UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Regentone A133 HT smoothing capacitors, V4 cathode resistor

space_charged · 20th Sep 2019, 8:53 pm

Apropos V4 cathode resistor, I like the idea of increasing it a bit. In my set, I have noticed the EL41 is very hot. I know its biased to class A and will get hot but thought I'd check the current through R14. The easy way, was to measure the cathode voltage, and check the resistor value. R14 measured 200 ohms and the cathode was at 10V, so according to ohm's law that means 50mA.

The data-sheet for EL41 says the max anode current is 36mA and a screen grid max of 5.2mA which gives 41.2. The measured current is significantly higher than I'd expect. I should say that the coupling capacitor (C15 0.01uF) has been replaced and disconnecting it completely makes no difference. The HT was also checked and measures 250V.

The control grid reads 1V to chassis which I wouldn't expect. There is still a fair negative voltage on the grid relative to the cathode (because of the current through R14). I have checked R12 (680K) that connects the grid to chassis. It is slightly high but not enough to be the cause. I'm wondering about the valve and have a spare on order.

20th Sep 2019, 8:53 pm	#18
space_charged Hexode Join Date: Dec 2014 Location: West Midlands, UK. Posts: 315	Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor Apropos V4 cathode resistor, I like the idea of increasing it a bit. In my set, I have noticed the EL41 is very hot. I know its biased to class A and will get hot but thought I'd check the current through R14. The easy way, was to measure the cathode voltage, and check the resistor value. R14 measured 200 ohms and the cathode was at 10V, so according to ohm's law that means 50mA. The data-sheet for EL41 says the max anode current is 36mA and a screen grid max of 5.2mA which gives 41.2. The measured current is significantly higher than I'd expect. I should say that the coupling capacitor (C15 0.01uF) has been replaced and disconnecting it completely makes no difference. The HT was also checked and measures 250V. The control grid reads 1V to chassis which I wouldn't expect. There is still a fair negative voltage on the grid relative to the cathode (because of the current through R14). I have checked R12 (680K) that connects the grid to chassis. It is slightly high but not enough to be the cause. I'm wondering about the valve and have a spare on order.