Hi, I have a zenith H500 transoceanic that needs it's selenium rectifier replaced, The main thing I need to know is what value and wattage of additional resistance that I need to add with the new diode. Thanks.

Kris: you might want to check the H500 page at:
http://oak.cats.ohiou.edu/~postr/bapix/H500_2.htm

They recommend 75 ohms to ensure that the filament voltage doesn't get too high. I would use a 1N4007 diode and at least a 2 watt, 75 ohm resistor. A 5 Watt would be even more conservative.

Rich

:Hi, I have a zenith H500 transoceanic that needs it's selenium rectifier replaced, The main thing I need to know is what value and wattage of additional resistance that I need to add with the new diode. Thanks.

:Kris: you might want to check the H500 page at:
:http://oak.cats.ohiou.edu/~postr/bapix/H500_2.htm
:
:They recommend 75 ohms to ensure that the filament voltage doesn't get too high. I would use a 1N4007 diode and at least a 2 watt, 75 ohm resistor. A 5 Watt would be even more conservative.
:
:Rich
:
::Hi, I have a zenith H500 transoceanic that needs it's selenium rectifier replaced, The main thing I need to know is what value and wattage of additional resistance that I need to add with the new diode. Thanks.
Rich: Every time I do one of these models I go ahead and install a silicon diode and leave the AC side of the diode unconnected and use a 1,000 ohm pot in series with the diode. The factory schematic calls for 105vdc at the output of the selenium rectifier. Therefore, I vary the 1K pot while monitoring the voltage and use that value of R that will give me 105 volts. I would caution you not to use too much wattage here since if the diode should ever short you want that resistor to burn up and act as a fuse. Also, to prevent transients from taking out the diode, place a .001 mf, 1Kv ceramic cap in paralell with the diode and it will greatly improve its longevity.
Good luck. Tom

::Kris: you might want to check the H500 page at:
::http://oak.cats.ohiou.edu/~postr/bapix/H500_2.htm
::
::They recommend 75 ohms to ensure that the filament voltage doesn't get too high. I would use a 1N4007 diode and at least a 2 watt, 75 ohm resistor. A 5 Watt would be even more conservative.
::
::Rich
::
:::Hi, I have a zenith H500 transoceanic that needs it's selenium rectifier replaced, The main thing I need to know is what value and wattage of additional resistance that I need to add with the new diode. Thanks.
:Rich: Every time I do one of these models I go ahead and install a silicon diode and leave the AC side of the diode unconnected and use a 1,000 ohm pot in series with the diode. The factory schematic calls for 105vdc at the output of the selenium rectifier. Therefore, I vary the 1K pot while monitoring the voltage and use that value of R that will give me 105 volts. I would caution you not to use too much wattage here since if the diode should ever short you want that resistor to burn up and act as a fuse. Also, to prevent transients from taking out the diode, place a .001 mf, 1Kv ceramic cap in paralell with the diode and it will greatly improve its longevity.
:Good luck. Tom

Tom: I believe the H500 applies the DC to the tube filaments, so that any kind of short is going to wipe out a tube before it kills a resistor. Also, when I worked at Westinghouse Semiconductor, we discouraged caps across diodes, except where multiple diodes are in a series stack. Sometimes the reverse recovery "snap" of the diode will "ring" and generate hash in the receiver audio. I like your idea about using a pot (with a hefty power rating) to ensure that you get the right voltages. The original selenium, however, had an aging characteristic such that its forward drop kept going up as it aged. You gradually get less B+ voltage, so maybe the designers simply ensured that the B+ voltage wasn't too high when the radio was new, and accepted the B+ degradation as a necessary evil.

:::Kris: you might want to check the H500 page at:
:::http://oak.cats.ohiou.edu/~postr/bapix/H500_2.htm
:::
:::They recommend 75 ohms to ensure that the filament voltage doesn't get too high. I would use a 1N4007 diode and at least a 2 watt, 75 ohm resistor. A 5 Watt would be even more conservative.
:::
:::Rich
:::
::::Hi, I have a zenith H500 transoceanic that needs it's selenium rectifier replaced, The main thing I need to know is what value and wattage of additional resistance that I need to add with the new diode. Thanks.
::Rich: Every time I do one of these models I go ahead and install a silicon diode and leave the AC side of the diode unconnected and use a 1,000 ohm pot in series with the diode. The factory schematic calls for 105vdc at the output of the selenium rectifier. Therefore, I vary the 1K pot while monitoring the voltage and use that value of R that will give me 105 volts. I would caution you not to use too much wattage here since if the diode should ever short you want that resistor to burn up and act as a fuse. Also, to prevent transients from taking out the diode, place a .001 mf, 1Kv ceramic cap in paralell with the diode and it will greatly improve its longevity.
::Good luck. Tom
:
:Tom: I believe the H500 applies the DC to the tube filaments, so that any kind of short is going to wipe out a tube before it kills a resistor. Also, when I worked at Westinghouse Semiconductor, we discouraged caps across diodes, except where multiple diodes are in a series stack. Sometimes the reverse recovery "snap" of the diode will "ring" and generate hash in the receiver audio. I like your idea about using a pot (with a hefty power rating) to ensure that you get the right voltages. The original selenium, however, had an aging characteristic such that its forward drop kept going up as it aged. You gradually get less B+ voltage, so maybe the designers simply ensured that the B+ voltage wasn't too high when the radio was new, and accepted the B+ degradation as a necessary evil.

Unless you do not have one, it is probably simpler to use a variac. Less of a high power issue here.