Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on January 18, 2012, 03:12:18 pm

Title: Bridge rectifier straight talk
Post by: firemedic on January 18, 2012, 03:12:18 pm
Construction of the bass amp has resumed.

I have a Twin Reverb non-CT 330-330 PT. I'm going to run 4-EL34s until I can get 4-KT88s (with an aux. heater tranny).

For the bridge rectifier, will 4-5408's, in a standard diamond configuration, cut it? Or should I use 8? I'm not even considering using IN4007s. Also, are the small caps I see across the diodes necessary? You guys use 'em?

This is the first bridge rectifier I've built so I want to get it right. Thanks!



 
Title: Re: Bridge rectifier straight talk
Post by: kagliostro on January 18, 2012, 03:27:27 pm
those 1N5408 are rated 1000v 3A so they can be used

about the use of the little cap they can have negative effects

nowadays you can use a fast scottky diode after the bridge (in series with B+) and you solve the problem about switching noise given by standard diodes (that was the purpose for the little caps)

alternatively you can build your bridge with fast diodes, but the first solution will have the same result at lower price

Kagliostro
Title: Re: Bridge rectifier straight talk
Post by: thermion on January 18, 2012, 03:32:55 pm
the small caps act as a hi-frequency switching noise filter. freds will not need these, but typical "slow" diodes may benefit.
these 5408 diodes are cheap; doubling up if nothing else gives you some redundancy in case one diode in the series fails.
Title: Re: Bridge rectifier straight talk
Post by: firemedic on January 18, 2012, 03:36:09 pm
Thanks for the quick reply gentlemen. I have plenty of board space so I'll go with 8.
Title: Re: Bridge rectifier straight talk
Post by: stingray_65 on January 18, 2012, 03:42:51 pm
Quote from: firemedic on January 18, 2012, 03:36:09 pm
Thanks for the quick reply gentlemen. I have plenty of board space so I'll go with 8.

Will look beefy and cool too :icon_biggrin:
Title: Re: Bridge rectifier straight talk
Post by: kagliostro on January 18, 2012, 03:53:22 pm
Quote
I have plenty of board space so I'll go with 8

in series diodes will double their voltage ability

in parallel they don't double their current ability

I think you mean to connect two by two in series than build your bridge, isn't it ?

Kagliostro
Title: Re: Bridge rectifier straight talk
Post by: firemedic on January 18, 2012, 05:39:20 pm
Trying to increase the PIV rating of the bridge, which series connection will do, yes?
Title: Re: Bridge rectifier straight talk
Post by: sluckey on January 18, 2012, 06:48:47 pm
Some bridge straight talk...

Bridge diodes work in pairs, ie, two will conduct on one half cycle of the applied AC voltage, then the other two will conduct for the next half cycle. If you draw  a simple bridge recto with a load  and put some polarity signs to represent the AC half cycles, you will see that you have one side of the AC (secondary) connected to a forward biased diode then the load, then another forward biased diode connected to the other side of the AC (secondary). This pair of diodes passes current to the load. The other pair of diodes is reverse biased and does not conduct. The diode pairs reverse roles every half cycle. IOW, you already have two diodes in series and you already have double the voltage rating. A bridge made from 4 1N4007 diodes will have a voltage rating of 2000VPIV. A 1N5408 bridge would also have a 2000VPIV rating.

So, if you use 8 1N5408s to build a bridge, it will have a 4000VPIV rating.
Title: Re: Bridge rectifier straight talk
Post by: firemedic on January 18, 2012, 10:14:27 pm
That was pretty straight. Thanks sluckey. Back to 4 diodes....