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Old 3rd April 2010, 04:01 PM   #1
TechnoBeaver is offline TechnoBeaver  Canada
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Default Gainclone Questions

Hey guys new member here. I have done a fair amount of reading, but still have a few questions. I have moderate experience, Cmoys, small guitar amps and distortion circuits.

1. What amp rating should the rectifier be? I have read people using any wheres from 3 to 35 amps. I can make one from diodes, or use a 4 amp one from a computer power supply.Would a 4 amp be enough?

2. I have what I think might be a decent transformer. It's not a toroid, it's from a Technics SA-222. The receiver says 185w on the back, I don't know the V.A. I tested the secondaries- Blues:5.8v, Yellows:36v and Orange(s) to green:26.6v. There are 2 oranges and one green together, I assume this is my dual secondary?

I also have a real old Hammond, says 40 v.a. on it, I don`t think this is suitable.

I will be buying some new parts, however I would like to use what I have. Part of the fun is scrounging through my heaps to see what kind of gems I can find.

I know there are similar threads and I will continue to read them. I appreciate anyone who wants to jump on board and help me with this gainclone build. Thanks. AL
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Old 3rd April 2010, 04:52 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
check to see if each of the windings are completely separate from the others.
Measure the resistance of each winding and where some are connected measure the series resistance as well.
Remember to subtract the Ohmeter's own resistance from the measured values.
Check the thickness of each of the windings. Try to estimate/measure the diameter of each.
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Old 3rd April 2010, 05:08 PM   #3
TechnoBeaver is offline TechnoBeaver  Canada
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Ok so Here's what I get minus 0.5 from my ohmmeter.

A/C input, Red/Black 3 ohm

Blues 1 ohm

Yellows 6.1 ohm

Orange to Orange 1.1 ohm
Orange to Green 0.5 ohm

Is this what you ment? I can't physically see if they are seperate. There does not appear to be any shorts in the windings

The wires say 22awg on the primary and secondary side, the insulation being a little thicker on the primary side.
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Old 3rd April 2010, 06:15 PM   #4
AndrewT is offline AndrewT  Scotland
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the primary @ 3r0 sounds about right.
The 5v8 1r0 blue indicate a fairly low VA and medium current rating.
The 36v 6r1 yell indicate a medium VA and a low current rating.

The orange to orange and orange to green continuity tell us these are a common winding and that green is the centre tap.
The 26v6 and 0r5 tell us it is a highish VA and a highish current rating.

The 26v6 windings are probably 22Vac to 24Vac windings and should be ideal for powering a chipamp or two.
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Old 3rd April 2010, 08:01 PM   #5
TechnoBeaver is offline TechnoBeaver  Canada
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Thanks Andrew I appreciate the help. I need to get this part of the project right as it deals with line voltage.

I thought people were using 100-200V.A. transformers for these gainclones. Would I see any benefit to looking for a larger one?

I also measured the Hammond 270 Z60, the numbers are quite a bit different. I assume it's intended for a valve amp, I don't know if it could/should be used in this application.

Here it is just for fun.

Larger Blacks: 12.2 ohms
Smaller Blacks: 0.3 ohms------Damaged??
Red/Red 408.0 ohms
Red1/Brown 197.0 ohms
Red2/Brown 211.0 ohms

Again I don't know if it's suitable, I just thought I would throw it out there because it is 40 V.A. It is from a very old HiFi, one of the first actually.
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Old 3rd April 2010, 10:35 PM   #6
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by TechnoBeaver View Post
Thanks Andrew I appreciate the help. I need to get this part of the project right as it deals with line voltage....

I good, safe and easy way to test the transformer, after you get to his stage where you _think_ you understand it is to get a 12V AC "wall wort" power supply. Or just any 1low voltage transformer you have. 9V to 12V is safe to work with.

Then connect the primary of your transformer up to 12V and measure the output on the secondaries. If you slip up sparks will not fly.

This technique matter a LOT more when working with transformers used with tube because they step up the AC to as much as 600V and can kill you. Chip amp transformers are a lot safer but still, even safer if you do alll yu testing with 12V AC. Use it to test your switch and fuse and everything up to the rectifiers for shorts and proper voltage
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Old 3rd April 2010, 11:58 PM   #7
TechnoBeaver is offline TechnoBeaver  Canada
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Quote:
Originally Posted by ChrisA View Post
I good, safe and easy way to test the transformer, after you get to his stage where you _think_ you understand it is to get a 12V AC "wall wort" power supply. Or just any 1low voltage transformer you have. 9V to 12V is safe to work with.

Then connect the primary of your transformer up to 12V and measure the output on the secondaries. If you slip up sparks will not fly.

This technique matter a LOT more when working with transformers used with tube because they step up the AC to as much as 600V and can kill you. Chip amp transformers are a lot safer but still, even safer if you do alll yu testing with 12V AC. Use it to test your switch and fuse and everything up to the rectifiers for shorts and proper voltage
Thanks for the tip, makes sense. Would the secondary voltage be 1/10th what it should be because I am using 1/10th line voltage?

Also right now I have 2 rectifiers I am trying to decide from.

D3SBA20 200V 4A
U4KB80R 800V 4A-This one says Audio equipment on the datasheet.

I also have some sets of Diodes

S2V 17 No datasheet, came with Technics transformer.
1N5401 I hear this is fairly typical.

I assume the U4KB80R rectifier would work if the amperage is OK. My gut tells me it will work, but then I see where people are using 35Amp rectifiers. For the life of me I can't fiqure why when a household breaker is 15 or 20 amps.

What do you guys think?
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Old 4th April 2010, 10:27 AM   #8
pacificblue is offline pacificblue  Germany
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The household breaker is on the primaries. The rectifier is on the secondaries. The same power at lower voltage means higher current.

The rectifier should withstand the highest current draw. The Gainclone IC LM3875 has a nominal output of 4 A. The LM3886 has a nominal output of 8 A. Then you need to take derating into account according to the ambient temperature. On top of that you need to watch the peak current drawn by the smoothing capacitors when you switch the amplifier on. All those factors call for rectifiers with high current rating.

What works in your favour is that music is not a continuous signal at nominal output and that rectifiers withstands short peaks of many times their nominal rating, so a 4 A rectifier will work more often than not even with big capacitors. If you prefer to listen loudly, a bigger rectifier will certainly calm your conscience though. And there is always the possibility to help the rectifiers with a heatsink.

If you take a look at the datasheets of your two rectifiers you will see how they can only withstand their nominal current with infinite heatsink at 25 C. The U4KB80R provides a slightly higher safety margin in all aspects.

One reason why people use 35 A rectifiers is, they are relatively cheap. Where I usually buy, a 4 A rectifier bridge costs 0,40-0,50 , just as much as a 6 or 8 A bridge. 35 A bridges start around 0,90 . As soon as the 4..8 A types need a heatsink, the 35 A version is the cheaper choice.
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Old 4th April 2010, 12:04 PM   #9
TechnoBeaver is offline TechnoBeaver  Canada
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You know after I posted I realized what you said about the amperage going up when the voltage is stepped down. Thank you.

Hmm, more to think about. I do have some nice heatsinks for it, however I would like to have it bulletproof. I rarely crank it up, but when I do, I want to know it's not going to turn into a smoke and light show.

When you say the LM3886 has a nominal output of 8 A, are you referring to a stereo application, or just one? I will have to get myself these datasheets.

I have been trying to fiqure out if I can get a higher amp rating by using 4 diodes as opposed to a pre-made rectifier. Google is usally pretty good, just missing the target on this one.
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