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plate voltage dropping resistrs

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Posted on March 29, 2022 at 21:56:19
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016



I am having difficulty working with this 7905 pentode. I usually work with triodes.
I want to reduce the b+ (550v) to 162v/~28ma on the plate. Ohms Law suggests something around a 10k resistor at (A), but my initial attempts show that the voltage at the plate remains unchanged.
Is it because of the resistor at (C)that controls g2?
In addition the voltage at g2 is also too high. How does Ohms Law work for g2 when it comes off the same B+ as the plate? Even when the estimated current for both locations when added, do not provide the correct voltage drop for either location.
What am I missing?
Thanks in advance.

 

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RE: plate voltage dropping resistrs, posted on March 30, 2022 at 01:44:02
Triode_Kingdom
Audiophile

Posts: 10042
Location: Central Texas
Joined: September 24, 2006
 If there's no voltage drop across a 10KΩ resistor, it's reasonable to think there's no current flow. Looks to me like you're applying +11V to the cathode, and that's shutting down the tube.




 

RE: plate voltage dropping resistrs, posted on March 30, 2022 at 07:20:35
Tom Bavis
Audiophile

Posts: 961
Location: Upstate NY
Joined: May 25, 2007
 Higher plate load resistor or higher screen voltage will bring plate voltage down.

 

RE: plate voltage dropping resistrs, posted on March 30, 2022 at 07:27:12
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 Thanks for the suggestion. Does this assume that I keep the voltage dropping resistor as well?

 

RE: plate voltage dropping resistrs, posted on March 30, 2022 at 09:44:41
Tom Bavis
Audiophile

Posts: 961
Location: Upstate NY
Joined: May 25, 2007
 Yes, keep the dropping resistor if you need the filtering. Higher plate resistor will give more gain, of course.

 

RE: plate voltage dropping resistrs, posted on March 30, 2022 at 10:12:39
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 Thanks again. And what if anything for the high g2 voltage? Will it come down with the change in plate resistor? I know that there will be a balancing act involved at some point.

 

RE: plate voltage dropping resistrs, posted on March 30, 2022 at 11:26:18
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 So I changed the plate resistor from 13k to 18k with no change in the plate voltage. Could the cathode resistor play a part in the problem? Too low? too high?

 

Do you think he knows about Ohms law?, posted on March 31, 2022 at 16:56:21
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012
 You are obviously correct, no current will result in no drop across a resistor. I would suggest that this type of DIY would be best served implementing a tried and true design.

 

RE: Do you think he knows about Ohms law?, posted on April 1, 2022 at 11:16:12
Triode_Kingdom
Audiophile

Posts: 10042
Location: Central Texas
Joined: September 24, 2006
 I would have thought it would be evident the tube isn't conducting. The comments above seem intent on ignoring the laws of physics. :)


 

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