From the schematic, you should be able to calculate it. You know the rated current through the filament string. You can then calculate the voltage drop across the original, fixed dropping resistor in the ckt (in series with the 50A1), and then the required voltage drop across the new resistor replacing the 50A1. That will give you the resistance and the power rating. Double the power rating for safety margin.
Doug
:In the past I have replaced a 50A1 tube on a Zenith transoceanic with an Ohmite 10W 1.3K ohm power resistor. I've run out of this size resistor and can't locate them anywhere. I have a couple of questions: Can I safely use a 10w 1.6K ohm and could I move up to as high as a 10w 2.5 ohm without running into any problems. These 2 sizes are readily available on E-bay. Thanks for any advice.
:Andy
Dave
________________________________________________________________________
:Andy: I've replaced a 50A1 (with a power resistor) in a T/O, but I don't recall the size of the resistor. I'm sure it depends a little on the condition of the selenium diode, i.e., its forward drop. I think it's a little bit trial and error to get the filament voltages right.
:
:From the schematic, you should be able to calculate it. You know the rated current through the filament string. You can then calculate the voltage drop across the original, fixed dropping resistor in the ckt (in series with the 50A1), and then the required voltage drop across the new resistor replacing the 50A1. That will give you the resistance and the power rating. Double the power rating for safety margin.
:Doug
:
:
::In the past I have replaced a 50A1 tube on a Zenith transoceanic with an Ohmite 10W 1.3K ohm power resistor. I've run out of this size resistor and can't locate them anywhere. I have a couple of questions: Can I safely use a 10w 1.6K ohm and could I move up to as high as a 10w 2.5 ohm without running into any problems. These 2 sizes are readily available on E-bay. Thanks for any advice.
::Andy
If you use two resistors of approximately the same general size, the power would be shared between them, so 5W for each should be OK. That would take up less room.
Doug
:You need a 1300 ohm 10 watt resistor, this will work 99% of the time. You can go with a larger wattage but don't change the resistance size.
:
:Dave
:
:________________________________________________________________________
:
::Andy: I've replaced a 50A1 (with a power resistor) in a T/O, but I don't recall the size of the resistor. I'm sure it depends a little on the condition of the selenium diode, i.e., its forward drop. I think it's a little bit trial and error to get the filament voltages right.
::
::From the schematic, you should be able to calculate it. You know the rated current through the filament string. You can then calculate the voltage drop across the original, fixed dropping resistor in the ckt (in series with the 50A1), and then the required voltage drop across the new resistor replacing the 50A1. That will give you the resistance and the power rating. Double the power rating for safety margin.
::Doug
::
::
:::In the past I have replaced a 50A1 tube on a Zenith transoceanic with an Ohmite 10W 1.3K ohm power resistor. I've run out of this size resistor and can't locate them anywhere. I have a couple of questions: Can I safely use a 10w 1.6K ohm and could I move up to as high as a 10w 2.5 ohm without running into any problems. These 2 sizes are readily available on E-bay. Thanks for any advice.
:::Andy
The only concern is the voltage to the tube filaments. Too much, and the tubes' life will be shortened. Too little, and the oscillator may not run reliably.
I think tube filament voltage within 5% of spec would be regarded as normal. So, if 1.3K puts the voltage "spot on," then a resistor within 5% of 1.3K should be pretty close.
I suspect the increased forward drop across the selenium diode (due to age) would cause a greater voltage variation.
Doug
:::
::::In the past I have replaced a 50A1 tube on a Zenith transoceanic with an Ohmite 10W 1.3K ohm power resistor. I've run out of this size resistor and can't locate them anywhere. I have a couple of questions: Can I safely use a 10w 1.6K ohm and could I move up to as high as a 10w 2.5 ohm without running into any problems. These 2 sizes are readily available on E-bay. Thanks for any advice.
::::Andy
:RadioDaze has a larger selection of power resistor values than what you found on eBay. Not 1.3K, but some closer than you found.
:
:The only concern is the voltage to the tube filaments. Too much, and the tubes' life will be shortened. Too little, and the oscillator may not run reliably.
:
:I think tube filament voltage within 5% of spec would be regarded as normal. So, if 1.3K puts the voltage "spot on," then a resistor within 5% of 1.3K should be pretty close.
:
:I suspect the increased forward drop across the selenium diode (due to age) would cause a greater voltage variation.
:Doug
:
:
:
::::
:::::In the past I have replaced a 50A1 tube on a Zenith transoceanic with an Ohmite 10W 1.3K ohm power resistor. I've run out of this size resistor and can't locate them anywhere. I have a couple of questions: Can I safely use a 10w 1.6K ohm and could I move up to as high as a 10w 2.5 ohm without running into any problems. These 2 sizes are readily available on E-bay. Thanks for any advice.
:::::Andy
:Thanks everyone for your suggestions. I was able to find basically what I wanted from "Justradios.com". I will use 2 of the 5W 25k ohm power resistors in parallel; 1/r= 1/r1 + 1/r2
: 1/r= 1/25,000 + 1/25,000
: 1/r= 2/25,000
: 1/r= 1/12,500
: r= 12,500
:I think this is the correct formula and when I get these connected I will check the voltage on pin 7 of the 3V4 tube which hopefully should not exceed 8.4 v
:Andy
:
:
::RadioDaze has a larger selection of power resistor values than what you found on eBay. Not 1.3K, but some closer than you found.
::
::The only concern is the voltage to the tube filaments. Too much, and the tubes' life will be shortened. Too little, and the oscillator may not run reliably.
::
::I think tube filament voltage within 5% of spec would be regarded as normal. So, if 1.3K puts the voltage "spot on," then a resistor within 5% of 1.3K should be pretty close.
::
::I suspect the increased forward drop across the selenium diode (due to age) would cause a greater voltage variation.
::Doug
::
::
::
:::::
::::::In the past I have replaced a 50A1 tube on a Zenith transoceanic with an Ohmite 10W 1.3K ohm power resistor. I've run out of this size resistor and can't locate them anywhere. I have a couple of questions: Can I safely use a 10w 1.6K ohm and could I move up to as high as a 10w 2.5 ohm without running into any problems. These 2 sizes are readily available on E-bay. Thanks for any advice.
::::::Andy
:Andy, the formula is correct. But you are looking for 1.3K ohms, not 12.5K ohms. You're off by a factor of 10.
:Doug
:
::Thanks everyone for your suggestions. I was able to find basically what I wanted from "Justradios.com". I will use 2 of the 5W 25k ohm power resistors in parallel; 1/r= 1/r1 + 1/r2
:: 1/r= 1/25,000 + 1/25,000
:: 1/r= 2/25,000
:: 1/r= 1/12,500
:: r= 12,500
::I think this is the correct formula and when I get these connected I will check the voltage on pin 7 of the 3V4 tube which hopefully should not exceed 8.4 v
::Andy
::
::
:::RadioDaze has a larger selection of power resistor values than what you found on eBay. Not 1.3K, but some closer than you found.
:::
:::The only concern is the voltage to the tube filaments. Too much, and the tubes' life will be shortened. Too little, and the oscillator may not run reliably.
:::
:::I think tube filament voltage within 5% of spec would be regarded as normal. So, if 1.3K puts the voltage "spot on," then a resistor within 5% of 1.3K should be pretty close.
:::
:::I suspect the increased forward drop across the selenium diode (due to age) would cause a greater voltage variation.
:::Doug
:::
:::
:::
::::::
:::::::In the past I have replaced a 50A1 tube on a Zenith transoceanic with an Ohmite 10W 1.3K ohm power resistor. I've run out of this size resistor and can't locate them anywhere. I have a couple of questions: Can I safely use a 10w 1.6K ohm and could I move up to as high as a 10w 2.5 ohm without running into any problems. These 2 sizes are readily available on E-bay. Thanks for any advice.
:::::::Andy
Here is the best way to approach this problem aside from replacing the 50A1 tube.
Use a 1200 ohm, 10 watt resistor and clamp it to the chassis using a metal cable clamp to hold it down tightly. This will transfer the heat to the chassis.
Next, get a 1 watt 8.2 volt zener diode and connect it to the BEGINNING of the heater string being careful to observe proper polarity. This will now insure that you will have 8.2 volts applied to the total heater string as it should be AND it is regulated better than the 50A1 tungsten regulator could ever do it. I have performed this on numerous T/O sets.
Best Regards,
Bill Grimm
You may also want to take a look at the following page:
http://www.dialcover.com/tubes.html
Info on a 50A1 substutute is there down on the page a ways.
Radiodoc
************
:In the past I have replaced a 50A1 tube on a Zenith transoceanic with an Ohmite 10W 1.3K ohm power resistor. I've run out of this size resistor and can't locate them anywhere. I have a couple of questions: Can I safely use a 10w 1.6K ohm and could I move up to as high as a 10w 2.5 ohm without running into any problems. These 2 sizes are readily available on E-bay. Thanks for any advice.
:Andy