05-21-2014, 05:48 PM
Depends on whether you are replacing a regular or mercury vapor rectifier.
The MV rectifier has a relative constant forward voltage drop like a SS device. Around 12V as I remember. A resistor will not be your friend. If you want to be weird put a batch of diodes in series for forward drop or add a couple of 5W zeners you will need to check output current to spec. to size the wattage. I ignored the extra 12V of B+ the few times I used silicon.
I feel much the same about regular rectifier tubes as well if there is not a design reason for voltage roll off with current. Which I don't ever recall seeing. Not much of a chance of roll off needed in a radio.
Hickok TT and the like are a special cases as they use a rheostat in series with the transformer primary and every amp down stream has an effect. Might be worth looking into a TRIAC frontend regulator for the TT rheostat. Anyone thought about or tried it? If there is much interest or discussion a new thread?
You can always pull all the other tubes and see a max B+. B+ will be lower with the tubes plugged in. About the only thing I might look at is the screen voltage of the audio output and not be too picky.
You can calculate the max B+ current (forget adding screen current - it is lost in the decimal point) and install a temporary bleeder if it makes your antenna rotate or just for the education factor before plugging the tubes back in.
I like the glow of the tubes, especially the MV purple / blue so I stick with tubes most of the time.
Any tube that runs in a series filament chain like the 35Z5 is another beast. The filament pins need to be bridged with the HOT resistance of the filament. A decade resistance box with a current rating as high as your filament current is your friend. Start with too much R and turn down to get 35V and add a resistor close in standard resistance with plenty high W rating. Resistor should be same / close to Hot resistance from a data book.
As Codefox says mains are higher today If you replace a 35Z consider using a resistor for about 45V or higher hot filament drop. That will do the rest of your series tube filaments a BIG favor.
High filament voltage is a top tube killer and REALLY BAD for outputs.
OK, shorted caps can kill your rectifier if you don't add a AC line fuse.
The flip side is weak tubes will get "new life" with high filament voltage. OH DRAT! nothing is absolute
There were LOTS of 35Z & 50C tubes at Dayton last week. Many in the $1 to $2 range. Not rare why bother? Must be a hamfest in your neighborhood to look for fleas at.
Selenium rectifiers are easy to restack to repair. Get a hand full of different plate dimensions as you can find them cheap. Count the plates on your original and stack one with the same number of plates. It is often possible to do a bit of scraping and air dry varnish to repair a bad plate or 2 or center tube on your original. May need to wiggle the plate to plate contacts during assembly. Plate area is current rating. I guess these days you need to be aware that Selenium is not good for you so take whatever precautions deemed prudent.
Line voltage today is held close to 120V. Vintage radios worked with a wide voltage variations as that standard. I remember 110 as normal and 100 regular low. Don't loose the trees for the forest. You're trying to make a vintage radio work and LAST not build a top fueller.
The MV rectifier has a relative constant forward voltage drop like a SS device. Around 12V as I remember. A resistor will not be your friend. If you want to be weird put a batch of diodes in series for forward drop or add a couple of 5W zeners you will need to check output current to spec. to size the wattage. I ignored the extra 12V of B+ the few times I used silicon.
I feel much the same about regular rectifier tubes as well if there is not a design reason for voltage roll off with current. Which I don't ever recall seeing. Not much of a chance of roll off needed in a radio.
Hickok TT and the like are a special cases as they use a rheostat in series with the transformer primary and every amp down stream has an effect. Might be worth looking into a TRIAC frontend regulator for the TT rheostat. Anyone thought about or tried it? If there is much interest or discussion a new thread?
You can always pull all the other tubes and see a max B+. B+ will be lower with the tubes plugged in. About the only thing I might look at is the screen voltage of the audio output and not be too picky.
You can calculate the max B+ current (forget adding screen current - it is lost in the decimal point) and install a temporary bleeder if it makes your antenna rotate or just for the education factor before plugging the tubes back in.
I like the glow of the tubes, especially the MV purple / blue so I stick with tubes most of the time.
Any tube that runs in a series filament chain like the 35Z5 is another beast. The filament pins need to be bridged with the HOT resistance of the filament. A decade resistance box with a current rating as high as your filament current is your friend. Start with too much R and turn down to get 35V and add a resistor close in standard resistance with plenty high W rating. Resistor should be same / close to Hot resistance from a data book.
As Codefox says mains are higher today If you replace a 35Z consider using a resistor for about 45V or higher hot filament drop. That will do the rest of your series tube filaments a BIG favor.
High filament voltage is a top tube killer and REALLY BAD for outputs. OK, shorted caps can kill your rectifier if you don't add a AC line fuse. The flip side is weak tubes will get "new life" with high filament voltage. OH DRAT! nothing is absoluteThere were LOTS of 35Z & 50C tubes at Dayton last week. Many in the $1 to $2 range. Not rare why bother? Must be a hamfest in your neighborhood to look for fleas at.
Selenium rectifiers are easy to restack to repair. Get a hand full of different plate dimensions as you can find them cheap. Count the plates on your original and stack one with the same number of plates. It is often possible to do a bit of scraping and air dry varnish to repair a bad plate or 2 or center tube on your original. May need to wiggle the plate to plate contacts during assembly. Plate area is current rating. I guess these days you need to be aware that Selenium is not good for you so take whatever precautions deemed prudent.
Line voltage today is held close to 120V. Vintage radios worked with a wide voltage variations as that standard. I remember 110 as normal and 100 regular low. Don't loose the trees for the forest. You're trying to make a vintage radio work and LAST not build a top fueller.
