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Old 9th November 2007, 05:01 AM   #1
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Default Three Simple Questions!!

Hi Folks,

Regarding the Dynaco MK III (diagram attached) which I am making from scratch, three simple questions if I may -

1. Can the selenium rectifier just be replaced with a silicon diode?

2. With respect to the 50 uF caps, is there any specific voltage rating or can I just use any old 50V electrolytic (the original ones in the photo's look huge!)

3. With respect to the power supply fuse, I am using a 240V primary. Should the fuse still be rated at 3 amps? The reason that I ask this is that if the voltage through the primary side of the transformer doubles then should the current not double also? This doesn't sound correct to me as the power output of the amp is still the same regardless of the input voltage of the transformer.

Appreciate any thoughts.

Thanks in advance,

Rob
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Old 9th November 2007, 05:31 AM   #2
Colt45 is offline Colt45  Serbia
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I don't see a schematic..?

1. can't see the schema, but selenium can be replaced with a Si diode. the selenium has more voltage drop, so I need to see a schematic, to see if some added resistance is required..

2. no schema..

3. With 240v primary, you should draw half the current.. so 1,5A fuse, I guess.
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Old 9th November 2007, 05:59 AM   #3
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Quote:
Originally posted by Colt45
I don't see a schematic..?
Click the image to open in full size.
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Old 9th November 2007, 07:27 AM   #4
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Sorry about the schematic, had to take the kids to tennis and fired off the email in a hurry. The schematic is the same as kindly posted above by planet 10.

I don't understand why half the current.

Cheers,

Rob
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Old 9th November 2007, 08:11 AM   #5
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Quote:
Originally posted by Colt45
2. With respect to the 50 uF caps, is there any specific voltage rating or can I just use any old 50V electrolytic (the original ones in the photo's look huge!)
They're big because the voltage rating is probably greater than 50 Vdc. Since the voltage rating of the capacitor isn't specified, you'll need to measure the AC output of the tap on the transformer. If your voltmeter reads RMS, be sure to multiply that value by 1.414 to determine the minimum rating for the capacitor.

Quote:
3. With respect to the power supply fuse, I am using a 240V primary. Should the fuse still be rated at 3 amps? The reason that I ask this is that if the voltage through the primary side of the transformer doubles then should the current not double also? This doesn't sound correct to me as the power output of the amp is still the same regardless of the input voltage of the transformer.
The amplifier will draw a certain amount of power. Assuming the power supply transformer is correctly connected, this doesn't change. Therefore, if the input voltage doubles, the current drops by a factor of two.

Now, you can't just plug 240Vac into a transformer designed for 120 Vac. The output voltage will be way too high, and you will damage the amplifier. The transformer needs to be designed to accept 240 Vac @ 50 Hz. Or, you need to have a step down transformer in front of it.
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Old 9th November 2007, 08:20 AM   #6
chrish is offline chrish  Australia
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Hi Rob,

Ohm's Law

(Power = Watts)

Watts = Volts x Amps

If power stays the same, then a doubling of volts = halving of amps.


Cheers
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Old 9th November 2007, 09:07 AM   #7
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Hi Chris,

Good to hear from you. I am going to use that great earth bus wire that you gave me for this project. Thanks for explaining the reason for the lower current - I understand now, thanks. Will keep you posted on progress.

Hi Rob, I am using a transformer with a 240v primary winding so no problems there. With respect to the 50 uF caps (there are 2 of them), I initially assumed that they were low voltage because the high voltage power supply caps have a specified voltage in the schematic therefore I presumed initially that the unrated ones were just low voltage units but when I saw the size of them in the Dynaco manual I had second thoughts - perhaps they are just big because of old technology. I will measure the voltage if it comes to that but I would prefer to get a specification so I can order some in advance if I need them - someone out there must know what voltage these caps should be rated at javascript:smilie('')
confused.

Rob
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Old 9th November 2007, 10:17 AM   #8
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well, they're both on the bias tap. normal bias for KT88/UL/480v is what, -65v?, and it's near the top of the tap, so I wouldn't expect to see more than a hundred volts there.

160v should be plenty of room for error, you can always go higher if you want to be real cautious, or you could just measure and see what voltage is at the bias tap :P

A silicon diode will work fine there.
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