Replacing Selenium [message #26077] |
Sun, 15 March 2009 15:44 ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
AstroSonic
Messages: 58 Registered: May 2009
|
Baron |
|
|
Hi, I am restoring a Zenith AM-FM table radio that uses a selenium stack as a half-wave rectifier it has no power transformer). I would like to replace the selenium diode with a modern silicon diode. My question is - what was the voltage drop across the selenium diode (when it was new). I figure I'll need to add a dropping resistor in series with the (replacement) silicon diode in order to get the design voltages to the tubes. Otherwise, the tubes will see higher (possibly excessive/damaging) voltages. A search (selenium) of this forum did not turn up anything. Surely others have run into this same problem, as these devices were used extensively in the 40s and 50s. TIA, Bob
|
|
|
Re: Replacing Selenium [message #26078 is a reply to message #26077] |
Mon, 16 March 2009 13:37 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
![](http://audioroundtable.com/forum/images/custom_avatars/5.gif) |
Wayne Parham
Messages: 18757 Registered: January 2001
|
Illuminati (33rd Degree) |
|
|
The forward voltage drop across a selenium rectifier plate is approximately 0.5v - 1.0v, so you could count the plates to estimate the drop produced by the stack.I have selenium stack in my TransOceanic radios. I'll power one of them up and measure across it. As I recall, the voltage drop was not huge - like a few volts, maybe 10. It would be similar to a series string of germanium or silicon diodes, each having 0.3v to 0.7v across it, each adding to the total voltage drop. You could mimic this behavior with a series string of more modern semiconductor rectifiers or a diode and resistor. If the circuit is low voltage, like a filament circuit, I might be inclined to do that. But if it's the B+ supply, I think that the DC voltage desired is probably high enough that a few volts loss isn't required for proper operation. In that case, I think you probably could replace it with a single diode having less than a volt drop across it.
|
|
|
Re: Replacing Selenium [message #26079 is a reply to message #26078] |
Mon, 16 March 2009 15:58 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
AstroSonic
Messages: 58 Registered: May 2009
|
Baron |
|
|
Wayne, I have a Naval electronics training manual that describes Selenium rectifiers as being 65-85% efficient. Not sure, but I thought it might mean that 15-35% of the applied voltage is dropped across them. It would be an easy fix if there were only about a one volt drop per plate: it has five 1x1 inch plates. I could then replace with a modern silicon diode as you suggested. Do let me know what you measure across the stack in the TransOceanic. Thanks, Bob
|
|
|
|
|
Re: 15v [message #26082 is a reply to message #26081] |
Fri, 27 March 2009 13:15 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
AstroSonic
Messages: 58 Registered: May 2009
|
Baron |
|
|
Thanks, thats very helpful. I am thinking that after replacement with a silicon diode, the extra 14.3 volts will not seriously challenge (stress) the tubes and caps. Bob
|
|
|
|
Re: 15v [message #26086 is a reply to message #26083] |
Sun, 05 April 2009 18:32 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
AstroSonic
Messages: 58 Registered: May 2009
|
Baron |
|
|
Wayne, It worked great. There is a little hum that was not audible before. I think it may be due to the detector/voltage amp tube I replaced. The old one was running on fumes (an original Zenith). There is a lot more gain now. Or the hum might be due to the difference in waveform between the Selenium rectifier and silicon rectifier. Could need a little more filtering. With music or voices at low to reasonable volumes, it is inaudible. The radio sounds remarkably good with its wood cabinet, alnico 4x6 full range speaker and an electrostatic tweeter (actually works!). Thanks for the help. Bob
|
|
|
|