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Old 5th April 2007, 03:39 AM   #1
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Question Substituting tube rectifiers by silicon

Hi,

If I have a shematic with a certain AC voltage PT & a tube rectifier,
Do I have to make any changes to the schematic to use silicons?? If yes, what are the changes? How (& on what basis) are the new values figured out?

Someone told me that the silicon rectified DC voltage will be higher than that with tubes. He also told me that adding resistors to decrease the voltage will affect regulation.

I got a 400-0-400 PT out of an HH Scott & I want to use it in a project with the same voltage but using a 5V4G rectifier. I want to substitute the tube with silicons & I want to know how to do this substitution myself for future projects, so please help.

Thanx
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Old 5th April 2007, 03:47 AM   #2
Tweeker is offline Tweeker  United States
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PSUD II is an extremely useful tool in helping figure such questions.

Silicon is likely to give ~15+ more volts than an indirect heated vacuum tube rectifier.
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Old 5th April 2007, 03:54 AM   #3
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You might want to add some delay on the B+ to prevent cathode stripping on the signal tubes.
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Old 5th April 2007, 08:54 AM   #4
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The voltage drop across the tube rectifier will be significant, depending on the current drawn - and it's not a linear relationship. I've seen a figure for the 5V4G of 25v drop for 17mA current. The voltage drop for an SS diode will be zero.
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Old 5th April 2007, 09:45 AM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
The voltage drop for an SS diode will be zero
the forward voltage drop for a silicon diode is approximately 0.7V but varies with current.
A rectifier bridge has two diodes operating in series and drops +1.4V.
Compared to estimates of 15V to 25V volts drop for a tube (valve) diode, the silicon drop is insignificant.

A FET controlled slow switch on using a circuit like a capacitor multiplier will achieve a delay that you can pre-set to your requirements.
Finally, a relay to short out the FET and send full voltage to your circuit, or keep the multiplier in circuit if you require the lower voltage. The output voltage tracks the mains voltage, but you can preset the volts drop across the multiplier. Use your highest quality caps after the multiplier.
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Old 5th April 2007, 10:56 AM   #6
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Can't you put a switch from the CT of the transformer to ground?
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Old 5th April 2007, 03:35 PM   #7
Tweeker is offline Tweeker  United States
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You can put a zener at CT to drop some volts without effecting regulation so much.
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Old 5th April 2007, 04:59 PM   #8
JoshK is offline JoshK  Canada
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Quote:
Originally posted by AndrewT
A FET controlled slow switch on using a circuit like a capacitor multiplier will achieve a delay that you can pre-set to your requirements.
Finally, a relay to short out the FET and send full voltage to your circuit, or keep the multiplier in circuit if you require the lower voltage. The output voltage tracks the mains voltage, but you can preset the volts drop across the multiplier. Use your highest quality caps after the multiplier.
Does anyone have a schematic that shows how this is done? I've heard it talked about but haven't seen it implemented.
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Old 5th April 2007, 10:46 PM   #9
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Sorry, I meant to say earlier that the voltage drop figure I've seen for 5V4G was 25v at 175mA. A half-way point between a normal tube rectifier and SS is to use TV damper diodes. They drop only about 8-10v.

BTW, if you intend using a regulator for B+ then it's very easy to build in a delayed ramp-up of the regulated voltage.
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Old 6th April 2007, 12:12 AM   #10
zobsky is offline zobsky  India
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Will these work well to delay B+? .. I see them pop up on ebay all the time
http://cgi.ebay.com/Power-Supply-Del...QQcmdZViewItem
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