Rectifiers: either rarer than hens teeth or I don't know what I'm looking for

oneinchside

Openly Baffled
probably the later. :nerd:

I'm trying to refurbish the amp section in an old Columbia c1457 console, I've found a source for the caps (and a isolation transformer as well) but it has a Selenium Rectifier (marked Britton 1305) and I can't figure out how to replace it.

The original idea was to just get a silicon rectifier and swap it out, but I have no idea what specs to look for or how to find them. (I'm new at this guys, I'm still just learning what a rectifier is)

As far as I can tell it's a half wave rectifier since it only has one wire coming in and one going out. (everybody that I have talked to has said "that's stupid, why would they put a half wave rectifier in there?!) I guess this is the simplest, low budget, scraping the bottom of the barrel power supply possible (or at least it's starting to seem that way) and that according to the schematic it is spec'd at 150 m.a., but 150 m.a. of what?

How do I figure out what the specs are on the rectifier and find a substitute?

max surge current?
reverse current ir?
forward voltage drop?

what about:
power dissipation?
peak reverse voltage?

Of course I'm writing this while I'm looking at Mouser's page of Bridge Rectifiers... Am I supposed to be looking for a bridge rectifier?
http://www.mouser.com/Semiconductors/Diodes-Rectifiers/Bridge-Rectifiers/_/N-2xpqk?Keyword=rectifier&FS=True


Should I just build a better/safer/more appropriate power supply?
If so, where do I find instructions/schematics to do it?

y'know, I wanted to tackle this and another small amp first before diving into my well loved Fishers and guitar amps... I guess this is the learning that I wanted... :eek:

The more I learn the more I realize I don't know a d4mn thing. This is very humbling. (probably good for me)

here's some photos and schems if that'll help:
columbia-c145guts800x600.jpg

columbia-c1457schem800x502.jpg

columbia-c1457layout800x502.jpg


Thanks
Mike
 

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1N4007 would be fine, I would change out the 22 ohm to a 100 ohm while your at it and go with a 5w sand resistor.

Get rid of those wax caps. I could not see a hot chassis, but it may have a cap from the line to the chassis somewhere.

Becareful of the chassis, it was prod designed to isolate every metal part from the user.

oh I reread your post good idea on the isolation tranny :)
 
Someting to keep in mind 1N4007's will replace 99% of the rectifiers that are found in tube amps ,in some cases a droping resistor will be needed to keep the B+ the same . One 1N4007 will replace a silicon diode or selenium rectifier, two will replace a tube rectifier such as a 5U4 or 6X4 and four will replace a bridge rectifier .A zener diode is one exception

If your are going to use that amp with a guitar or mic an isolation transformer is a life saving idea .
 
Someting to keep in mind 1N4007's will replace 99% of the rectifiers that are found in tube amps ,in some cases a droping resistor will be needed to keep the B+ the same . One 1N4007 will replace a silicon diode or selenium rectifier, two will replace a tube rectifier such as a 5U4 or 6X4 and four will replace a bridge rectifier .A zener diode is one exception

If your are going to use that amp with a guitar or mic an isolation transformer is a life saving idea .


DOH!!!!

The same guy that sent me the I.T. taped a 1N4007 to the side of it! It was so small I didn't know what it was. :eek:



Why is the zener diode the exception?
 
DOH!!!!

The same guy that sent me the I.T. taped a 1N4007 to the side of it! It was so small I didn't know what it was. :eek:



Why is the zener diode the exception?

(1) A rectifier is a Diode, plain and simple. "Rectification" descibes what it does, such as chopping off half the AC waveform to extract DC or detect an AM radio signal.

(2) A Zener is a voltage-control device, clamping to a designed voltage when reverse-biased. There are many different types, thus a simple diode that has no reverse conduction at all can not be used for such a specific purpose (other than .6v forward-conduction clamping or dropping.)
 
1N4007 would be fine, I would change out the 22 ohm to a 100 ohm while your at it and go with a 5w sand resistor.

Get rid of those wax caps. I could not see a hot chassis, but it may have a cap from the line to the chassis somewhere.

Becareful of the chassis, it was prod designed to isolate every metal part from the user.

oh I reread your post good idea on the isolation tranny :)

I'm not completely clear on what a hot chassis is, other than potentially deadly and therefor not very good. (but it might make a great mother in law gift)

I am putting a 3 wire cord on it with the 3rd prong grounded to the chassis (I also just found out that the outlets in my apartment have that 3 prong in the sockets hooked to... nothing it seems. My tester says that the ground is open. I think that makes the I.T. either more important or a moot point, I'm not sure which)

And in looking at the little amp again I did notice that one of the caps is hooked to a ground which is hooked to the chassis... does that make this a hot chassis?

The cap is circled below in the photo and schem:
columbia-c1457ampdeathcap.jpg

columbia-c-1457-schemdeathcap.jpg



Thanks again
Mike
 

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I'm not completely clear on what a hot chassis is, other than potentially deadly and therefor not very good. (but it might make a great mother in law gift)

I am putting a 3 wire cord on it with the 3rd prong grounded to the chassis (I also just found out that the outlets in my apartment have that 3 prong in the sockets hooked to... nothing it seems. My tester says that the ground is open. I think that makes the I.T. either more important or a moot point, I'm not sure which)



Do NOT connect this radio to the wall with a three-prong system if your wall grounds are open! The result is that you'll be putting AC through that .05 capacitor plus its DC leakage onto the case of anything else that has a ground pin on that same U-ground circuit, - maybe through piping to the screw on wall switches etc. It can be a dangerous mess. FIX that issue immediately. Open a wall socket and see WHY the u-ground is open.. It may go to piping which is supposed to have an Earth stake, which has been disconnected or even removed during renovation.

IF the U-grounds were properly connected and therefore this amplifier safe to plug in, then when adding a three-prong plug to this unit make sure the Neutral (white wire, silver screw, wider blade) is at that common point the other side of the .05uf capacitor, NOT the bottom of the filaments, that's your high side (black wire, copper/brass screw, narrower blade)

This is an incredibly dangerous unit, it should be rewired for a proper transformer supply. PLUS you have a DEADLY timebomb waiting in your house wiring.

DEAL WITH YOUR DANGEROUS HOUSE WIRING FIRST, THEN TACKLE THIS CRAPPY AMPLIFIER!
 
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1N4007 would be fine, I would change out the 22 ohm to a 100 ohm while your at it and go with a 5w sand resistor.

Get rid of those wax caps. I could not see a hot chassis, but it may have a cap from the line to the chassis somewhere.

Becareful of the chassis, it was prod designed to isolate every metal part from the user.

oh I reread your post good idea on the isolation tranny :)

Whenever there is no power trafo the danger of a hot chassis ALWAYS exists - and it's a serious danger. What if the outlet you plug the amp into is wired wrong? That's a very common occurrence, and can be lethal. Did you notice there is no fuse either? If your person becomes a path to ground the only current limit would be the breaker in the fuse box.

There is also a .05 "death cap" to chassis ground.

This amp has one of the most dangerous power supply setups I've ever seen in a commercial product!

I urge you to make a new transformer isolated supply for it.

Other than the heater voltage/current, the 6CU5 or 12CU5 are both identical to the 50C5, and would plug right in. The 6CU5 needs 1.2 amps of heater current, the 12CU5 neds 1/2 that. So a 6 volt filament winding of 6.3 volts at 2 amps or a 12 volt winding of 1 amp would do fine. The B+ is likely about 130-150 volts - you could use a small isolation transformer. A Triad N-51X would be ideal and it's only $12.24 from Allied. A trafo for heater power would be no more $$, probably less.

Then use a full wave bridge to rectify the B+, hook up a safe 3 wire line cord and fuse the primary. You'll need to do the cap replacements others spoke of too, they're old and tired - including the power supply triple cap.

There are other issues in the way (what's with that output trafo setup??) but the changes I mentioned are required for safe use. If you aren't prepared to make those changes I urge you to abandon the project for your own safety! :yes::yes::yes::yes::yes:
 
I think you need to replace that .05 Cap with a safety capacitor. This link will tell you something about what they are, what they do, and why you need a specific type of capacitor rather then an ordinary run of the mill cap:

http://www.justradios.com/safetytips.html

If I'm not correct here will someone chime in and let me and the original poster know.

Thanks

Shelly_D
 
This amp has one of the most dangerous power supply setups I've ever seen in a commercial product!

How is this any more dangerous than the myriad of other transformerless radios/phonos of that time period. I don't see the normal 220k resistor from B- to chassis but other than that it looks like all the others including the switch in the B- leg. Just trying to learn to see the difference.

I've always thought that a fuse was there to protect an expensive transformer not to protect someone from grounding the circuit through their body.
 
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Do NOT connect this radio to the wall with a three-prong system if your wall grounds are open! The result is that you'll be putting AC through that .05 capacitor plus its DC leakage onto the case of anything else that has a ground pin on that same U-ground circuit, - maybe through piping to the screw on wall switches etc. It can be a dangerous mess. FIX that issue immediately. Open a wall socket and see WHY the u-ground is open.. It may go to piping which is supposed to have an Earth stake, which has been disconnected or even removed during renovation.

IF the U-grounds were properly connected and therefore this amplifier safe to plug in, then when adding a three-prong plug to this unit make sure the Neutral (white wire, silver screw, wider blade) is at that common point the other side of the .05uf capacitor, NOT the bottom of the filaments, that's your high side (black wire, copper/brass screw, narrower blade)

This is an incredibly dangerous unit, it should be rewired for a proper transformer supply. PLUS you have a DEADLY timebomb waiting in your house wiring.

DEAL WITH YOUR DANGEROUS HOUSE WIRING FIRST, THEN TACKLE THIS CRAPPY AMPLIFIER!

They just did a renovation on another unit in the building, I hope that the house is a simple fix for the electrician that I called today (told the landlord 3 weeks ago, no response. grrrr.)

I agree, the house wiring has to be done before I can work on anything else, it didn't occur to me that whatever went out through that 3rd prong would go through the rest of a house looking for a ground... of course this applies to all the other items in the house that are on that circuit... a failure on any of them would put voltage through that path (including switchplate screws, etc...) right? Ack.

In the meantime maybe I should learn how to build a proper power supply...
 
They just did a renovation on another unit in the building, whatever went out through that 3rd prong would go through the rest of a house looking for a ground... of course this applies to all the other items in the house that are on that circuit... a failure on any of them would put voltage through that path (including switchplate screws, etc...) right? Ack.

Yes, I know of a Lady who was killed precisely because of this after the NEIGHBOUR'S driveway was redone and the ground stake disconnected. Want the details?

I'm only asking permission because otherwise it would be a threadjack.
 
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Whenever there is no power trafo the danger of a hot chassis ALWAYS exists - and it's a serious danger. What if the outlet you plug the amp into is wired wrong? That's a very common occurrence, and can be lethal. Did you notice there is no fuse either? If your person becomes a path to ground the only current limit would be the breaker in the fuse box.

There is also a .05 "death cap" to chassis ground.

This amp has one of the most dangerous power supply setups I've ever seen in a commercial product!

I urge you to make a new transformer isolated supply for it.

Other than the heater voltage/current, the 6CU5 or 12CU5 are both identical to the 50C5, and would plug right in. The 6CU5 needs 1.2 amps of heater current, the 12CU5 neds 1/2 that. So a 6 volt filament winding of 6.3 volts at 2 amps or a 12 volt winding of 1 amp would do fine. The B+ is likely about 130-150 volts - you could use a small isolation transformer. A Triad N-51X would be ideal and it's only $12.24 from Allied. A trafo for heater power would be no more $$, probably less.

Then use a full wave bridge to rectify the B+, hook up a safe 3 wire line cord and fuse the primary. You'll need to do the cap replacements others spoke of too, they're old and tired - including the power supply triple cap.

There are other issues in the way (what's with that output trafo setup??) but the changes I mentioned are required for safe use. If you aren't prepared to make those changes I urge you to abandon the project for your own safety! :yes::yes::yes::yes::yes:

The reason I took the project up is because I knew it had a nightmare power supply and I wanted to learn how (and why) to correct it. I have gotten an isolated power supply to use with it (120v-120v) but learning to build one is a good idea. I'm not an EE, just a music listener and guitar player, so I am having to take this in baby steps and learn as I go. The main goal is to not kill my self or other family members in the process.

I had an ARC Guitar amp that had a grid leak chassis, it was small and the kids always wanted to use it. I moved it to another room, must have plugged it in backwards and got zapped. It was promptly removed from the house and sold to someone who know what they were doing with it. All I could think of is one of my kids getting zapped. I've been terrified of hot chassis' ever since, so I want to learn how to really fix the problem. In the process I'm learning a lot about amps, design, components, everything. The learning process is great.

It sounds like the next step is learning how to build a proper, safe, power supply. Any tutorials or links out there?

If I changed the two 50C5's to 12CU5's they would have the same voltage requirement as the 12AX7, (12v) correct? And then the grid voltage would be (probably) 130-150v... and what I would need to get would be the two 12CU5's, 3 prong plug, a fuse, and a new transformer with a 150v tap and a 12v tap, correct? Or should I be thinking of 2 separate transformers?

It also sounds like it might be a good idea to move all this to another chassis so I can see what I'm doing.

The output trafo is odd... it was setup for one mono 10" in the console, plus 2 twin 4" satellite for stereo. I would like to set it up with two 10" inside the console. (considering I haven't been willing to plug it in until the rectifier and caps were replaced redoing the OT's for the speakers I wanted seemed a bit far down the road)

The death cap... I figure it should be replaced with a safety cap (which Shelley_D also mentioned) but I have heard that they should be removed entirely... somehow that doesn't seem like a good idea.
 
Yes, I know of a Lady who was killed precisely because of this after the NEIGHBOUR'S driveway was redone and the ground stake disconnected. Want the details?

I'm only asking permission because otherwise it would be a threadjack.

Ack. Not really a thread jack IMO, the idea here is to learn how to stay alive.

"a word to the wise is sufficient"
in my case it often takes many words, italicized, in bold 24 point type...
 
Are you rebuilding this to put back in the console? If so have at it but ONLY if you run it thru an isolation transformer while you working on it. If your trying to use it for something else I would lobby that it is not worth the time and effort to rebuilt it so safely use. That is one funky output arrangement. I take it the console has one large speakers with two small remotes?

Steve

EDIT, Looks like we posted simultaneously. I'd say toss this amp and get something more suitible for Guitar amp modification. It should be too hard to find something that already has a power transformer.
 
How is this any more dangerous than the myriad of other transformerless radios/phonos of that time period. I don't see the normal 220k resistor from B- to chassis but other than that it looks like all the others including the switch in the B- leg. Just trying to learn to see the difference.

I've always thought that a fuse was there to protect an expensive transformer not to protect someone from grounding the circuit through their body.

1. No fuse of any kind - regardless of why it would be put there, there is nothing to interrupt current flow whatsoever in this design.

2. A metal chassis - on his work bench he has to regard the metal chassis as hot unless he uses an isolation trafo.

3. Death cap - if it shorts the chassis will be 117 volts above earth ground. Touch an earth ground and the amp chassis and you are in big trouble. BTW, tell me that cap doesn't look swollen at both ends.

4. A single pole on-off switch! I strongly recommend that power switches break both hot and neutral (on gear with power trafos too!). Otherwise if you shut the unit off the circuitry is still connected to the wall AC on the hot lead. Touch the chassis and a neutral or earth ground and you get shocked.

I've seen lots of trafo-less units, but I've never seen one before that has so many scary features. It might have been somewhat safer when in it's original enclosure - but on the work bench it's a killer - literally.
 
If you use an isolation transformer, you can use a bridge rectifier to get better filtering. Now that it is no longer tied to a side of the line, the bridge rectifier no longer has the downside of making the chassis look like it's always riding an 85VAC source. Note that many switching power supplies, and solid state TV sets do this. And I'd reconnect the heaters so the 12AX7 is in the middle of the string, as the 12AX7 heater will look to be at a bias of half the B+ voltage DC (which also reduces hum leakage). In the diagram, the isolation transformer is inserted between the powerline and the tube heaters.
bridge.gif

This diagram was drawn with a hot chassis radio in mind, with tube heater #8 being a 50C5 and the others all 12V tubes. But in the amp case, heaters 1 and 2 would be the 12AX7.
 
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Ok, so nothing more scary than virtually every AA5 radio schematic I've seen. I agree in its current configuration that touching anything metal on this amp while it's plugged in is a form of Russian roulette. A polarized cord with the hot lead being switched is always a good idea as is an isolation transformer especially when working on it outside the enclosure.

If he's going to put it back in the console and no metal is exposed anywhere then I can't see how completely rebuilding a power supply is going to help him vs. just using an isolation transformer. If it is truly insulated from the user then there is no shock hazard even if he doesn't use an IT.
 
Are you rebuilding this to put back in the console? If so have at it but ONLY if you run it thru an isolation transformer while you working on it. If your trying to use it for something else I would lobby that it is not worth the time and effort to rebuilt it so safely use. That is one funky output arrangement. I take it the console has one large speakers with two small remotes?

Steve

EDIT, Looks like we posted simultaneously. I'd say toss this amp and get something more suitible for Guitar amp modification. It should be too hard to find something that already has a power transformer.

Yah, keep it as a console, if at all, and find something MUCH safer and better quality for the guitar. I'm a Steve too.. (heheh)
 
The reason I took the project up is because I knew it had a nightmare power supply and I wanted to learn how (and why) to correct it. I have gotten an isolated power supply to use with it (120v-120v) but learning to build one is a good idea. I'm not an EE, just a music listener and guitar player, so I am having to take this in baby steps and learn as I go. The main goal is to not kill my self or other family members in the process.

Okay, I understand. But look at my last post before this one to be sure you understand the situation.

I had an ARC Guitar amp that had a grid leak chassis, it was small and the kids always wanted to use it. I moved it to another room, must have plugged it in backwards and got zapped. It was promptly removed from the house and sold to someone who know what they were doing with it. All I could think of is one of my kids getting zapped. I've been terrified of hot chassis' ever since, so I want to learn how to really fix the problem. In the process I'm learning a lot about amps, design, components, everything. The learning process is great.

Then what I posted before will be just what you want to do.

It sounds like the next step is learning how to build a proper, safe, power supply. Any tutorials or links out there?

A million, no doubt. But you aren't trying to build anything exotic, you just want to build a good quality safe supply. Try Hammond Transformers' site, IIRC they have some design tutorials. Google it and see what turns up...

If I changed the two 50C5's to 12CU5's they would have the same voltage requirement as the 12AX7, (12v) correct?

Correct.

And then the grid voltage would be (probably) 130-150v...

Not the grid voltage, the B+ voltage - meaning the 115 volt AC from the new power trafo rectified with a full wave bridge and capacitor filtered with a reasonable size capacitor.

and what I would need to get would be the two 12CU5's, 3 prong plug, a fuse, and a new transformer with a 150v tap and a 12v tap, correct? Or should I be thinking of 2 separate transformers?

You may find a single trafo that can do the job, but it'll probably be more costly than two separate. Plus you may want to reuse the iron in other projects and separates are more versatile.
You would need to get:

1. the required transformer(s)

For the B+...

2. (4) rectifier diodes or a diode bridge rated at 400 volts minimum (600 is better)
3. A 3 wire line cord, fuse holder and fuse
4. a double pole on-off switch rated at 3 amps or more - a couple .01 or so 250 VAC caps across each switch contact stops pops on turn on/off and extends switch contact life.
5. New filter caps (3) and dropping resistors - the caps can be relatively small, maybe 40-50 uf each at 250 volts; the size of the dropping resistors will depend on the trafo you use. The stock values probably are pretty close if you use the trafo I mentioned - but I'd double the wattage ratings for safe operation.

For the tube heaters just the transformer is all you need. The primary on it can hook to the same place as the B+ transformer.

For the tubes...

5. (2) 12CU5 tubes
6. A new cathode bypass cap (the stock one is 20 uf, you can use that or larger value at 25 volts or higher) and I'd replace the cathode resistor just to be safe.

I think that covers everything, I may have left something out but if I did I'm sure someone will catch it.

It also sounds like it might be a good idea to move all this to another chassis so I can see what I'm doing.

Yes, by all means!

The death cap... I figure it should be replaced with a safety cap (which Shelley_D also mentioned) but I have heard that they should be removed entirely... somehow that doesn't seem like a good idea.

I use a .01 1600 volt cap certified for that use. I highly recommend you use that cap in that location.

Let me know if you have questions, either here or email me.
 
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