5 watt sand resistor

[vts1134]

I'm preparing for a restoration on a Teletone TV-149 and am taking some resistance measurements prior to digging into the set. I found that I need a replacement for the surge limiter, which is an 18 ohm 5 watt sand resistor. Does any one know a source for replacement on this piece?

[ctc17]

I always replace those with the modern ceramic ones. You can put a 15 and 3 ohm in series.
I hate they way they look but they are easy to get.

[pearsonk]

Think I have an exact.
I'll pm

[sean]

If you decide to replace the selenium rectifier with a silicon diode you would need to add a series resistor to compensate for the lower voltage drop. You might as well replace that 18 ohm resistor with one of a larger value to combine both resistors into one.

[pearsonk]

In mail......

[vts1134]

Is there a way to test a selenium rectifier other than in operation? Is there a reason to simply replace without question, besides the malodorous side effect of total failure ?

[Einar72]

Unlike a modern silicon diode, which drops 0.7 volts across itself when conducting, a selenium-stack typically drops a bit more, similar to a vacuum tube, so the "diode check" function on modern DMM's is useless here. If they work, leave 'em alone. If you do replace them, you can use the DMM to trim the series resistors to exactly the value needed to get the proper B+ voltage.

As for aesthetics, I look long and hard in junker chassis', swap meets and ePay to avoid those cheesy-looking ceramic resistors we shamefully import these days. An IRC ceramic or Ohmite "Brown Devil" looks so sweet and close-to or actually period-correct as well!

Now here's a question for all those who rave about modern low-ESR electrolytics:

Does the lack of this "effective series" resistive component actually result in a greater surge current at turn-on? Were the 1940's originals low-ESR as well when new? It would seem to me that this scenario, IF that is correct, would stress the aged selenium device even more...

[DavGoodlin]

FWIW I add thermistors in my restorations to limit transformer filament and/or silicon rectifier inrush current. The more reliable sets in my collection have them as original equipment.

120 ohms cold - 2 amps works super. CL-90 or Allied #837-0090
you have to accept lower cold resistance for higher current apps