I’ve gone through this before on champs. Raise the cathode resistance and the plate voltage will go up so VxI may not drop significantly. These high plate voltages along with insufficient ant primary inductance in the OT leads to tube arcing under high signal. It is also imperative to measure the screen current and subtract that from the cathode current to get the plate current. If the 470 cathode resistor it truly almost 1K it should be replaced as a carbon comp resistor that has drifted that much may exhibit other problems. I have fried more than one OT on more than one of these gems. One I fixed by using a large power resistor to drop the supply voltage. I sold that on to a friend and it is considered one of his best sounding amplifiers. The other I fixed by replacing the OT that smoked with one rated for more power (considerably higher primary inductance). It didn’t sound quite as raw with the beefier OT but has been rock solid since.
08:39 "this sucker is hot" yes even with double the stock resistance, imagine what was going on when they came from the factory and the resistor was still in spec 470 ohms. Been looking for the answer for years, and can only figure that Leo thought: I want them as loud and clean as possible and tubes are cheap. If you have a nice NOS 6V6, don't put it in your Champ! Also would never leave a resistor that has doubled in value in the amp -- would at least have the amp run for some time and then measure again. What if it dies?
Great info, thanks for sharing
Nice explanation. Thank you.
I’ve gone through this before on champs. Raise the cathode resistance and the plate voltage will go up so VxI may not drop significantly. These high plate voltages along with insufficient ant primary inductance in the OT leads to tube arcing under high signal. It is also imperative to measure the screen current and subtract that from the cathode current to get the plate current. If the 470 cathode resistor it truly almost 1K it should be replaced as a carbon comp resistor that has drifted that much may exhibit other problems. I have fried more than one OT on more than one of these gems. One I fixed by using a large power resistor to drop the supply voltage. I sold that on to a friend and it is considered one of his best sounding amplifiers. The other I fixed by replacing the OT that smoked with one rated for more power (considerably higher primary inductance). It didn’t sound quite as raw with the beefier OT but has been rock solid since.
Don’t forget about screen current
Very good explanation, thank you.
08:39 "this sucker is hot" yes even with double the stock resistance, imagine what was going on when they came from the factory and the resistor was still in spec 470 ohms.
Been looking for the answer for years, and can only figure that Leo thought: I want them as loud and clean as possible and tubes are cheap. If you have a nice NOS 6V6, don't put it in your Champ!
Also would never leave a resistor that has doubled in value in the amp -- would at least have the amp run for some time and then measure again. What if it dies?
Very good explanation, thank you.