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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.
soulmerchant said:

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

Hi ,

I’ 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 I’ 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 ,

soulmerchant said:
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.
soulmerchant said:
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 )

I’ll 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 , you’ll 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 ,

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...
JoshK said:
Where did you get this value for minimum impedance? I've never seen that before and it isn't in the aforementioned datasheet.

andyjevans said:

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

Hi Andy , Hi Josh ,

No!! The use of protective resistor is a "good practice" when
you are using tube rectifiers , no matters if it is a damper diode
or double diode or even a general rectifier like 5U4 , GZ34 , etc .

The protective resistor aims to prevent , the cathode stripper ,
limiting the cold inrush current , thus increasing the tube life
and its reliability .

I ALWAYS do the calculation that I showed above , ALWAYS
using the data furnished by the manufacturer or the designer
of that individual tube ( no problem about that because I have
a lot of Tube Manuals and the Frank's Tube Page , to consult )

In the specific case of 6BY5GA , I could get this precious
information on my colection of Tube Manuals . It is a reliable
information , you can be shure about that , as follows :
6BY5GA - High perveance damper diode
Minimum effective plate impedance when used as
a general rectifier = 100 ohms ( per plate )
Ideal value for the filter input capacitor = 10 uf
Maximum output CC = 175 mA

You are correct , there is no provision on PSU Designer to calculate or insert these protective resistors .

IMHO , The PSU Designer is an excellent tool , to simulate the
conditions , the voltages , the currents and the general behavior
of any planned power supply , BUT in the real life , things happen
a " bit " different and then there is nothing more reliable than
the experience and the data acquired in the laboratory condition
from an old manufacturer or an old designer of tubes .

I think that these old engineers and old lab technicians , did know
exactly what they were doing . Or not ??
And they did their job very , very , very , well !!!!
Because all of us , even in the Very Large Scale Integration era ,
prefer the tube sound ( that was born in the 1900's ) instead
the modern and hyper-miniaturized " silicon " sound ,

Regards for all ,