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FRED diode change for Tubelab se

wianieq

Member
2008-12-23 5:02 pm
Thank you. I did search, but for some reason didn't find those threads.

I Know from when I was working on solid state stuff a while ago, that trying to garner the appropriate specs from diode datasheets does not work so well for me. I cannot wrap my head around it.
 
trying to garner the appropriate specs from diode datasheets does not work so well for me.

The spec that is important here is avalanche energy rating. This spec is not always mentioned or even tested. What this means is how much energy (if any) the diode can eat when breaks down in the reverse direction.

All diodes will conduct if enough reverse voltage is applied. This is called the avalanche effect. If the diode structure is not uniform across the entire chip the avalanche current will flow in a small area causing damage. Zener diodes are designed to operate in reverse conduction but an ordinary diode is not. We try to specify diodes such that they will never see enough voltage to cause reverse breakdown. This is not so easy anymore.

Many of us have learned about stored magnetic energy by being zapped. I learned at a very young age by measuring the field coils of several speakers with a Simpson 260. The current from the ohm meter created a magnetic field in the coil. When the current was disconnected the collapsing magnetic field created a voltage in the coil that found a discharge path through me!

What does this have to do with our tube amps? Well the power transformer is a magnetic coil capable of storing energy in its core, and this energy will be released when the current is abruptly shut off. This energy has to have a path and there is a path through the tube heaters. This has worked well for years. As modern power transformers have been cost reduced the coupling between the windings isn't as good as it used to be and the leakage inductance is higher. This can cause a very short high voltage spike to appear on the high voltage winding (most inductance) as the power switch is turned off. The spike can be less than a microsecond wide and 2500 volts or more! I have measured this on a Hammond 274BX. Vintage transformers are usually much lower.

This effect is magnified when a standby switch interrupts the secondary winding itself. I have seen rectifier tubes arc over when the standby switch is opened. I no longer recomment a standby switch with modern power transformers.

With a big old 1N5408 diode that has several hundred pF of capacitance the spike gets absorbed. With a high speed FRED the spike blows through the silicon zapping the diode. Well it turns out that this is a problem in SMPS's too, so the diode manufacturers have specified the amount of energy that the diode can eat in a short spike. The Fairchild Stealth diodes are avalanche rated and I have never seen one blow.

What happened to my FREDs? At the time the TSE and SSE's were designed the FREDs were the hot ticket. The DSEI 12-12A was a good low cost FRED that worked well in my amps. I bought 100 of them 6 years ago and to this date none of them have failed. Most of the second batch that I bought blew up in short order. Clearly something has changed.

It is common for semiconductor to modify the design of a device during its production life. Higher volume devices are moved to more efficient fabs and transfered to larger wafers. The modified devices are tested to conform to the published specifications. Often large volume customers are given advanced samples of the modified devices to verify performance in their designs. Many FREDs including the DSEI12-12 are not avalanche rated so the modified devices did not violate the non spec.
 
No Fairchild ISL9R8120P2 till June.

Hello All,
I have been a long time lurker, and have gotten some great information from all of you, but alas, it looks like my lurking days are done...
I decided to finally build that Tubeleb SE that I have been eyeing for some time, I got my boards from George, (Thanks BTW) ordered my iron from Edcore, ebayed me up some tubes, but things just came to a grinding halt at digi-key. The DSEI12-12A from the parts list is on backorder for a month or so, I went over to mouser and looked for the replacement that Ty suggested and it seems they are out with a 41 week lead time listed. Does anyone have any ideas? Is there a suitable replacement availble, are there other suppliers I should check? It will probably be till the end of the year before my transformers get here, so I am not in that big of a hurry, but I am not sure I can make it 41 weeks.

Any suggestions would be much appriciated.
Jpeg
 
It looks like all the Fairchild rectifiers are out of stock everywhere (Mouser, Digikey, Newark). If you want something now, I guess you'll have to try a STMicro part. There's three with similar specs, but I don't know if anyone has ever tried them. Let us know how they work out?

STTH812FP STMicroelectronics Rectifiers
STTH812D STMicroelectronics Rectifiers
STTH812DI STMicroelectronics Rectifiers

Mouser also claims to have a "new" part from Ixys. Maybe it's improved over the old version?

DSEP12-12B Ixys Rectifiers
 
Thanks everyone.

Thanks for the info! I got a little overwelmed trying to use the search feature to find a replacement. I never knew there where that many different diodes out there. Is this a part that has an impact on the sound quality? Or does somthing downstream blow up if it fails? I built an S-5 kit last year, and I replaced the solid state rectifier with 4 4007 (I think) fast recovery diodes, as stated on the forums, to keep AC from leaking past and effecting the sound.
 
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I have been using a pair of HER108's that were left over from another project. They are 1000V fast diodes. I have the amp set up with KT88's and will be setting up some Edcor CXSE 5K 4 ohm OPTs over the next week or so.

With an Allied 6K7VG B+ is running around 450 and the KT88's are biased at about 95mA.

I have about 30-40 hours on this setup and it has not hiccuped once. The previous OPTs were James 6123s. I was able to hook up the Edcors for a couple hours yesterday and they are extremely impressive.
 
I have been using a pair of HER108's that were left over from another project. They are 1000V fast diodes.

With an Allied 6K7VG...

The 6K7VG is a 750VCT power transformer. Your diodes will see 750V * sqrt(2) = 1060 peak inverse volts. Technically, you're over their limit. Unlike tubes (which have only guidelines), solid state devices tend to fail when you exceed their ratings. I think George put a 1200 PIV rated diode on the parts list because most builds use a PT around that voltage range.
 
I think George put a 1200 PIV rated diode on the parts list because most builds use a PT around that voltage range.

As stated the math puts 1060 volts on the diode if the transformer voltage is accurate. The Allied transformers are made by Hammond which means that they tend to run on the high side. It is possible to see close to 1200 volts across the diode in many situations. I would have specified 1600 volt diodes if they were readilly available.

On the other side of this coin the PIV spec of a diode usually has margin. I routinely stick 1N5408's (1000 PIV) in my experiments because they are really cheap and I bought a bag full. I have not blown one yet.

As stated once the diode breaks down in the reverse direction it will blow. Diodes almost always fail to a short which will blow the primary fuse with little chance for collateral damage. Understand this and check the diodes with an ohmmeter if the fuse ever blows. If one diode is dead, replace both since the good one was responsible for blowing the fuse, since it shorts the power transformer on every half cycle and ate a lot of current!
 

Mark Johnson

Member
Paid Member
2011-05-27 3:27 pm
Silicon Valley
With a big old 1N5408 diode that has several hundred pF of capacitance the spike gets absorbed. With a high speed FRED the spike blows through the silicon zapping the diode. Well it turns out that this is a problem in SMPS's too, so the diode manufacturers have specified the amount of energy that the diode can eat in a short spike. The Fairchild Stealth diodes are avalanche rated and I have never seen one blow.

If that's really the problem then why not solve it with one of these 560pF, 3000v, $0.36 capacitors across each diode?

By the way I've copied the capacitance vs voltage plots for the 1N5408 from three different manufacturers. They all say the capacitance is below 100pF, and at 1000V reverse bias, WAY below 100pF.

_
 

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