Hexfreds- Any help from my friends in the EU?

[Funkalicousgroove]

Hello All!

I am building an amp to send to the UK, and with the 50Hz power I figured that I ought to use Hexfreds for rectifier diodes. Here is the situation:

The amp will have a 700V on the plates, and use a voltage doubler setup like a Music Man. What rating of HEXFRED do I need to use, and do they still need a snubber cap

I've read about guys smoking 1,3,and5 amp FREDs in Marshalls, do I need to go as large as 1200v 8A? or should I go even larger?

Thanks!
Funk

[paulster]

Brandon

I don't like the TO-220 packaged ones a lot so I use FR107s, which look and act just like 1N4007s, but have a recovery time of 30ns rather than 2us. I don't use snubber caps with these.

They'll handle the voltages you're looking at, and you shouldn't be drawing more than 1A off your HT supply (unless this is an absolute beast), but you could always use a pair on each leg of the bridge if the voltage is getting a little marginal (take a look at an Ampeg SVT schematic to see what I mean).

Here's the specs:

DIODE, FAST RECOVERY 1A 1000V
Case style:DO-41
Current, If av:1A
Current, Ifs max:500A
Time, trr max:30ns
Voltage, Vrrm:1000V
Voltage, forward at If:1.3V
Current, If Vf:1A
Current, Ifsm:500A
Diameter, External:2.7mm
Length / Height, external:5.2mm

[skyboltone]

This in no way answers your question Brandon but I've just bought 10 of these at $2.71 each. From Mouser.

http://www.fairchildsemi.com/ds/IS/ISL9K8120P3.pdf

These are dual diodes for a CT full wave.

[Funkalicousgroove]

I decided to go with the 8A 1.2KV, the TO-220 doesn't bother me a bit

[Fredaxis]

I have used those ones in my 6V6plexi, with good results: DSEI806 8A 600V

I remember I had the recommendation from ax84 to keep doubling them.

I plan on using those ones in my Express build: MUR860 8A 600V

[paulster]

If you need an 8A fast-recovery diode then they're great, but we don't need 8A of HT in guitar amps and the recovery time is slower than the one I quoted anyway, which fits on a regular board without having to worry about ensuring you've got eyelets properly spaced for the TO-220 pins.

I've never understood why everyone seems to think these are the only FRED diodes that are out there.

[Fredaxis]

True that I bought some Hexfred for some test/comparison reasons, Installed them in the plexi (with correct turret spacing), it works silently...
But, as works other amps with 1N4007, I do not hear a tremendous difference.

Still have to try some UF4007.

I guess we have another kind of urban legend in this.

I only think that the 8A rating will not hurt reliability wise.

[skyboltone]

If you need an 8A fast-recovery diode then they're great, but we don't need 8A of HT in guitar amps and the recovery time is slower than the one I quoted anyway, which fits on a regular board without having to worry about ensuring you've got eyelets properly spaced for the TO-220 pins.

I've never understood why everyone seems to think these are the only FRED diodes that are out there.

It would be interesting to take a picture of the instantaneous inrush current on 325 volts unfiltered DC into an empty 100uf electrolytic cap. I've always thought that the reason we gang 1N4007s together was for inrush current not peak voltage. Technically peak voltage thru a rectifier should only be 1.41 times RMS voltage, yet you'll always find 1n4007s ganged at least two in a guitar amp. The dual Hex diode who's data sheet I cited in my previous post has a smaller footprint than 6 1N4007s.

Whatever
Dan H

[paulster]

Dan

Those are neat, but this needs a bridge rectifier so the common cathode would only help on one half of the bridge, and you'd need another pair (or a common anode version) for the other half.

I think you're right about the diodes being used in pairs to spread the load over a pair of diode junctions rather than having one junction have to suffer the hit but you don't often get 1A diodes dying without another failure in the amp having caused it.

What I don't get is how everyone wants to use fast recovery diodes to get rid of switching noise, yet all seem to think that you have to go to one of these 8A packages to get them. They're available in all shapes and sizes to suit the application is what I'm saying.

Paul