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Hexfred question

sloober

sloober

Super Member
I have used hexfreds in place of a regular diode in a few radios and amps. I used one on my recent MC30 rebuild. Since the hexfred had a TO-220 case, I bolted it to the circuit board with a nylon bolt and nut. After a few days, I was checking it over and noticed the nylon bolt had turned yellow brown. It is obviously getting hot. Way hot. I hooked up a digital thermometer and it is over 150°F. I have since removed it from the board so it is floating and made a small heatsink. It is rated at 600v and 16A so I think it should be up to spec, but should it be so friggin hot?
 
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Usually, if they're free-floating (all sides exposed), they'll be OK in a tube amp. Not that much B+ current... at most a few hundred milliamps. The reason it got hot, is that by being bolted to a board, it was effectively insulated from heat rejection from one side. Get the backside (the metal plate) exposed to air, that'll usually do it.

OTOH, in a solid-state amp... YES, a heatsink is required! Several amps through one of these, and they will get HOT...

Regards,
Gordon.
 
So, the other question is . . . .
Do hexfreds live up to their reputation or should any fast recovery diode do the job.
What do hexfreds offer that's special?
I ask this as someone who has used more than a dozen.
The first set I used were the big ones from tubesnmore. A friend sent a set of fast recovery diodes. I thought I detected an improvement in sound, but I didn't have a scope at the time to check for power supply ripple on the outputs in the head-to-head test.

Any recommendations? What's your experience?
 
Last edited:
audiodon657 said:
So, the other question is . . . .
Do hexfreds live up to their reputation or should any fast recovery diode do the job.
What do hexfreds offer that's special?
I ask this as someone who has used more than a dozen.
The first set I used were the big ones from tubesnmore. A friend sent a set of fast recovery diodes. I thought I detected an improvement in sound, but I didn't have a scope at the time to check for power supply ripple on the outputs in the head-to-head test.

Any recommendations? What's your experience?
Click to expand...

First off, let me dispel a myth - diodes have no effect on AC ripple. A 1N4007 and a high $$$ HexFred will show the same ripple all else being equal.

The difference between high performance diodes and standard diodes are in the way the diode turns off. Without getting into a super-high technical discussion, suffice to say that hi-po diodes produce much less noise at turn-off than standard types.

HexFred is not a diode type - it is the trademarked name owned by International Rectifier for their particular line of "FRED"s - Fast Recovery Epitaxial Diodes. A lot of companies make FREDs, it is not a proprietary product, only a trademarked name. IR was early out of the gate with a marketing blitz, and somehow all FREDs became known as HexFreds (similar to how in the USA most people call any type of facial tissue "Kleenex" or any cotton swab a "Q-Tip").

I have some issues with HexFreds:

1. They are ridiculously overpriced.

2. They are more difficult to mount than a typical DO-41 or similar package diode.

3. Heat considerations are present, so...

4. To avoid the heat issues you need to mount them in free air or heat sinked. In that configuration the leads make dandy RF antennae.

Some years ago Rick Miller and a few guys from Tektronix (? IIRC) did a GREAT study of fast recovery diode performance. They found fast AND soft turn off were BOTH required for the best diode performance. I've used the diode Rick & Co. recommended for years and I've been totally pleased with the results. And these diodes are in a "conventional" axial leaded package, making them VERY east to work with.
 
So I swapped it with a 1n4007. No heat at all. The thought of a 150°f chunk of heatsink in this sealed chassis is just not good. It was so hot my temp probe maxed out around 170°F a few times. John
 
Jim McShane said:
Some years ago Rick Miller and a few guys from Tektronix (? IIRC) did a GREAT study of fast recovery diode performance. They found fast AND soft turn off were BOTH required for the best diode performance. I've used the diode Rick & Co. recommended for years and I've been totally pleased with the results. And these diodes are in a "conventional" axial leaded package, making them VERY east to work with.
Click to expand...

Care to share which diode that is?
 
Something doesn't sound right here?? How could a tube amp possibly draw enough current to make a 16 amp diode heat to 170 degrees? If that was the case, why wouldn't the 1N4007 diode get hot as well? I have used many, many hexfreds and crees in that case and never had an issue with heat (as in not even warm to the touch). Are you absolutely sure you had the cathode properly identified on that package?:scratch2:
 
Well, it did work, the amp that is. I too remember them not getting hot at all. That is why I bolted it to the board. What I am not sure of is how I used it. I looked up the spec sheet and found it was 2 in one package. I was only using 2 pins, or one of the diodes. It may just be the wrong one for this application. Either way, I think I am sticking with the 1n4007 for now. Johnhttp://www.google.com/url?sa=t&source=web&ct=res&cd=1&ved=0CAsQFjAA&url=http%3A%2F%2Fwww.irf.com%2Fproduct-info%2Fdatasheets%2Fdata%2Fhfa16ta60c.pdf&ei=fF3-StzLKKS1tweZj8zIDA&usg=AFQjCNFkkxPhzQ0CoXs6SIl7JZicqGU6aw&sig2=5NDgk-_-dxTUdjHfX67t2Q
 
I've used those "double" hexfreds too. I wonder what type of rectifier would use both sides at the same time. I guess it's always possible that you had a bad one too. Did you happen to measure your B+ with the diode getting hot, compared to now? There had to be a bunch of resistance through that dude to make that much heat. Here's a handy link that shows connections.

http://www.vishay.com/docs/88892/mbr60100.pdf
 
This diode goes to the bias supply. I did have to adjust the resistor to get the bias negative voltage set.
 
I'd be looking close at the bias cap if the diode is getting hot.... with that much of a load, you are looking at loosing a transformer winding.

vishay hexfred 1200v 6amp to220 have done an excellent job for me, digikey VS-HFA06TB120PBF-ND at $1.21 each in quantity 10....
 
I think I am OK. Not sure why, but in this application the hexfred is a no go for me. I either used a bad one or the wrong one. Went back to the 1n4007 and all is good and much cooler. And switching to the 125v tap on the power tx made a big change too. John
 
It never occurred to me that there would be two diodes in the three pin package. The diagram on the package does not mention that. In fact, it's fairly cryptic.
The larger ones, the 1600 volters, have pins 2 and 3 tied together and the function of pin 1 and 2/3 is very obvious.

Last time I bought at tubesnmore, I bought the 600 volt three pin jobbies. I tied pins 2 and 3 together like their larger bretheren do at the factory and have had no end of trouble with the integrated amp I put the pair in. It never occurred to me that the diodes could be the problem. Now it does.
Live and learn.
 
I wonder if that is the one I should have used. Not sure what is off, but for now it is working and the bias is set dead on. Terry was good enough to relay a setting for the plate voltage I have so I think I am all done. It is not nearly the oven it was before the change in input tap and removing this fred.
 
When I rebuilt my pair of MC-30's, I took Terry's advice: 1n4007 and the resistor values he recommends in the McIntosh hints post. They have been working like champs for more than a year now.
 
Is your amp wired with the 125v or 117v tap? I too used Terry's advice, but tweaked the resistor value to get bias where he recommends. Using the 125v tap, I am getting 420v plate voltage instead of 445v. For this he recommended bias at -40v. Used a couple resistors to come out to 9.8k for the bias resistor. John
 
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