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Old 6th January 2013, 02:25 PM   #1
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Default Help with main transformer specs

Hi,

I'm looking at this schematic to power a parallel se 300b monoblock.
Please verify the specs for the mains tranformer I have come up with.

400v for parallel 300b and one 6sh7:
300b draws about 100mA, 6sh7 about 10mA
300b heaters draws max 1.5A.
5u4GB draws 3A
6sh7 heaters draw 0.3A

So with some margin that would be:

400v-0-400v @ 300mA for B+
2.5v-0-2.5v @ 4A for 5u4GB heaters
3.1v-0-3.1v @ 1A for 6sh7 heaters
2x 5v-0 @ 3A for 300b heaters

Does that seem right? Any under/over -rating?

Thanks in advance
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Last edited by CoolJazz; 6th January 2013 at 02:45 PM.
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Old 6th January 2013, 04:33 PM   #2
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400 - 0 - 400, with a cap input, is going to give you approx 560 volts before the losses of the 5U4. Spec sheet for the 5U4 says a loss of 50 volts/plate @ 275 ma /plate. Your draw is under 200 ma total, so ball park loss in the 5U4 maybe 50 volts? That would give you 560 - 50 volts = 510 to 500 volts. If I'm right, you'll still have a bunch of volts to drop.
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Old 7th January 2013, 07:39 PM   #3
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Seems I was a little premature with the schematic, ignore.

A more general question:
Do you spec a transformer purely on the current drawn from the tubes, or are there other things to consider?
So if I want a winding that can power 2 300b's, can I just say '300b draws max 100mA so i need a 2x100mA = 200mA rating on that winding'?

Is there a rule of thumb as for what margins to apply?
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Old 9th January 2013, 03:02 PM   #4
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Quote:
Originally Posted by prairieboy View Post
400 - 0 - 400, with a cap input, is going to give you approx 560 volts before the losses of the 5U4
From what I understand, when half wave rectification used, the voltage isn't raised to peak voltage. Do I understand that correctly?
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Old 9th January 2013, 03:18 PM   #5
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The 5U4 in the OP is full wave rectification.

Do you really want the 6SH7 heaters to be at HT ? Is there not a danger that the valve might operate in reverse with current flowing from the Cathode to the heater.

Last edited by KatieandDad; 9th January 2013 at 03:22 PM.
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Old 9th January 2013, 04:03 PM   #6
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I was looking at the schematic at this page
Audio Note Kits - Legend Mono Block

Seemed like a good startingpoint.
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Old 9th January 2013, 04:06 PM   #7
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I'll bow down to the valve experts but I wouldn't put the heaters at full HT.
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