Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on April 24, 2010, 11:51:55 am
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Hi friends
I find the advertising of an "Ultimate Rectifier Board" as a commercial product
as you can see it is a full wave rectifier with 3 diodes in series in each branch
in parallel with each diode there are one cap (.01uf - 1Kv) and one resistor (470k)
I'm a bit confused about the presence of the resistors
I can understand the presence of the cap
but the presence of the resistor in my mind give the way to the AC to go on the B+ rail
isn't it ??
and also is a "must to do trick" ??
I have never seen that before
Kagliostro
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// caps act as roughy snubbers and can cut HF noise, // resistors balance currents betwen the diodes.
Such a topology is widely used in indus power supplies. Is it worth the bother with modern UF diodes and guitar amps? I don't think so but YMMV as usual.
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Back in 1966 we did throw resistors across series-diodes to equalize the reverse voltage. Otherwise the leaky ones would take less voltage, the least-leaky one would get most of the voltage, and fail.
Modern diodes have higher breakdown voltage, very much lower leakage, and break-down gently. It's not necessary to add "equalizing" resistors on series diodes for any reasonable B+.
Those inexpensive ceramic caps look like an added failure point.
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Adding snubbers without doing the math can make noise worse in my experience. Here's the classic article on snubbers (that I don't understand):
http://www.hagtech.com/pdf/snubber.pdf
{EDIT: URL clean-up --PRR}
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If you want 2 diodes in series just get a small 1000V bridge rectifier and snip off the AC leads.
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This does look like a "me too!" product.
If the builder knew what they were doing, they would know that you don't need 3 series diodes anymore, even though the old amps had 3 series diodes per leg. Unless they're using something smaller than a 1N4007, of course... But I doubt they can buy a smaller diode any more cheaply than that one.
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MANY THANKS TO ALL
what you say have sense
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if I snip the AC leads in a bridge
I obtain a single diode with double reverse voltage ability
and also a doubled current ability - right ?
Kagliostro
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Here's the classic article on snubbers (that I don't understand):
http://www.hagtech.com/pdf/snubber.pdf
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If you snip the AC leads you do not get double current because one series-ed pair will always hog most of the current.