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Old 8th February 2009, 11:21 PM   #1
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Default 6BY5GA question

Hi all,

I am planning to use a 6BY5GA as a full wave rectifier with two silicon diodes as an artificial center tap on HT of an Aikido style phono stage I am building.

I want to use a CLCLC filter on the HT (around 300V) but am not sure about how high I can go with the first capacitor. Any advice would be most appreciated.
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Old 8th February 2009, 11:31 PM   #2
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I've left the input cap at max 10uF. Mind, quasi choke input sounds better anyway if you don't need the extras volts.

andy
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Old 9th February 2009, 03:03 AM   #3
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Default Re: 6BY5GA question

Quote:
Originally posted by soulmerchant

........ but am not sure about how high I can go with the first capacitor. Any advice would be most appreciated.
Hi ,

Ií ve used the 6BY5 GA , in many projects , preamps and low
power amps . Its full-wave rectifier behavior is amazing , giving
a very sweet sound .

In all projects Ií ve always used CLC or CRC filter , without any
problem BUT you can not go far with the first capacitor .

A good value stays between 10 uf and 20 uf , with a minimum
effective plate plate supply impedance around 100 ohms each
plate .

An 8 uf paper-in-oil ( preferably ) or even a 15 uf polypropilene
motor-run for the first cap would be nice .

Regards ,

Carlos
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Old 9th February 2009, 06:17 AM   #4
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Thanks andyjevans and refference for the advice.

One more question refference, is there anything I should consider when calculating the minimum value for protective plate resistors if I do an artificial center tap?
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Old 9th February 2009, 06:59 AM   #5
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Default Re: 6BY5GA question

Quote:
Originally posted by soulmerchant
I want to use a CLCLC filter on the HT (around 300V) but am not sure about how high I can go with the first capacitor. Any advice would be most appreciated.
According to Frank's the 6BY5GA has an Isurge rating of 525mA / plate. That's what you have to stay under. So far as how big a resevoir capacitor, that depends on what your load current is.
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Old 10th February 2009, 12:42 AM   #6
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Quote:
Originally posted by soulmerchant
One more question refference, is there anything I should consider when calculating the minimum value for protective plate resistors if I do an artificial center tap?
Hi soulmerchant ,

There is a very simple ( but with good results ) formula to calculate the impedance :

Rt = Rs + Rp . N2 where :

Rt = Total impedance ( transformer natural impedance )
Rs = Secondary DC resistance , measured with a multimeter
and :
Rp . N2 = The influence of primary impedance over the secondary
( called reflected impedance )
where :

Rp = Primary DC resistance , measured with a multimeter
N = Transformer ratio ( Secondary Volts / Primary Volts )

Iíll give you an example :

Rs = 35 ohms
Rp = 6 ohms
Transformer ratio ( N ) = 300 VAC output / 117 VAC input = 2.56

Then Rt = 35 + 6 . ( 2.56 )2 = 74 ohms , that way , youíll need TWO 27 ohms resistors ( preferably , wirewound 5 watts resistors ) between each secondary tap and each plate of the 6BY5GA , because 74 + 27 = 101 ohms > 100 ohms ( minimum impedance per plate )

If you use this formula with a split secondary ( center tap ) , you must to consider that :
( Rs ) will be the secondary DC resistance between one tap
and the center tap .
( N ) Transformer ratio will be the
HALF total secondary voltage / primary voltage

Did you understand ? You need to complete the transformer
natural impedance with resistors , to reach the minimum impedance value per plate .

Best Regards ,

Carlos
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Old 10th February 2009, 10:02 AM   #7
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Thanks for that post, Carlos. I was unaware of the need for extra resistors. Is this particularly the case with 6BY5, or is it particular to damper diodes in general or what?

andy
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Old 10th February 2009, 01:32 PM   #8
JoshK is offline JoshK  Canada
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Where did you get this value for minimum impedance? I've never seen that before and it isn't in the aforementioned datasheet.
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Old 10th February 2009, 06:07 PM   #9
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Some data sheets do list this, as some list maximum input capacitance. But the REAL limit on the rectifier is peak inrush current and peak repetitive current. Before modern test equipment and computers, this was not easy to calculate or measure. Now, you can just use PSU Designer (download from duncanamps.com) and / or stick a current probe in the circuit...
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Old 10th February 2009, 06:48 PM   #10
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Now, you can just use PSU Designer (download from duncanamps.com)>>

But there's no provision for inserting resistors before the rectifier in PSU Designer unless I'm missing something. Could you explain further?

andy
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