UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

PJL · 14th Jul 2019, 9:30 pm

I would agree with Lawrence, you probably measured the screen grid current (Ig2) which is typically around 10% of the anode current.

We can get the combined cathode current from Ik=V/R = 15/470 (31mA).
The anode current based on the 25V across the output transformer = 25/680 (37mA) or 25/915 (27mA) as you don't say which is the good valve. The most likely is the 915 ohm winding.
To see if you are over-running the good EL42 we will roughly calculate the combined anode and screen grid dissipation as follows D = (Va-Vk) * Ik = (288-15) * 0.031 = 8.4W. This is a bit high as the max is 7W according to the datasheet but not as bad as I expected.
Again looking at the datasheet, the anode current of say 29mA seems about right with 273V on the anode and screen grid so that valve is still good. The anode and screen grid should be less than 250V but it will be high due to the lower current being drawn from the HT supply due to only one valve working.

14th Jul 2019, 9:30 pm	#61
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Voltages on ECC83 in BlackBox I would agree with Lawrence, you probably measured the screen grid current (Ig2) which is typically around 10% of the anode current. We can get the combined cathode current from Ik=V/R = 15/470 (31mA). The anode current based on the 25V across the output transformer = 25/680 (37mA) or 25/915 (27mA) as you don't say which is the good valve. The most likely is the 915 ohm winding. To see if you are over-running the good EL42 we will roughly calculate the combined anode and screen grid dissipation as follows D = (Va-Vk) * Ik = (288-15) * 0.031 = 8.4W. This is a bit high as the max is 7W according to the datasheet but not as bad as I expected. Again looking at the datasheet, the anode current of say 29mA seems about right with 273V on the anode and screen grid so that valve is still good. The anode and screen grid should be less than 250V but it will be high due to the lower current being drawn from the HT supply due to only one valve working.