Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Nov Mon 29, 2021 10:35 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 4 posts ] 
Author Message
 Post subject: screen bias for the control grid of a 12sk7 tube?
PostPosted: Sep Sat 18, 2021 5:53 pm 
New Member

Joined: Sep Sat 18, 2021 5:15 pm
Posts: 2
I have been fighting this 12sk7 tube for several days, and have lost badly! I need help here: I have a 198 v power supply, and am using a 15K ohm resistor on the plate, and have 13.2 ma showing on the load line. My idle spot chosen shows 100 v , 9ma, and a grid bias voltage of -3 v. I will list my voltage and resistance values: My supply voltage reads 191 volts, My plate voltage reads 115 volts with the 15k ohm resistor. My screen, with a 56k ohm resistor, reads 112 volts, My control grid is grounded because that is the only way that I could get the control grid to be 191 volts { -3 volts} while the cathode, with a 470 ohm resistor, is 188 volts. According to my calculations, the tube is biased ok. But: If I place a 1 meg or even a 2 meg resistor on the control grid, the voltage there,186v, shows 2 volts lower than the voltage on the cathode,188v. You figure! Any input appreciated........................william


Top
 Profile  
 
 Post subject: Re: screen bias for the control grid of a 12sk7 tube?
PostPosted: Sep Sun 19, 2021 1:04 am 
Member
User avatar

Joined: Sep Mon 16, 2013 2:42 am
Posts: 5178
Location: Tucson, Arizona U.S.A.
How about a schematic with the voltages marked? A verbal description only leads to guesses for answers.

And what are you trying to do? Is this part of a radio or an experiment or what? Are you following some instruction or making it up?

If you are using the internet for information, be aware that many people posting there have no idea about what they are talking about.

_________________
Jim Mueller

Who's that swimming in the punch bowl?
It's Walter Wart, the freaky frog!


Top
 Profile  
 
 Post subject: Re: screen bias for the control grid of a 12sk7 tube?
PostPosted: Sep Mon 20, 2021 12:22 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 18895
Location: Utica, NY 13502 (USA)
I am confused by the title of your post. "Screen bias for the control grid..." Huh?

Then there is this: "My control grid is grounded because that is the only way that I could get the control grid to be 191 volts..." Shouldn't a grounded control grid be be zero?

Is this an amplifier, a receiver, an oscillator, or what?

Please post a schematic.

Dave


Top
 Profile  
 
 Post subject: Re: screen bias for the control grid of a 12sk7 tube?
PostPosted: Sep Tue 21, 2021 5:29 pm 
New Member

Joined: Sep Sat 18, 2021 5:15 pm
Posts: 2
Thanks for the help everyone! I put in a 1 meg resistor on the control grid and put in a 15000 ohm resistor on the cathode, with a result of 6 volts on the control grid, which is perfect. Mission accomplished. Again. Thanks for the help!!!!!!!!!!!!!!!!!!!!!!


Top
 Profile  
 
Post New Topic Post Reply  [ 4 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 11 guests



Search for:
Jump to:  




































Privacy Policy :: Powered by phpBB