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06-01-2023, 11:12 AM
Hi Guys
First thing: The cap connecting the grid to anywhere other than the signal input source (splitter) is absolutely undesirable.
You might see caps that tie to the bottom of the grid-leak as post-filters of the bias voltage in a fixed-biased supply, often because the raw bias supply is not well filtered to begin with, or simply because in the hifi ideal this point should look like an AC ground for signals. In the case of your observation with the cathode-bias adjust circuit, the cap is likely causing oscillation which make the tube red-plate.
As I suggested earlier, simply changing the value of Rk to a higher value should show decreased Ik BUT you might always see the same Vk. When you tried the overall circuit with the stock Rk, you indicated that there was control over the currents. Did you achieve hum balance at that point?
What is the supply voltage in this amp? Ik should never exceed a specific value based on the tubes in use, and specifically, based on the tube's plate power rating. For EL84 this is 12W. If Va=300V, then 12W of heat is achieved with just 40mA. Rk determines how hot the tube will get in the given voltage environment, and for any Rk you can calculate a target maximum Vk.
Lots of hobbyists will argue that Rk carries plate plus screen current, which is true, and that you should use the plate dissipation plus the screen dissipation for calculations. Their goal is to run the tube at its maximum heat. Mine never is. For the sake of the 2W of screen dissipation, I prefer to leave that out and err on the side of "slightly cooler"especially since EL84 amps tend to have skimpy screen protection. With any tube you can use a Pd value UP TO its max and anywhere below that value.
First thing: The cap connecting the grid to anywhere other than the signal input source (splitter) is absolutely undesirable.
You might see caps that tie to the bottom of the grid-leak as post-filters of the bias voltage in a fixed-biased supply, often because the raw bias supply is not well filtered to begin with, or simply because in the hifi ideal this point should look like an AC ground for signals. In the case of your observation with the cathode-bias adjust circuit, the cap is likely causing oscillation which make the tube red-plate.
As I suggested earlier, simply changing the value of Rk to a higher value should show decreased Ik BUT you might always see the same Vk. When you tried the overall circuit with the stock Rk, you indicated that there was control over the currents. Did you achieve hum balance at that point?
What is the supply voltage in this amp? Ik should never exceed a specific value based on the tubes in use, and specifically, based on the tube's plate power rating. For EL84 this is 12W. If Va=300V, then 12W of heat is achieved with just 40mA. Rk determines how hot the tube will get in the given voltage environment, and for any Rk you can calculate a target maximum Vk.
Lots of hobbyists will argue that Rk carries plate plus screen current, which is true, and that you should use the plate dissipation plus the screen dissipation for calculations. Their goal is to run the tube at its maximum heat. Mine never is. For the sake of the 2W of screen dissipation, I prefer to leave that out and err on the side of "slightly cooler"especially since EL84 amps tend to have skimpy screen protection. With any tube you can use a Pd value UP TO its max and anywhere below that value.
