10-28-2021, 08:52 PM
HELP, PLEASE ! Well, things are to the point of testing now, and I have run into a problem right off to bat. It centers around the type 75 tube and the 2nd detector circuit....
[Image: https://64.media.tumblr.com/cb2ba248f2a4...fa99da.jpg]
If I feed an audio signal into the 42 output tube either directly to the control grid through a .01 condenser, or feed that signal in to the plate of the first AF, the 75, I get the sound loud and clear through the speaker. If I try feeding the signal in to the control grid of the 75, I get almost nothing. This is the pinout of the 75, followed by the voltages on the pins.
[Image: https://64.media.tumblr.com/97dcf3bb5f0a...d60a22.jpg]
Between 1 and 6 I have heater voltage just fine. K is +0.8 mv. GT is -1.6 v. PD1 is -520 mv. PD2 is -520 mv. PT is -550 mv.
Looking at the schematic, I should think the plate on the 75 should be strongly positive to work, but it is not connected to B+ in any way that I can see. Is there a dot missing on the schematic? What am I missing?
The plate voltages on the other tubes seem fine. The bias voltages coming from R44 are -1.85 V and -22.08V to ground.
[Image: https://64.media.tumblr.com/cb2ba248f2a4...fa99da.jpg]
If I feed an audio signal into the 42 output tube either directly to the control grid through a .01 condenser, or feed that signal in to the plate of the first AF, the 75, I get the sound loud and clear through the speaker. If I try feeding the signal in to the control grid of the 75, I get almost nothing. This is the pinout of the 75, followed by the voltages on the pins.
[Image: https://64.media.tumblr.com/97dcf3bb5f0a...d60a22.jpg]
Between 1 and 6 I have heater voltage just fine. K is +0.8 mv. GT is -1.6 v. PD1 is -520 mv. PD2 is -520 mv. PT is -550 mv.
Looking at the schematic, I should think the plate on the 75 should be strongly positive to work, but it is not connected to B+ in any way that I can see. Is there a dot missing on the schematic? What am I missing?
The plate voltages on the other tubes seem fine. The bias voltages coming from R44 are -1.85 V and -22.08V to ground.
