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PostPosted: August 11th, 2018, 11:06 pm 
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Joined: July 15th, 2016, 10:02 am
Posts: 69
I came across this post recommending forward-biased diodes in series with the input to (or output from) a rectifier tube:
http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod

The poster recommends this for all tube rectifiers to prevent flashing. Do folks here think this is worth doing? My amp can deal with the diode drop.


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PostPosted: August 12th, 2018, 6:48 am 
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Joined: January 15th, 2015, 7:19 am
Posts: 651
Location: Baltimore MD
I have seen this before in a kit from Lampizator. I really never understood the reason. What is "flash over"
I was curious if this mod added noise to the system.
Further down in the thread one talks about placing diodes on the output of the rectifier just befor the big caps.
If this is really a plus and SS diodes are cheap why not?


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PostPosted: August 12th, 2018, 9:54 pm 
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Joined: July 15th, 2016, 10:02 am
Posts: 69
My thoughts exactly. Was hoping someone could shed some more light on if/how this works and whether it's good practice.


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PostPosted: August 12th, 2018, 10:10 pm 
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Joined: July 15th, 2016, 10:02 am
Posts: 69
This thread shed some light on "flashover," but still doesn't have a lot of detail.
http://dynacotubeaudio.forumotion.com/t2525-rectifier-tube-flash

"The diode mod prevents reverse current flashover like when the amp is suddenly turned off, caps are fully charged up, and the rectifier cathode gets cooler losing its negative charge. The conduction path in this special case (no diodes) is back through the heater windings to ground which is very low impedance."

I don't think I understand. How does the current get through the heater windings to ground?


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PostPosted: August 12th, 2018, 10:42 pm 
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Joined: July 15th, 2016, 10:02 am
Posts: 69
Just found this:
https://www.premierguitar.com/articles/the-immortal-amplifier-mod-1

I am now inclined to believe that the diode mod actually has nothing to do with "flashover," it just protects the PT if the rectifier fails short (which causes the filter caps to fail short, shorting the secondary to gnd).

What's weird though is if you put diodes on the input to the rectifier tube, I don't think the rectifier tube does anything anymore other than providing soft-start and sag.


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