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Old 30th April 2011, 01:14 PM   #1
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Default RH84 Power supply

I have only a small knowledge of electronics and initially I am trying to get to understand PSU Designer II. I need to come up with a different power supply for a RH84 SE amp build because my main transformer is a little low powered. ( RH 84 - Tube Audio ...... RH DESIGN )

Secondary 1. 325-0-325v at 100 mA
Secondary 2. 6.3v at 3A
Secondary 3. 5 or 6.3v at 2A

I hoped to use a 5Y3 rectifier but continually came up against the ‘forward current has been exceeded’ (0.44A) warning message. Substituting a 5V4G has improved things, see below. The target is 300v so the B+ is still a little high. I will add the 0.47uF by-pass capacitor and a bleed resistor but I expect to still have to add additional resistance to get the voltage down.
The figures for the transformers have been measured but the choke figures come from a Hammond data sheet.
Am I on the right track or should I add additional filters to improve things?
Any advice would be appreciated.

Graham
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Old 30th April 2011, 03:57 PM   #2
ryuji is offline ryuji  United States
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firstly, to properly model your transformer you need to either measure or look up the winding resistance and use the source impedance calculator to get an appropriate value plugged in for it. did you do that already?

secondly, how much current you looking to get out of it? is 100ma a placeholder for maximum transformer current? due to misc losses its going to be slightly higher then 100mA on the transformer windings if you have a 100mA load

Last edited by ryuji; 30th April 2011 at 03:59 PM.
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Old 30th April 2011, 05:43 PM   #3
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Hi ryuji,

Thanks for your response.
I measured the transformer and used the impedance calculator.
The load of 100 mA was obtained by adding the requirements for both EL84's and the ECC81 (about 2x48 + 3 mA).
My concern is that this is already at the max for the transformer?

Graham
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Old 30th April 2011, 06:13 PM   #4
ryuji is offline ryuji  United States
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Your likely running past the limit of your transformer. Set the simulation recording delay to 1 second run sim and then scroll to the right to find rms current requirment of transformer, I bet it will be significantly overloaded. If your buying a new transformer, try out in your sim a 400v transformer and no input cap, just direct connection to inductor, you should get really close to 300v out and the inductor input will reduce rectifier current surges
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Old 30th April 2011, 06:37 PM   #5
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I'm also building this circuit and trying to figure out my power supply, though I'm not using simulation software. It has a 47uF cap before the choke and 220uF after that. I understand that the 220uF value is not critical and can be increased, but that increasing the 47uF to, say, 100uF will have a damaging effect on the choke or the power transformer? I don't quite understand why since both are 'smoothing' the waveform of the DC, right?
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Old 30th April 2011, 06:50 PM   #6
ryuji is offline ryuji  United States
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Higher capacitance values put more strain on the rectifier, if you are tube rectifying 100uF is probably a bad idea
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Old 30th April 2011, 06:53 PM   #7
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Hi ryuji,

I hope I have carried out your instructions properly.

Running the one second delay I get 117.3 mA at the transformer and 100.22 mA at the load.
What is also confusing me is that I have previously read on this forum that others have successfully used the same transformer for the RH84?

Hi Carbondated,

From my limited knowledge, I have established that the value of the first cap can significantly change the B+ value.
Lower cap, lower B+.
Higher cap, higher B+.
I am not conversant with this damaging the choke.

Graham
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Old 30th April 2011, 07:00 PM   #8
ryuji is offline ryuji  United States
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You get diminishing returns on higher cap values before choke. Only way to hurt a choke to my knowledge is to overheat it by pulling too much continuous current through it

Rms voltage is 1.4142 times the transformer secondary voltage, subtract the rectifier droop from that value and that gets you your ' ideal' voltage out

Larger input cap means less ripple and in its place more dc

it could be ok to slightly overload that transformer -- perhaps its conservatively rated
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Old 30th April 2011, 07:06 PM   #9
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Hi ryuji,

Thanks for your help.
Perhaps someone using a similar transformer will post on the forum.

Graham
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Old 30th April 2011, 07:26 PM   #10
ryuji is offline ryuji  United States
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I say if others used it give it a go, just make sure you fuse the primaries and if it kills the transformer eventually just replace it with a higher rated one worst that can happen is you eventually melt through the insulation and the windings short out
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