Hey Mark, I believe that since there is no coupling cap or resistor and the input to the phase inverter is direct coupled at 50V this creates a sensitivity to oscillation. The negative feedback loop is tight with few components and low loss. The cap in question bleeds off the high frequencies that cause oscillation (low pass filter). This is common in RF or microwave amplifiers but not in audio amps. "Bleed em if you've got them oscillator blues" Keep em glowing!
I have the same problem: saying volts when I mean to say amps in my videos. That’s ok, just don’t say ‘Ann’ when you mean to say ‘Nélia’ (thats what my wife’s name is). TH-cam viewers are more forgiving than your wife. :)
Question: I've seen people recommend removing the death cap and then grounding the chassis by installing a three prong cord with a ground wire. Watching Mr. Carlson's Lab and this video, it seems there are good reason NOT do that, but I don't know the reasons. I kind of get why the modern safety cap that is designed to fail open addresses the danger of component failure, but I don't understand the reasons for and against removing the death cap/grounding the chassis versus replacing the death cap with a modern saftety cap. If anyone can explain, that would be great.
I hadn’t thought about the cap that is on the grid of the PI as being a low pass filter but I see your point. I do however, wonder why there is no coupling cap between the pre amp plate and the PI grid. How do you keep the b+ off the grid of the PI?
It's not the voltage which happens to be on the grid which matters; it's the voltage on the grid with respect to the cathode. The grid is at +50V with respect to ground, but current flowing in the 100k cathode resistor is developing +55V with respect to ground at the cathode so that the grid is at -5V with respect to the cathode, exactly what is required for the tube to function.
Hi Mark; Great video series! Enjoyed as always. Just curious, are you going to replace the electrolytics with new multi section can caps or discrete caps under the chassis?
Interesting. I'm not very familiar with direct coupled amps. It sounds like they are low pass by nature. In my electronics engineers handbook it makes reference to that cap being high frequency compensation but the example it gives has a resistor in series with it and offers no way of calculating this that I have found.
It's simple enough to just move the decimal point 4 place to the left, or 4 places to the right. 10 to the 6th, or 10 to the 9th -- I'm glad you put the safety-cap. in, some youtuber's just cut them out. *DUMN!!!*
Hey Mark, I believe that since there is no coupling cap or resistor and the input to the phase inverter is direct coupled at 50V this creates a sensitivity to oscillation. The negative feedback loop is tight with few components and low loss. The cap in question bleeds off the high frequencies that cause oscillation (low pass filter). This is common in RF or microwave amplifiers but not in audio amps. "Bleed em if you've got them oscillator blues"
Keep em glowing!
I have the same problem: saying volts when I mean to say amps in my videos. That’s ok, just don’t say ‘Ann’ when you mean to say ‘Nélia’ (thats what my wife’s name is). TH-cam viewers are more forgiving than your wife. :)
Great video Mark, thank you for all of the hard work.
Thanks for the chart and another good video. 73's KI5CBM
Question: I've seen people recommend removing the death cap and then grounding the chassis by installing a three prong cord with a ground wire. Watching Mr. Carlson's Lab and this video, it seems there are good reason NOT do that, but I don't know the reasons. I kind of get why the modern safety cap that is designed to fail open addresses the danger of component failure, but I don't understand the reasons for and against removing the death cap/grounding the chassis versus replacing the death cap with a modern saftety cap. If anyone can explain, that would be great.
I hadn’t thought about the cap that is on the grid of the PI as being a low pass filter but I see your point. I do however, wonder why there is no coupling cap between the pre amp plate and the PI grid. How do you keep the b+ off the grid of the PI?
It's not the voltage which happens to be on the grid which matters; it's the voltage on the grid with respect to the cathode. The grid is at +50V with respect to ground, but current flowing in the 100k cathode resistor is developing +55V with respect to ground at the cathode so that the grid is at -5V with respect to the cathode, exactly what is required for the tube to function.
Hi Mark; Great video series! Enjoyed as always. Just curious, are you going to replace the electrolytics with new multi section can caps or discrete caps under the chassis?
Interesting. I'm not very familiar with direct coupled amps. It sounds like they are low pass by nature. In my electronics engineers handbook it makes reference to that cap being high frequency compensation but the example it gives has a resistor in series with it and offers no way of calculating this that I have found.
It's simple enough to just move the decimal point 4 place to the left, or 4 places to the right. 10 to the 6th, or 10 to the 9th
--
I'm glad you put the safety-cap. in, some youtuber's just cut them out. *DUMN!!!*
The chart!