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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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5th Aug 2017, 1:41 pm | #1 |
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Replacement rectifier.
Hi.
First posting, so my apologies for mistakes. I am re capping a Rock-Ola valve amp from my jukebox. I want to change the old metal rectifier, which is a WESTALITE type 18RD 2-2-8-1, which is chassis mounted. Can anyone please advice on a modern replacement, the only info on the schematic is B-250 C-250. Thanks Last edited by exlecy1; 5th Aug 2017 at 2:05 pm. |
5th Aug 2017, 1:47 pm | #2 |
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Re: replacement rectifier
I don't know this circuit, but in general you would replace it with a suitably rated silicon diode or bridge followed by a 100-200 ohm resistor. A 1N4007 will be adequate in most cases, or 4 for a bridge.
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5th Aug 2017, 2:00 pm | #3 |
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Re: Replacement rectifier.
Hi Paul. Thanks.
I am new to the electronics so I was informed to replace it with 600V 4A bridge rectifier. Same connections AC and +ve and -ve DC. You mention a resistor, if this is required where in the circuit would it go? |
5th Aug 2017, 2:06 pm | #4 |
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Re: Replacement rectifier.
A 1N4007 is good for 1A forward current and 1000 peak inverse volts. How much current does your juke box amplifier need?
The resistor goes in series with the positive supply.
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5th Aug 2017, 2:10 pm | #5 |
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Re: Replacement rectifier.
The resistor is to mimic the voltage drop of the original selenium rectifier and limit the inrush current. It isn't essential but fitting one is good practice. It would normally go between the rectifier +ve connection and the reservoir electrolytic.
Using a 4A rated part won't do any harm but is unlikely to be necessary if it's just supplying HT to the amp, even a big juke box amp. It's possible it's supplying something else though, such as a motor or solenoid - as I said, I don't know the circuit, and am not a juke box expert anyway. |
5th Aug 2017, 2:30 pm | #6 |
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Re: Replacement rectifier.
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5th Aug 2017, 2:31 pm | #7 |
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Re: Replacement rectifier.
Thanks for help. Is it possible to attach pics so I could post a photo of the schematic?
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5th Aug 2017, 2:38 pm | #8 |
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Re: Replacement rectifier.
It is possible, but I don't think it's necessary. The thread linked to shows that the original rectifier is rated at around 150mA, so 1N4007's will be fine, or you could use a 1A bridge rectifier.
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5th Aug 2017, 2:42 pm | #9 |
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Re: Replacement rectifier.
To repeat, it will do absolutely no harm to use a 4A bridge, so go ahead and use one if it makes you feel more comfortable. A 4A part will cost a bit more and be physically larger, but they're not expensive anyway.
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5th Aug 2017, 3:58 pm | #10 |
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Re: Replacement rectifier.
Is this the one that supplies the motors with 25VDC or the HT? Sorry I now see it is the HT rect should be a simple repair. Mick.
Last edited by vinrads; 5th Aug 2017 at 4:05 pm. |
5th Aug 2017, 5:04 pm | #11 |
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Re: Replacement rectifier.
I have doubts about it being an HT rectifier. Only 250 volt rating is very low as most Jukebox valve amps will have 350 volts or more HT. Many models I've seen have lower voltage supplies for motors, solenoids etc. built into the amp chassis and take their AC power from the amp transformer.
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5th Aug 2017, 5:11 pm | #12 |
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Re: Replacement rectifier.
I don't think it matters what the rectifier is doing so long as the replacement has the same or better spec than the original.
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5th Aug 2017, 10:33 pm | #13 |
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Re: Replacement rectifier.
thanks for all the input ,i feel ok now about replacing the rectifier ,just don't fully understand the resistor part if the original rect did not have one
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6th Aug 2017, 8:06 am | #14 |
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Re: Replacement rectifier.
A replacement silicon rectifier will have a lower forward voltage drop than the original selenium one, considerably so, hence the resistor.
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6th Aug 2017, 8:16 am | #15 |
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Re: Replacement rectifier.
At one stage I restored a WWII German cipher machine at Bletchley Park. That used two selenium rectifiers, which were about 70 years old and slightly rusty. Because originality was important, I produced engineering drawings of the originals and had replica selenium rectifiers manufactured by http://www.gdrectifiers.co.uk/ . They even painted the units in the dull red of the originals. The only difference is that the originals used round plates, and the replicas have square plates (since that is consistent with GD's manufacturing process). And they deposit selenium on aluminium plates, so they don't suffer corrosion in the long term.
This is the performance of the old original and new selenium bridges: Old bridges: 1: 5 plates per diode, 35mm diameter, threshold voltage 2V, slope resistance 40 ohms at 0.3A 2: 9 plates per diode, 25mm diameter, threshold voltage 5V, slope resistance 100 ohms at 0.18A Interestingly reverse leakage was totally negligible. So the old rectifiers were probably still viable. New bridges 1: Threshold voltage 2.4V, slope resistance 4 ohms at 0.3A 2: Threshold voltage 4.6V, slope resistance 12.4 ohms at 0.24A The key thing is that the threshold voltages are about the same, but the slope resistance is an order of magnitude lower for the new bridges. Anyway, assuming you are not going to have new selenium bridges made, a silicon bridge can be made to look the same by putting something like a 220 ohm series resistor in the transformer feeds. Or if you are using individual diodes a 100 ohm resistor in series with each diode. I'm also implicitly assuming that an extra 5V or so on the HT rails is not a problem; although there would be a solution somewhere by using several silicon diodes in series in each bridge leg to get the voltage drop, and then a resistor too. Depends on how gilded you want the lily. Craig Last edited by Craig Sawyers; 6th Aug 2017 at 8:41 am. |
6th Aug 2017, 7:17 pm | #16 |
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Re: Replacement rectifier.
i have managed to check a same amp as mine that has been rebuilt, the rectifier used is KBU8J,, the ac side is feed from the seperate power supply,as mine is, the other ac leg goes to the slow blow inline fuse, the line has a 4ohm resistor in series, the dc + feeds a 120uf electrolitic cap an also a pin on 4 off EL84 valves, the - goes to chassis, there is no resistor on DC + side .thought this might be of interest to all that gave me help
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6th Aug 2017, 9:32 pm | #17 |
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Re: Replacement rectifier.
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6th Aug 2017, 10:05 pm | #18 |
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Re: Replacement rectifier.
To repeat yet again, it does no harm to fit a higher rated bridge. You can fit a 20A part if you want to. However, if the original part was only rated at 150mA then there's not much point in massively overrating any replacement.
The series resistor is unrelated to the current rating of the bridge. Again, to repeat, it is there to mimic the electrical characteristics of the original selenium bridge. The value isn't critical. |
7th Aug 2017, 9:25 am | #19 |
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Re: Replacement rectifier.
Exactly which model of jukebox is this from, or amp model if it's marked. We may then be able to see what it does. Pin 4 of an EL84 is a heater pin so if it's HT it won't connect there unless you are quoting the wrong pin number.
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7th Aug 2017, 9:34 am | #20 |
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Re: Replacement rectifier.
I don't think a pin number is quoted, just a pin on four of the valves, probably all of the screen grids of the output valves connected to HT.
Lawrence. |