[Top] [All Lists]

Re: [Amps] HV Diodes

To: "Glen Zook" <>, "Jim Thomson" <>, <>
Subject: Re: [Amps] HV Diodes
From: "Carl" <>
Date: Mon, 13 Oct 2014 12:22:22 -0400
List-post: <">>
I use the 1N4007 for simple things such as replacing the 0Z4, 6X5, 6X4, 12X4 in auto radios as well as a few test equipment, house radios and others that are known for 6X4 and 6X5 problems.

For items such as the HT-32/HT-37 family I use the 4007 in LV and the 1N5408 in HV since those transformers are known for the 5V rectifier winding shorting to the HV winding due to poor insulation.

I AM NOT a wholesale replacer of tube rectifiers with SS as they can cause other problems.

A string of 1N5408's are fine for replacing troublesome 866A's and becoming expensive 3B28's in vintage amps as the voltage drops are almost identical.


----- Original Message ----- From: "Glen Zook via Amps" <>
To: "Jim Thomson" <>; <>
Sent: Monday, October 13, 2014 10:29 AM
Subject: Re: [Amps] HV Diodes

Who is paying 9.99-cents for a 1N4007? I pay 2-cents for them in 100 quantity. I keep a drawer full to use anywhere I need a silicon power diode.

I was paying like 10-cents each for 1N5408. However, they recently went to 12-cents each. Again, in quantity of 100. I have another drawer full of those diodes.

Being retire, and on a "fixed budget", I don't like to squander money. However, $2.00 for a quantity of 1N4007 and $12.00 for a quantity of 1N5408 is not going to "kill me"! Being able to open a parts drawer and have something to do the job at hand, is certainly worth it!

As to which diode to use: It depends! I don't like to have to repair equipment so I do tend to "go overboard" sometimes with the parts used. But, not that far from the boat! If, for some reason, I don't have the desired part right then, I may go ahead and used a "heavier" part just to get things going. However, that is for convenience and not for any financial reason.

Glen, K9STH


On Sunday, October 12, 2014 8:08 PM, Jim Thomson <> wrote:

Date: Sat, 11 Oct 2014 16:35:39 +0000
From: Manfred Mornhinweg <>
Subject: Re: [Amps] HV Diodes

Dear all,

I wonder why the diode selection is so totally overblown here.

Let's assume a pretty big amplifer, solid legal limit, CCS, which is more than any ham needs. The power supply might deliver 3500V at 0.8A. Each diode string in a bridge rectifier will then see a peak voltage that might reach 4000V in the event of line overvoltage, and an average current of 0.4A at full output. The
current will be very peaky, but rectifier diodes are rated to take that.

## Commercial broadcast HV supplies will typ use TRIPLE the piv rating for each leg of a FWB. The theory here is.... the MOVs across the 240 vac input,or 208vac, 3 phase input will not start to clamp until the ac line V has doubled.
For a 3500 vdc no load B+  supply, use 10 kv piv per leg.

The cheapy 1N4007 diode is rated for 1000V reverse voltage, and 1A continuous average forward current, when mounted in a normal way that will allow conducting about 1W of heat away, through its leads. Current-wise this diode has all the safety headroom you might need. Voltage-wise you need 4 of them in series, and that's it. For a bridge rectifier you need 16 of those diodes, which cost $0.099 each at Digikey, if you buy just those 16. For $1.58 you get all the diodes you need! Or be generous, use 5 in series in each leg, and spend two bucks on them.

### A 1N5408  runs pretty damn warm to hot  with 1A  CCS  flowing, when
I tested em for bias use.... using a variable dc power supply + resistor in series with the string of 1N5408s. A 6A10 runs warm to hot with just 2A CCS flowing.
And that’s with full lead lengths on each end of each diode.
Try running 1A  CCS  through a 1N4007, and see hot hot it gets.

I hear some of you cry "and the voltage transients?" Well, what transients? The diode bridge sits right across a big capacitor! Any voltage transients will be
clamped to the capacitor voltage.

Of course some of you will now cry "and the current transients?" Sure, any
voltage transients on the power line will translate into current transients when the filter capacitor clamps the voltage. In addition there will be a big inrush current at switch-on, if no step start or slow start circuit is used. So there
will indeed be some current transients. But how large can they be? Small
transformers have so much resistance in their wire, that just the resistance
would limit the short circuit current to about ten times the nominal load
current. Larger transformers like those used in legal limit amps have relatively smaller winding resistances, so that the resistance might limit the current to 20 times the nominal value. But then there is leakage inductance, which also reduces the current transients. So I would not expect such a transformer to
produce an inrush current stronger than 10 times the nominal current, even
assuming a zero impedance power line, which none of us has! So, the inrush
current with no step start circuit might be around 11A, considering that a
transformer for 0.8A output from a capacitor-input filter needs to be rated for
about 1.1A.

And a cheapy 1N4007 has a 35A inrush current rating. More than three times what
we need.

### Nice try. My dahl xfmr has a 6 ohm dc resistance across its 5200 vac winding, and only 3 ohms across the 2600 vac tap. Pri resistance is just .002 ohm.
 My L4B xfmr has a 10 ohm sec winding.
The big B+ caps across the B+ will only clamp the excess V so much. See carls comments. That 35A inrush rating is for a half cycle of 60 hz.... or 8.3 msecs to be exact.

And how much transient current could we see during lightning? This is harder to tell, because such fast, extremely strong hits are hugely attenuated by line impedance, and clamped by all sorts of electronic devices in the homes, so that the current spike resulting in your amp's rectifier diodes depends a lot on those impedance values. But if such a spike exceeds 35A, it would have to exceed roughly 350A on the 240V power line feeding the amp, which means that in a typical home it would need to far exceed a kiloampere at the service input. The voltage at that point would need to be VERY high, causing flashovers everywhere and thus limiting the transient current into the home. I would expect the final transient in the amp's rectifier diodes to remain well below the 35A rating of 1N4007 diodes. You might see your house catch fire from the flash-overs and the
molten wire, before those diodes blow up.

Of course everyone is free to use 1N5408 or 6A10 diodes, at $0.278 and $0.368 respectively at Digikey. And if somebody wants to use 10 in series in each leg,
who am I to forbid that? But technically it's pointless. Strings of 4 to 5
1N4007 diodes in each leg, properly mounted for decent heat sinking, are
perfectly safe and sound.

### I use 6A10s for the HV supply, also use 6A10s for the bias..and also use em for safety diodes, between B- and chassis. I have seen 6A10s short out when used as safety diodes, so now use 3-4 x 6A10s in parallel, to increase the surge rating. Since I use strings of diodes for the FWB / FWD..and also 10-60 of em in series for bias.... tapped of course to vary the bias...and also for safety diodes.... its dirt cheap to just buy 200-250 of the damned things.. 6A10s..and be done with it. I also put a rvs connected 6A10 across each HV lytic.

In my National NCL2000 I use 1N5408 diodes, but that amplifier has a voltage
doubler, so the diodes work at twice the current, compared to a bridge
rectifier. While 1N4007 diodes would still have been operating within ratings, the current headroom would have been rather tight. So I chose the bigger diodes.


## If you cant afford to buy 1N5408s or 6A10s.... you shouldn’t be in this hobby. Even if you are in the poor house, you can still afford something better than the
$.0990  1N4007 pos diode.

## I spend way more for coffee + 94 octane every day. How cheap can you get ? Blow up some 1N4007s and you will be cursing that you didn’t use a real diode. Having to pull an amp all apart, unsolder some stupid 1N4007s, then replace em with 1N5408s is a pita. And don’t even think of doing something silly.... like replacing with more 1N4007s. Even if your time is free, its still a pita.

Jim   VE7RF
Amps mailing list
Amps mailing list

No virus found in this message.
Checked by AVG -
Version: 2014.0.4765 / Virus Database: 4040/8381 - Release Date: 10/13/14

Amps mailing list
<Prev in Thread] Current Thread [Next in Thread>