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PostPosted: Sep Wed 17, 2008 1:25 am 
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The 1N5399 should be 1000 PIV and the 1N5398 should be 800 PIV and the datasheet shows them as 1.5A. Ya never know about The Shack and how they might label things. They could be off-spec rejects so they put their own ratings on them. Shouldn't matter, either would work in this application. The 1N914/4148 is an entirely different animal for low voltage switching applications.


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PostPosted: Sep Wed 17, 2008 1:32 am 
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...doesn't RS also have 1N4007s (1000 PIV, 1A)? I'm pretty sure they do. You can use one of those without any difficulty. I did that with an S-120 a few years back; just got a two-lug ungrounded terminal strip and put that where the selenium was mounted, then put a 1N4007 across the two lugs and wired it up...B+ jumped from maybe 90V soaking wet to around 130V...
...1N4148s are not intended to be used as rectifiers, especially not B+ rectifiers (although I have seen them used as detectors)...there is no way you could use one of those for this purpose...


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PostPosted: Sep Wed 17, 2008 10:04 pm 
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Thanx guys!FM refugee, I don't understand your B+ voltage measurements-what do you mean by soaking wet?Did you use just the 1N4007?Please explain.FWIW, RS advertises a 25 diode pack that MAY include 1N4001-1N4007-what's the lowest I could safely use?Still, RS has 3 of the 1N5399's for $2.00 or so, so that's what I will probably use. Per Mr. Shirley's instructions, I will then replace the old selenium rectifier with a 5 position terminal board, solder in a 1N5399 with the anode pointing towards the filament string and the cathode connected to power in. I will then solder the 100 ohm 10 W wirewound resistor to the 1N5399 anode and then solder the other end of the 100 ohm 10 W resistor to the existing 33 ohm fusible resistor. I will then measure B+ from the far end of the 33 ohm resistor to the - of one of the filter caps, correct?:)-Gearhead


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PostPosted: Sep Wed 17, 2008 10:33 pm 
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Try Radio Shaft # 276-1114 which is a type PTC205 diode rated 2.5 amp @ 1000 PIV. Comes in a 3 pack.


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PostPosted: Sep Wed 17, 2008 10:48 pm 
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Do take a moment to download the schematic if you haven't already done so. I think we might be getting confused with 'terminology'. We seem to be explaining it in entirely different ways.

http://bama.edebris.com/manuals/hallicra/s120/

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Per Mr. Shirley's instructions, I will then replace the old selenium rectifier with a 5 position terminal board, solder in a 1N5399 with the anode pointing towards the filament string and the cathode connected to power in. I will then solder the 100 ohm 10 W wirewound resistor to the 1N5399 anode and then solder the other end of the 100 ohm 10 W resistor to the existing 33 ohm fusible resistor. I will then measure B+ from the far end of the 33 ohm resistor to the - of one of the filter caps, correct?Smile


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PostPosted: Sep Wed 17, 2008 11:58 pm 
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Thanx again guys-Appreciate the feedback, Mr. Layton, but we are both referring to the same rectifiers:).I understand that I want the 1N5399 diode installed , followed by the 100 ohm 10 W replacement resistor followed by the stock inline 33 ohm resistor.I will either remove or tap off of the power lead coming INTO the Selenium rectifier. I do have the schematic at home (where I'm not), but will totally understand this once I remove the circuit from the chassis:)-Gearhead


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PostPosted: Sep Thu 18, 2008 2:58 am 
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RE: Replacing selenium rectifiers. I know that we have two schools of thought about this and I respect both. I will tell you that HH Scott amplifiers used a Siemens selenium bridge that was used for the DC bias on the output tubes and to put DC voltage on the first pre-amp filaments. They usually are weak or fail open. That will take out the output tubes, expensive GZ34 and/or output transformers quickly if it stops working.

For my equipment, that selenium will always be replaced before the first power is applied to my amps. Just kinda the way it is...

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PostPosted: Sep Thu 18, 2008 3:45 pm 
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gearhead222 wrote:
Thanx guys! FM refugee, I don't understand your B+ voltage measurements-what do you mean by 'soaking wet'?-Gearhead

...'soaking wet' here was just a figure of speech; it usually means something like 'just barely'. You didn't really think I meant literally 'soaking wet', as in, I had hosed the radio down before I measured it, did you? :shock: :lol:
...this was before I joined this forum and learned about the common and recommended practice of putting a dropping/limiting resistor in when replacing a selenium with a silicon rectifier. So yes, I was just running the 1N4007 'naked'. Had I known then what I know now, I would have added the resistor...


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PostPosted: Sep Thu 18, 2008 11:51 pm 
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Dear Don and FM Refugee-Thanx again guys!Dear FM, I was an Interior Communications Electrician in the USN and have done Industrial Electrical Construction and commercial HVAC-NEVER heard the term "soaking wet" as applied to electrical/electronics work-is that a Yankee expression?;)-Gearhead


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PostPosted: Sep Fri 19, 2008 2:04 am 
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Can I give you and example of what happens with Scott amplifiers when the selenium bias bridge goes open please?

OK, let me tell you about a stupid thing that I did due to not paying attention...

I was restoring an LK-72 Scott amp. I had replaced the Siemens selenium bridge, removed the first resistor after it, an 18Ω 1-watt resistor. Nine out of ten times, that resistor should be changed to a 33Ω resistor, 5-watt. I had not installed the resistor yet. I intended to tack it in, measure voltages and make it permanent.

I applied power to the amplifier and when the filaments got warm and the output tubes started to conduct (with no bias at all), those strong 7591 tubes overloaded the Mullard GZ34 and immediately, shorted the cathode to one of the plates. $50 down the tank for that tube.

So, as you can see, when those seleniums go open (and they will) it can cause some real grief.

Just a little story, your results will be different.....

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PostPosted: Sep Fri 19, 2008 2:22 am 
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1N4007 Diodes are about 4 bucks per 100 of them from Mouser.com. Stock up on them as they are quite useful diodes. If the guy that replaced his selenium ... the voltage was at 90 volts... jumped up to 130 after diode change.... then your selenium was an OWO ...
that On the Way Out! ........ about to croak!

On a Halli S-120 I would NOT worry about the B+ being a little higher with the diode than a brand new selenium would have been .. it doesn't need the added resistor in other words.... not if you playing around with a Zenith TO or some other critter where filaments are picked up from some strange place like the cathode of another tube... then exact correct B+ is definitely recommended...

John k9uwa


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PostPosted: Sep Fri 19, 2008 2:52 am 
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Why change oil in a car if it still runs?

When did they put oil in sel. rectifiers :?:


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PostPosted: Sep Fri 19, 2008 2:52 am 
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gearhead222 wrote:
Dear FM, I was an Interior Communications Electrician in the USN and have done Industrial Electrical Construction and commercial HVAC-NEVER heard the term "soaking wet" as applied to electrical/electronics work-is that a Yankee expression?;)-Gearhead

...I wouldn't say it's a 'Yankee' expression; I don't know where it actually comes from. And that may actually have been a misuse of it. It's actually used more often to describe, for example, someone who is very thin, as in 'she can't weigh more than 90 pounds, soaking wet'...


Last edited by FM Refugee on Sep Fri 19, 2008 3:08 am, edited 1 time in total.

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PostPosted: Sep Fri 19, 2008 3:06 am 
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k9uwa wrote:
1N4007 Diodes are about 4 bucks per 100 of them from Mouser.com. Stock up on them as they are quite useful diodes. If the guy that replaced his selenium ... the voltage was at 90 volts... jumped up to 130 after diode change.... then your selenium was an OWO ...
that On the Way Out! ........ about to croak!

John k9uwa

...that's exactly why I replaced it. That B+ reading (and I might have been generous calling it 90 volts!) seemed awfully low from what I remembered from working on 'AA5 version 2' radios; typical B+ on those was usually on the order of 115-125 volts or so...and the performance seemed a bit 'sluggish', even for a low-end set like the S-120. And this was after I had replaced the electrolytic and paper caps. Since I was seeing the low voltage right out of the rectifier, and having been taught that seleniums were 'notoriously inefficient' and 'failure prone' (that was the mindset at the time I was learning the trade; I know it's not necessarily or automatically true), I reasoned that the rectifier was indeed 'OWO' and went ahead and replaced it...


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PostPosted: Sep Fri 19, 2008 6:24 pm 
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Dear Don, FM and K9-Thanx again for the enlightening responses. I guess if my S-120 Selenium rectifier shorts, then there's a possibility of the filaments getting fried in the 4 tubes? Called ALL of the Radio Crap's in Corpus Christi and NONE had all of the components in one store.Thankfully, my hometown Radio Crap in Austin has all of them, so I will be improving the half-wave rectifier circuit next week:)Thanx again all!:)-Gearhead


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PostPosted: Sep Fri 19, 2008 9:06 pm 
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...no, if the selenium rectifier shorts, it will NOT fry the tube filaments. They don't run off the rectifier. The S-120 used the selenium rectifier and a separate voltage dropping resistor for the filament string to replace the 35W4 rectifier tube usually found in this tube lineup. The resistor and filament string are a separate circuit from the rectifier and B+ source and run directly off the AC line.


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PostPosted: Sep Fri 19, 2008 9:27 pm 
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Thanx FM, but let me rephrase that question. What COULD happen if the Selenium rectifier shorts or opens?Just curious.I guess that alot of what happens depends on the circuit design, eh?-Gearhead


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PostPosted: Sep Sat 20, 2008 12:27 am 
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Hi

You are lucky if it opens. Nothing will happen.

If a selenium rectifier shorts pull the plug and leave the room. You will never forget the smell of a burning selenium rectifier.

Electrically the input filter cap will most likely be damaged. A circuit should have a series resistor which burns open.

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PostPosted: Sep Sat 20, 2008 12:30 am 
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Thanx Norm!I will definitely be replacing the rectifier next week!:)-Gearhead


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PostPosted: Sep Sat 20, 2008 12:34 am 
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Seleniums usually don't just die suddenly unless there's an outside reason. What typically happens is that they develop leakage resistance over time and begin heating excessively. Thats what all those plates are - heat sinks.

The unit itself is comprised of a number of small voltage diodes, 5 or 7 is typical in small radios. One could go shorted by fate and then its up to the others to pick up the slack. Fate in this scenario typically means it failed from overheating because of the increased leakage resistance. Then it heats up more and you know the rest of the story.

They let off a very foul smelling gas when they overcook and then generally wind up going open at the end of the event.

Their typical failure condition is usually first noted by low B+. If the B+ is low then one needs to investigate why. Since the selenium rectifier has some amount of inherent resistance, along with the 22-33 ohm resistor associated with it, a leaky filter cap can easily contribute to excessive current draw which will be noted by a drop in voltage and maybe some heating. These things were typically rated only at 100-150ma when brand new so they don't have much wiggle room once degradation sets in. If they drop voltage excessively with new caps and a perfectly good operating radio otherwise then they are on their way towards failure. Thats almost a guarantee because if they are dropping voltage beyond spec then they are heating internally beyond spec.

There's plenty of good ones still out there on the road. I'm of the mind to generally replace them on sight if I'm truly refurbing a piece of gear but I have no problem leaving them in place if there are no signs of imminent demise. You often hear tales of "I replaced one in my working radio and the B+ shot up 30 volts". There's only about 6-7 volts difference between the selenium stack and a new diode so go figure. If it is dropping 20-30 volts it WILL crap out sooner than later.


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