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Old 6th Dec 2008, 7:52 pm   #4
jjl
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Join Date: Jan 2003
Location: Ware, Herts. UK.
Posts: 1,082
Default Re: Sound Sales PX4 Amplifier

Hello Alan

As others have stated, your cathode resistor must have a value of around 470 or 500 ohms.
The rated anode current for a PX4 is 30mA to 60mA depending on anode voltage, bias etc, this means that one of your PX4s has a fair bit lower emission than the other and also means that your amplifier appears to be running the PX4s at fairly low anode dissipation. What anode voltage do you measure at pin 1 on each PX4 with normal main voltage?
In principle the anode currents of the PX4s should be equal, this is more important where there is a shared cathode resistor as in your circuit. The PX4 with lower emission current may need to be replaced one day but will probably carry on working for a long time.
I'd advise aginst reducing the mains supply voltage to the amp as this will underrun the heaters of all of the valves and is likely to shorten their life.


John
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