Where: VC = Voltage across capacitor (Volts)
: Vin = Supply voltage (Volts)
: Vh = Heater voltage (Volts)
:We know the supply voltage and the heater voltage, so by rearranging the formula we can calculate the voltage across the capacitor:
:-----------------------------
:
:Now your radio requires 68 volts for the filament string and if you add a #44 6v lamp the total is 74 volts @ 300ma
:
:1.) take the filament voltage (Vh) and square it =74sq= 5476
:
:2.) Take the input voltage (Vin) = 125v and square that too. = 125sq =15625
:
:3.) subtract (Vh-sq from Vin-sq) step #1 from Step#2 = 15625-5476=10149
:
:4.) Take the square root of that answer = 10149 SQRT= 100.75 volts AC RMS across that dropper cap (Vc)
:
:..but.... BIG BUT HERE....
:
:5.) NOW ( 100.75v = Vc) that's just the RMS voltage so you now have to get the PEAK voltage = 100.75 x 1.414 = 142 volts PEAK AC
:
:So the dropper cap will have to deal with as much as 142 volts AC peak.
:
:If the cap that you have that is only rated for 100vac ....is it actually able to handle 142v Peak is the question.
:
:I would suggest that you you buy an 8uf non-polar cap rated for 200vAC or better.
:
:ALTERNATE idea:
:NOW it is also possible that you CAN safely also use 2 back to back electrolytic caps of the same value and rating to create a NON-POLAR cap... providing that you AGAIN observe the voltage exposure calculations above.
:
:----------------- reference ----
:
:http://electrochem.cwru.edu/ed/encycl/misc/c04-appguide.pdf
:
:CORNELL DUBILIER
:Excerpt:
:Application Guide, Aluminum Electrolytic Capacitors
:If two, same-value, aluminum electrolytic capacitors are connected in series, back-to-back with the positive terminals or the negative terminals connected, the resulting single capacitor is a non-polar capacitor with half the capacitance. The two capacitors rectify the applied voltage and act as if they had been bypassed by diodes. When voltage is applied, the correct-polarity capacitor gets the full voltage. In non-polar aluminum electrolytic capacitors and motor-start aluminum electrolytic capacitors a second anode foil substitutes for the cathode foil to achieve a non-polar capacitor in a single case.
:-------------------------------------------
:
:So if you use two 16uf 450vDC electrolytics back to back ( plus to plus or neg to neg)
:The total capacitance will be 8uf.
:
:HOWEVER when used back to back the voltage rating DOES NOT double like it would if used in series (plus to minus) normally.
:"Each cap will see the full voltage" as noted in the excerpt above.
:
:Now ALL DC rated caps MUST be de-rated down to 60% of their DC rating for use in AC non-polar applications like this.
:
:So a 450v DC rating can be used in non-polar AC applications at 60% of the DC rating...or only up to 270 volts AC
:
:In your case the PEAK AC is 142 volts so de-rating a 450v DC cap down 60% to 270V for AC should be safe.
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