Results 1 to 8 of 8

Thread: Fundamental tube questions

  1. #1
    Member
    Join Date
    Jul 2007
    Posts
    100

    Fundamental tube questions

    Hey all, new kid here. I have a couple of questions about tube parameters.

    For example, the typical number used to describe a 12AX7's internal Plate Resistance is 62.5K ohms, which seems to work perfectly in the standard gain equation(s). I'd like to know how that number was derived.

    I use P-Spice, and have a number of pretty accurate tube models. I've simulated a set of Plate Characterist Curves, and the results are virtually identical to the published PCC's for the 12AX7.

    I was told that this number comes from delta Eb over delta Ib, however, I've not been able to get that particular number. I was also told that this delta Eb/Ib needs to be done using a very small incremet of Eb change.

    Thought?

  2. #2
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    21,263
    Get the RCA RC-30 tube manual or something similar and read up on the tube function. Change in voltage at the grid causing change in current through the tube is pretty much the definition of gain isn't it?

  3. #3
    Senior Member
    Join Date
    Aug 2006
    Posts
    127
    Quote Originally Posted by Enzo View Post
    Change in voltage at the grid causing change in current through the tube is pretty much the definition of gain isn't it?
    No, that is the transcountance. You could multiply the transconductance by the load impedance to get the gain, though.

  4. #4
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    21,263
    Thanks.

  5. #5
    Member
    Join Date
    Jul 2007
    Posts
    100
    Actually, transconductance is a change in Plate Current / a change in Plate Voltage with the grid held constant. It's Gm, and is Amp/Volts described in mhos.

    I have RC19, I believe, and will have to get it open and see what I've managed to forget. I've also found a good web page that talks about this (ra) in Load Line terms.

  6. #6
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    21,263
    RC-19 is cool, but it is real old. If you want to get into it, I really suggest getting the RC-30 as well. The 19 has a lot of older tube stuff, but the 30 will include modern tubes we use a lot, as well as the tutorials and component value charts and stuff. The 30 will have all the data for tubes we currently use.

  7. #7
    Senior Member
    Join Date
    Aug 2006
    Posts
    127
    Quote Originally Posted by Earl View Post
    Actually, transconductance is a change in Plate Current / a change in Plate Voltage with the grid held constant. It's Gm, and is Amp/Volts described in mhos.
    That's the plate's conductance, not the mutual conductance.

  8. #8
    Member
    Join Date
    Jul 2007
    Posts
    100
    Right you are, Author...thanks for the correction.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Tube Cad / Spice Questions
    By bob p in forum Tubes (Valves)
    Replies: 11
    Last Post: 08-31-2007, 06:15 PM
  2. big lot of questions
    By black_labb in forum Pickup Makers
    Replies: 5
    Last Post: 06-01-2007, 11:42 AM
  3. dumb tube questions
    By davoux in forum Maintenance, Troubleshooting & Repair
    Replies: 13
    Last Post: 12-30-2006, 04:58 PM
  4. tube amp problem. loose tube maybe?
    By bumcharge in forum Maintenance, Troubleshooting & Repair
    Replies: 5
    Last Post: 10-16-2006, 01:50 PM
  5. 5E3 Questions
    By MarkL8 in forum 5 E 3
    Replies: 2
    Last Post: 09-07-2006, 05:13 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •