M-125 new(er) build - copper cap rectifier

Those small diodes only handle a continuous 1 amp load...  When you say they will handle a 30 amp surge you have to keep in mind that's only 8.3ms (milliseconds) and then it's toast.

That's not completely correct. 1.0 amps is the AVERAGE current they can handle. If you look at the On Semiconductor data for the 1N40** series here:

https://www.mouser.com/datasheet/2/149/1N4007-888322.pdf

Or the Vishay data sheet for their UF54** series here:

https://www.vishay.com/docs/88756/uf5400.pdf

and scroll down to the figure 3 chart on either datasheet, you'll see that while the maximum of 30 or 150 amps (depending on which diode you are looking at) is sustainable for 1/2 of a 60Hz sine wave (which lasts 8.3 milliseconds), you'll see that after 100 cycles (830 milliseconds, almost one full second) both diodes are still rated to carry 10 times or more of their rated average current. So the diodes will NOT fail instantly - so as long as the average is within the device's limits.

None of this has anything to do with how the amp is fused. I'm only trying to point out what it takes to prevent diode failures from overvoltage/PRRV or overcurrent conditions.

If you want to use larger/higher rated devices that's fine, go ahead. Look at an amp with a 350-0-350 volt AC power trafo secondary feeding a full wave center tapped diode rectifier with a capacitor input filter setup  configuration. That amp will have about 490 volts DC B+ voltage on one side of the diode; and on the other side of the diode you have an AC sine wave that varies from about +350 volts at the "top" of the AC wave to about -350 volts at the "bottom" of the AC wave. At the bottom of the AC wave therefore, you'll have -350 volts on one side of the diode and +490 volts on the opposite side. So the total voltage across the diode will be about 840 volts! For me, that's too close to the PRRV limit of the UF5408, so I use 2 in series and I'm now covered in case of a surge, a high AC line voltage, etc. For about $0.50 for a UF5408 diode it's cheap insurance.

The "yellow sheet" mod puts the heaviest voltage and switching loads on the diodes and in doing so makes life for the tube rectifier much easier. So use good diodes!

"For me, that's too close to the PRRV limit of the UF5408, so I use 2 in series and I'm now covered in case of a surge, a high AC line voltage, etc. For about $0.50 for a UF5408 diode it's cheap insurance. The "yellow sheet" mod puts the heaviest voltage and switching loads on the diodes and in doing so makes life for the tube rectifier much easier. So use good diodes!" wrote:

With 2 UF5408 diodes in series that makes 6 amps continuous well above what is needed to stay in the safty zone so your not replacing more diodes and/or other components. Yep.... a little insurance is better then non. Im using 10A10 10A diodes. I know thats a lot of overkill but in the long run they will take a lot of stress and abuse, keep voltages stable, and should last a long time.

"Jim McShane (830 milliseconds, almost one full second)" wrote:

And just to let you know 8.3ms is not almost 1 full second. 8.3ms is 0.0083 seconds.

Tom Pickett wrote:

And just to let you know 8.3ms is not almost 1 full second. 8.3ms is 0.0083 seconds.

That's not what I wrote! Here's what I wrote:

you'll see that after 100 cycles (830 milliseconds)

If you go back and reread my post carefully, you will see I am referring to the figures on the data sheet graphs (figure 3) showing current capability  - the graphs display capability up to ONE HUNDRED of the 8.3 millisecond cycles. And 100 8.3 millisecond cycles is 830 milliseconds.

Please - I'm sure you mean to be helpful, but your last two posts are incorrect and can mislead people - possibly leading them into making a mistake in how they configure their amps.

It’s one of the problems with many audio sites — where advise is dispensed too readily. Some people like to help, while others are a little overzealous (at times) offering their “knowledge”. In a black and white area like this, there can only be one right answer.

I truly believe there is a misunderstanding here Jim McShane. No worries and have a good day.

Tom

Earl wrote:It’s one of the problems with many audio sites — where advise is dispensed too readily. Some people like to help, while others are a little overzealous (at times) offering their “knowledge”. In a black and white area like this, there can only be one right answer.

Earl... yes that is correct.

Even though I have been doing electronics most of my life I don't claim to know everything because I don't. Electronics has always been a learning process no matter how many years of experience you have.

I founded, run and operate, my own astronomy forum/group on Facebook where I have over 50,000 members from all over the world talking and discussing astronomy. Sometimes people know what they're talking about and sometimes they don't. Even though I have been doing astronomy since I was seven years old I don't tend to lend or give advice on my site because I always run into those that believe they know it all and sometimes I try to do my best to avoid any misunderstandings and just stay silent.

Sometimes it's best to just stay silent and keep your knowledge to yourself to avoid any arguments or misunderstandings.

Have a good day Earl and stay safe.

Tom

Jim McShane...

I stand corrected.... You are correct about two diodes handling twice the amperage when they are in parallel. It's only doubles the resistance when you have two diodes in series.

At the time I believe I was thinking resistance and said amperage but you stand correct.

Have a good day Jim.