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Questions on Audio Note Kit One
Questions on Audio Note Kit One
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Old 17th February 2019, 12:09 PM   #1
aslee is offline aslee  Hong Kong
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Default Questions on Audio Note Kit One

Hello,

I am planning to build an Audio Note Kit One amplifier. When I view the circuit, I have a puzzle.

The power transformer HT secondary is 425-0-425 at 250mA. The PS uses CLC filter. The first cap is around 24uf, second is 220uf. The resistance of the choke is 80R. How is the B+ of 415V come about? It seems to me that the B+ will be approaching 500V.

Please kindly educate me.

Thanks and best regards,
Francis
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Old 17th February 2019, 12:14 PM   #2
petertub is offline petertub  Sweden
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Questions on Audio Note Kit One
5u4 has a substantial ( 40V ?) voltage drop. even when not loaded. See this thread : A handy table of common Rectifier specifications
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Old 17th February 2019, 02:25 PM   #3
waltube is offline waltube  Italy
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The 5U4 has an high internal impedance; following the data sheet you can understand the final Vdc available for a specific dc current.
I know very well this project and it is working very fine.
In my opinion is one of the best SE for 300B available for different reason, first of all the robust drive stage.

Walter
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Old 17th February 2019, 09:55 PM   #4
PRR is offline PRR  United States
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Questions on Audio Note Kit One
> How is the B+ of 415V come about?

Power transformer has significant winding resistance, 100r-200r.

If winding resistance were less, peak rectifier current limit is exceeded.
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Old 17th February 2019, 10:40 PM   #5
6A3sUMMER is online now 6A3sUMMER  United States
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425V rms x 1.414 = 600V peak
0.25A x 200 Ohms = 50V drop, 1/2 secondary winding DCR
40V drop, rectifier
0.25A x 80 Ohms choke = 20V drop

600 - 50 - 40 - 20V = 490V.

I also do not see how the B+ is shown as only + 415V.
Where is the additional 75V drop?
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Old 17th February 2019, 11:27 PM   #6
piano3 is online now piano3  United Kingdom
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Presumably the transformer spec is under load conditions. Ripple voltage is 106V peak to peak, using the usual approximation.
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Old 18th February 2019, 02:32 AM   #7
45 is offline 45  Italy
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Quote:
Originally Posted by 6A3sUMMER View Post
425V rms x 1.414 = 600V peak
0.25A x 200 Ohms = 50V drop, 1/2 secondary winding DCR
40V drop, rectifier
0.25A x 80 Ohms choke = 20V drop

600 - 50 - 40 - 20V = 490V.

I also do not see how the B+ is shown as only + 415V.
Where is the additional 75V drop?
There is no additional voltage drop. The output voltage at C8 is about 420V. Then add 5-6V volts dropped into the OPT primary and that's 415V.

The typical DC voltage from datasheet with 450V per plate, 67 ohm total plate-supply resistance, 40 uF input cap is "just" 460V for 275 mA current. It would be some 50V more for 150 mA.

Here the total current draw is closer to 150 mA but the total series resistance is a lot more, the input cap is about 1/2 and each AC plate supply is 25V lower.
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Old 18th February 2019, 05:11 AM   #8
6A3sUMMER is online now 6A3sUMMER  United States
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45,

You proved my point. With no additional voltage drop, you can not start with 600V peak, and have 415V at the plate of the output tube. That is a total of all the drops of 185V.

With no additional voltage drops, the B+ would have to be a modified to have a much smaller input capacitance than 47uF/2.

Last edited by 6A3sUMMER; 18th February 2019 at 05:13 AM.
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Old 18th February 2019, 03:16 PM   #9
waltube is offline waltube  Italy
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The official manual of kit One says that the Dc voltage at pin 8 of 5U4 must be 450 volt.
At the out of the choke there are 435 Vdc; if it has 80 ohm of Rdc and there are 200 mA ( it is real) of max dc current we got the value listed on manual. And it is in line with the datasheet.
The current for each 300B is around 80 mA. At the anode of 300B we must found around 425 Vdc so the Rdc of OT is around 125 ohm

This is , less or more, what I also found in the past.


Walter

Last edited by waltube; 18th February 2019 at 03:22 PM.
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Old 18th February 2019, 05:06 PM   #10
45 is offline 45  Italy
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Walter, 415V instead of 425V is within tolerances and doesn't make any noticeable difference in practice.
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