So this looks fun: Matchless Spitfire

Discussion in 'Shock Brother's DIY Amps' started by JuneauMike, Jan 4, 2019.

  1. Ten Over

    Ten Over Tele-Meister

    Posts:
    328
    Joined:
    May 13, 2015
    Location:
    Central California
    The tail resistor doesn't have anything to do with gain. Figure 1 in post #49 doesn't even have a tail resistor, but it has the same gain as in Figure 3 in post #50. The tail resistor is just there to get a voltage drop without using very much current.

    Figure 1 needs to be understood because this is the basis for all the LTP splitters. The LTP does not act like a cathode follower, nor a cathodyne, nor a common-cathode gain stage. It works by balancing the current fluctuations between the two triodes. No matter whatever else happens with a LTP splitter, the balancing effect is always there and it will be the dominant effect.
     
    Last edited: Jan 9, 2019
  2. Ten Over

    Ten Over Tele-Meister

    Posts:
    328
    Joined:
    May 13, 2015
    Location:
    Central California
    Thanks, Mike
     
    Last edited: Jan 9, 2019
  3. JuneauMike

    JuneauMike Friend of Leo's

    Posts:
    3,559
    Joined:
    May 5, 2015
    Location:
    Alaska
    @Ten Over:

    Yes, you are right. I was conflating gain with headroom.

    "The amount of current set by the bias resistor, along with the value of the tail resistor, determines the DC voltage dropped across this resistor, which, in turn, partly determines the headroom of the circuit. If no global negative feedback is used, the tail resistor should be made as large as practical, with respect to the amount of current being drawn, and the desired headroom of the amplifier. This will give the best balance to the PI outputs. This resistor has little effect on gain, but a major effect on balance and headroom."

    http://www.aikenamps.com/index.php/the-long-tail-pair



     
    Last edited: Jan 10, 2019
  4. Ten Over

    Ten Over Tele-Meister

    Posts:
    328
    Joined:
    May 13, 2015
    Location:
    Central California
    Here is what happens to Figure 1 if you add negative feedback.

    LTP NFB Constant Current Fig 2.png
    LTP Figure 2 PNG.png
     
    robrob likes this.
  5. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    7,261
    Joined:
    Dec 29, 2012
    Location:
    United States
    Ten Over, what is your source for these LTP diagrams and interesting discussion?
     
  6. Ten Over

    Ten Over Tele-Meister

    Posts:
    328
    Joined:
    May 13, 2015
    Location:
    Central California
    Me, Myself & I et al
     
    robrob likes this.
  7. Ten Over

    Ten Over Tele-Meister

    Posts:
    328
    Joined:
    May 13, 2015
    Location:
    Central California
    I read over Aiken's bit on LTP's because he seems to be considered an ultimate authority by some folks. By and large, it is compatible with what I say.

    LTP actual w voltages Fig 3B.png
    LTP Figure 3B PNG.png
     
    JuneauMike likes this.
  8. JuneauMike

    JuneauMike Friend of Leo's

    Posts:
    3,559
    Joined:
    May 5, 2015
    Location:
    Alaska
    One of the things I have trouble tracking with is the notion that a drop in current results in a drop in voltage. I have it in my head that when current goes down, voltage goes up.
     
  9. Ten Over

    Ten Over Tele-Meister

    Posts:
    328
    Joined:
    May 13, 2015
    Location:
    Central California
    Maybe it's because there are voltage drops and drops in voltage. Voltage drops across a resistor are the result of current and they a proportional to the current -- when the current increases, the voltage drop increases.

    Drops in voltage depends on how and what you connect the resistor to. If you hook one end to +300V and reduce the current, the voltage on the other end increases. If you reduce the current through a resistor with one end connected to ground, the voltage on the other end decreases.
     
    King Fan, JuneauMike and robrob like this.
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    7,261
    Joined:
    Dec 29, 2012
    Location:
    United States
    Ten Over, I'd like to read all of this. Do you have the complete dissertation posted somewhere?
     
    JuneauMike likes this.
  11. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    7,261
    Joined:
    Dec 29, 2012
    Location:
    United States
    JM, it might help if you focus on the upper grid. When a positive voltage guitar signal is on the grid there are less electrons on the grid to block electron flow through the tube. As electron flow increases through the tube all those electrons have to flow through the tail and cathode resistors too. More flow = more voltage drop across the tail which increases the voltage at the grid leak and cathode resistor junction, and on the cathode too. A negative voltage signal on the upper grid will do the opposite and lower the voltages. Once you understand what the upper triode is doing you can then turn your attention to the lower triode.
     
    JuneauMike likes this.
  12. JuneauMike

    JuneauMike Friend of Leo's

    Posts:
    3,559
    Joined:
    May 5, 2015
    Location:
    Alaska
    Feel like the dirty faced kid at the back of the short bus, just sittin' there eatin' my glue.
     
    robrob and Prophetsnake like this.
  13. Ten Over

    Ten Over Tele-Meister

    Posts:
    328
    Joined:
    May 13, 2015
    Location:
    Central California
    This is the first time I have posted this series. There are only two more at this point. I just quickly drew up Figure 6 to show Mike what happens with applying a signal to grid 2. I might go into that a little more after all this year end dust settles.

    LTP NFB actual w voltages Fig 4.png

    LTP Figure 4 PNG.png
     
    robrob likes this.
  14. Ten Over

    Ten Over Tele-Meister

    Posts:
    328
    Joined:
    May 13, 2015
    Location:
    Central California
    I'm not quick on the uptake like you, so I had to think about that for a few seconds. I decided you meant that you feel like a mentally challenged child on a special ed bus. There are two things that struck me right out of the gate:

    1. There is a difference between intelligence and knowledge. You are clearly intelligent, so you shouldn't run yourself down like that because it is an impediment to gaining knowledge. Instead, use that effort to gain more knowledge. Tube knowledge is cumulative just like most things, so you need to know the most basic things first. Don't jump around.

    2. Never criticize those that are in a situation that is no fault of their own.
     
  15. peteb

    peteb Friend of Leo's

    Posts:
    3,750
    Joined:
    Apr 25, 2003
    Location:
    Cascadia

    they are nice descriptions, I was also interested where they came from, they look very professional.


    Awesome!




    can I ask one question?





    I have read your excellent descriptions of the LTPI more than once. I have one question or need for clarification.



    the purpose of the LTPI is to produce equal and opposite signals for the power stage.



    they are opposite because the signal goes to one grid, the other is grounded, and the cathode is some where in between. that part is clear.






    why are the two outputs balanced? I read your descriptions with this question in mind and I did not see this part explained. I am sorry if I missed it.







    is it because the two one meg resistors keep the signal on the cathode exactly half way between the two plates?
     
  16. noah330

    noah330 Friend of Leo's

    Posts:
    3,681
    Joined:
    Feb 10, 2009
    Location:
    Maryland
    Look for a schematic for the Gibson GA-15R. It's very similar to a Spitfire and has reverb.
     
    JuneauMike likes this.
  17. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    39
    Posts:
    158
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    Because the grid signals are opposite and the plate resistors are (roughly) equal, the outputs are (roughly) equal/balanced.

    I'm not sure though whether this is the answer you are looking for.
     
    peteb likes this.
  18. Ten Over

    Ten Over Tele-Meister

    Posts:
    328
    Joined:
    May 13, 2015
    Location:
    Central California
    I couldn't find that schematic. Where did you find it?
     
  19. Ten Over

    Ten Over Tele-Meister

    Posts:
    328
    Joined:
    May 13, 2015
    Location:
    Central California
    The voltage difference between the grid and cathode (Vgk) controls how much current flows through a triode.

    If the cathode in a gain stage is fully bypassed, the cathode is connected to ground for AC purposes. Applying a signal to the grid forces changes to the Vgk because the cathode voltage cannot change.

    If the cathode is not bypassed, we cannot force changes to Vgk because the cathode voltage can and will change. The triode will set its Vgk all by itself to suit the situation.

    If we connect the cathode of a gain stage to a constant current source, the triode will set its Vgk all by itself to allow that amount of current to flow through it. If we raise the voltage on the grid, the triode will raise the voltage on its cathode so that the Vgk will allow the same amount of current to flow. Nothing happens at plates because there is no current change.

    If we connect that gain stage to another gain stage as shown in Figure 1, Stage 1 is no longer free to change its Vgk at will because the change will also alter the Vgk for Stage 2. If Stage 1 tries to adjust its Vgk for the same amount of current like it did in the preceding paragraph, it will cause a change in the Vgk of Stage 2 that will decrease the current through stage 2. This isn't going to happen because then there would be an overall change in current and we are holding the current constant.

    Stage 1 and Stage 2 are going to adjust their Vgk's all by themselves so that the increase in current through one stage is countered by a decrease in current through the other. The magnitude of the changes would have to be the same for both stages in order to come up with no net change in current. If the plate resistors you are running these currents through are of the same value, the outputs are going to be balanced.
     
    peteb likes this.
  20. JuneauMike

    JuneauMike Friend of Leo's

    Posts:
    3,559
    Joined:
    May 5, 2015
    Location:
    Alaska
    image.jpg
     

    Attached Files:

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.