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Old 27th December 2012, 02:52 PM   #1
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Question 6AX4GT TV damper

I want to make a hybrid Graetz rectification with two tubes 6AX4-GT & two diodes UF4007, are the attached schematic correct?

What's the maximum capacitance 6AX4-GT can accept?
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Old 27th December 2012, 03:20 PM   #2
chip647 is offline chip647  United States
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That circuit is correct. I can not find any published limits on capacitance, any reasonable amount should be fine.

What you have to look out for is the load you are going to place on the rectifiers. While the data sheet says 125ma per plate, it also says max of 5.0 plate watts for each plate in the tube pair. At 950 volts this would limit this supply to 10mA.
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Old 27th December 2012, 03:20 PM   #3
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Hi Felipe,

The 6AX4 was never characterized for mains rectifier use. So no solid data exists for the max capacitance. You could sim it with PSUD2 and make sure the peak currents stay below the limits. But I would apply some substantial derating since the peak currents in the datasheet are given for damper use.

I'd say you should be save with 20uF. However choke input would be the better solution also usually in terms of obtainable sound quality. Or low value capacitances. It is a common mistake to load the rectifiers with the maximum possible capacitance they can withstand. Maybe this saves some cost in the filter chain, but quality is compromised.

Some other remarks: Only one heater winding is needed in this configuration. Connect it to the cathodes. Make sure the power transformer has a sufficiently isolated heater winding. With the voltage in your schematic, use at least 2 silicon diodes in series for each leg, otherwiese PIV rating will be exceeded. Otherwise your schematic is correct.

Best regards

Thomas
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Old 27th December 2012, 03:45 PM   #4
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Quote:
Originally Posted by chip647 View Post
That circuit is correct. I can not find any published limits on capacitance, any reasonable amount should be fine.

What you have to look out for is the load you are going to place on the rectifiers. While the data sheet says 125ma per plate, it also says max of 5.0 plate watts for each plate in the tube pair. At 950 volts this would limit this supply to 10mA.
Quote:
Originally Posted by Vinylsavor View Post
Hi Felipe,

The 6AX4 was never characterized for mains rectifier use. So no solid data exists for the max capacitance. You could sim it with PSUD2 and make sure the peak currents stay below the limits. But I would apply some substantial derating since the peak currents in the datasheet are given for damper use.

I'd say you should be save with 20uF. However choke input would be the better solution also usually in terms of obtainable sound quality. Or low value capacitances. It is a common mistake to load the rectifiers with the maximum possible capacitance they can withstand. Maybe this saves some cost in the filter chain, but quality is compromised.

Some other remarks: Only one heater winding is needed in this configuration. Connect it to the cathodes. Make sure the power transformer has a sufficiently isolated heater winding. With the voltage in your schematic, use at least 2 silicon diodes in series for each leg, otherwiese PIV rating will be exceeded. Otherwise your schematic is correct.

Best regards

Thomas
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Old 27th December 2012, 03:53 PM   #5
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Hi!

Quote:
Originally Posted by chip647 View Post
While the data sheet says 125ma per plate, it also says max of 5.0 plate watts for each plate in the tube pair. At 950 volts this would limit this supply to 10mA.
This is not really correct, the plate dissipation calculates as current multiplied by the voltage drop within the tube. Not the supplied voltage. The voltage drop within a 6AX4 is quite low, depending on current draw maybe 25V.

A supply like this can easily deliver 200mA at 950V

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Old 27th December 2012, 03:58 PM   #6
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Thomas do you refer first cap 20uF or the total capacitance PSU?
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Old 27th December 2012, 04:00 PM   #7
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Hi Felipe,

first cap. After any choke you can put as much capacitance as you like (within reason, monster caps of 1000s uF can have issues at start up)

Thomas
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Old 27th December 2012, 04:05 PM   #8
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Thanks Thomas YHPM.
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Old 27th December 2012, 04:17 PM   #9
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Default 4 * 6AX4's

As the heater is well isolated from the cathode, you also can use 4 of them to form a full tube bridge with the same heater winding supposing that it can support the 1.5A per tube (6A total)..
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Old 27th December 2012, 05:31 PM   #10
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Would 6DW4B's work also?
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