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 Post subject: Replacing tube recitifiers with solid state
PostPosted: Jun Wed 08, 2016 8:31 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1453
Location: Santa Rosa, CA
This isn't something I do often when restoring vintage equipment but in some cases
it makes sense. Like when you're trying to reduce the amount of heat generated.
Or reduce stress on hard to find transformers.

I'm not going to get into choosing the correct diode (PIV) or selecting inrush
current resistors here but something most folks don't do. Add snubber caps
across the diodes. The purpose of adding snubbers it to bypass high frequency
spikes that can occur and exceed the PIV specs even if there is a lot of margin.
High frequency, high voltage spikes can be cause by a number of things including
arcing switches, relays or come in from the power lines.

Adding snubbers is easy. A .01 uF ceramic capacitor is placed across each diode.
Of course the voltage rating should be much greater than the reverse voltage
each diode in the string (if more than one is used) will see. This is standard
procedure in large transmitters but apples to smaller electronic equipment as
well. Was reminded of that yesterday.
Steve

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 Post subject: Re: Replacing tube recitifiers with solid state
PostPosted: Jun Thu 09, 2016 6:37 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12549
Location: Mpls, Minnesota
You may want to read this, sometimes you increase the transformer load going to solid state.
viewtopic.php?f=8&t=301136

Dave


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 Post subject: Re: Replacing tube recitifiers with solid state
PostPosted: Jun Thu 09, 2016 3:10 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1453
Location: Santa Rosa, CA
easyrider8 wrote:
You may want to read this, sometimes you increase the transformer load going to solid state.
http://www.antiqueradios.com/forums/vie ... 8&t=301136

Dave


I'm very aware of all of the discussions and arguments against and for
going to solid state rectifiers. As I said its not something I normally do
however there are instances where it does make a lot of sense.
And in those cases snubbers are a good thing to add.
Steve

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 Post subject: Re: Replacing tube recitifiers with solid state
PostPosted: Jun Fri 10, 2016 12:41 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12549
Location: Mpls, Minnesota
When I use solid state I always use snubbers and two rectifiers in series, it doesn't take much of a line surge or spike to short them out.

Dave


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 Post subject: Re: Replacing tube recitifiers with solid state
PostPosted: Jun Sun 12, 2016 7:03 pm 
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Joined: Dec Thu 10, 2015 7:01 pm
Posts: 319
Location: Aurora, IL
This subject reminded me of my bench tech days decades ago in that this time of year in early summer we always had a very robust TV repair business. Why? Lots of sets coming in with mostly the same problem... blown fuse/breaker caused by a shorted SS rectifier. Seems transients on AC lines from electrical storms was responsible.
Can't recall many sets having snubber caps across diodes. Business might have been a lot lighter if they did.


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 Post subject: Re: Replacing tube recitifiers with solid state
PostPosted: Jun Wed 15, 2016 1:10 am 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 9956
Location: Santa Rosa, CA
In the olden days.... when I was selling 212-76 rectifiers (like a large 1N4006) to Zenith, we had lots of problems with diode failures. Mostly due to the poor construction of diodes back then. The junction edges were coated with varnish. Today, that would be nitride or glass. Moisture penetrated the varnish raising the leakage and creating contamination paths.

Also in the olden days, we processed rectifiers in a way that caused a "hard" or "snap" recovery.... that is going from conducting forward current to trying to block reverse voltage.

The rapid recovery waveform caused all kinds of problems with any circuit inductance (v = L di/dt). Also you can see "ringing" that caused voltage transients.

Image

Modern rectifiers are processed for soft recovery and should not need any parallel capacitance in most cases. I have seen a few audio amps with parallel capacitors and I am told that some radios used them, but in my 30+ years in the business, I saw very few cases where this was necessary. One exception.... if you put a bunch of diodes in series, adding capacitors will swamp out the recovery time differences and ensure that all diodes block the applied peak voltage. 10 nF (0.01 mF) is more than enough.

Rich


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