Using the different inputs in LTP inverter?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by CitizenCain, Sep 1, 2008.

  1. CitizenCain

    CitizenCain Member

    Messages:
    4,831
    Joined:
    Jan 10, 2004
    Location:
    Magdalena, NM, US
    Reading about the LTP design on Aiken's site, he says there are three inputs you can use in this design. I have a learning mule that I like to play with and have been thinking about trying something out. Just want to run it past you guys to see if there's anything obvious I should be looking out for.

    I have a Sundown head that I've converted the clean channel on to handwired tagboard. It's a single-channel amp now because of that. Reading some things in the Aiken article gave me these ideas. Use the "std" input as the input from the preamp. Use the 2nd coupling cap input as the input from the reverb circuit. Use the 3rd input (at the bottom of the tail resistor) as the return for the neg feedback line, running the presence control from there.

    Will the amp blow up if I try this? :rotflmao

    I don't know if it will make any difference in the sound, but I like to try stuff out just to see how it works, you know? :D
     
  2. reaiken

    reaiken Member

    Messages:
    1,798
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC
    Yes, this is what I did in my Invader series amplifiers. Note that the "third input" is not really where the feedback is returned to, as it adds to both phases, it is merely a way of better balancing the PI than feeding it to one side only, so the feedback is actually summed into the "second" input with a bit added to the tail for balance.

    You will also have to figure a way to sum the reverb signal and the feedback signal into the second input without loading or attenuating either one too much. There is an easy way to do this, but I'm not tellin'. :)

    Randall Aiken
     
  3. brad347

    brad347 Member

    Messages:
    4,813
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    hey, anyone else ever notice that whenever Aiken's name is invoked he replies to the thread? Can you set some sort of notification up to notify you whenever certain words come up?

    :)

    Now I know that if I ever want a good answer to a tube amp related question, just type the word "aiken" in the post somewhere.
    :banana
     
  4. reaiken

    reaiken Member

    Messages:
    1,798
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC
    Ha! I regularly search the name, just in case someone is having a problem with one of my amps, or a question regarding them. It's part of the overall customer service package. ;)

    Otherwise, I try to stay out of most threads to avoid the appearance of spamming.

    Randall Aiken
     
  5. CitizenCain

    CitizenCain Member

    Messages:
    4,831
    Joined:
    Jan 10, 2004
    Location:
    Magdalena, NM, US
    Hahaha! Words from the man himself :D

    OK, here's what I just edited up on my in-progress schematic.

    http://www.visi.com/~sstolle/sundown/sd1000h-mod4.GIF

    So you're saying that I really can't do it this way, I need to return the feedback to the 2nd input, and sum the two signals.

    Are you not saying because it's a trade secret, or are you just trying to make me think? :confused:

    'Cause I'm a noob at this stuff and I don't think too good yet :messedup

    BTW, here's the current schematic that I started from (I'm playing around with the preamp, too), http://www.visi.com/~sstolle/sundown/sd1000h-mod3.GIF
     
  6. reaiken

    reaiken Member

    Messages:
    1,798
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC

    No, you can't do it the way you have it drawn. That will give you reverb, but not the correct global feedback, because you've broken the main path for the feedback, which summed into that 0.02uF cap node. Put that back the way it was in the original schematic, and think of some way to sum your reverb into that second grid along with the feedback signal, without excessively loading down either one.

    Either that, or study up on opamp theory and treat the PI/output tube stage as a vacuum-tube opamp and figure out how summing junctions work in inverting and non-inverting configurations.

    That's all the hints you're getting out of me! :)

    Well, maybe one more hint:

    When you do get it built, here is how you determine whether or not your global negative feedback is working: Disconnect the feedback wire from the output transformer and apply a 1kHz sine wave to the amp input, setting the volume level so the output is not clipping. Reconnect your feedback wire, and if the signal drops, you have negative feedback. If want 6dB of feedback, the signal voltage should drop by half, so a 10Vp-p signal will drop to 5V p-p. If the signal increases, you have positive feedback.

    Also scope both phases of the phase inverter - you should see fairly closely matched levels when the feedback is applied. You may or may not see matched levels without the feedback, depending on how well the PI is designed. Ideally, the levels should be identical both with and without feedback.



    RA
     
  7. CitizenCain

    CitizenCain Member

    Messages:
    4,831
    Joined:
    Jan 10, 2004
    Location:
    Magdalena, NM, US
    OK, I'll take a crack at figuring this out and give it a try. I'll post back on how well it works, or badly I screwed up!
     
  8. CitizenCain

    CitizenCain Member

    Messages:
    4,831
    Joined:
    Jan 10, 2004
    Location:
    Magdalena, NM, US
    So, summing the reverb and NFB at the 2nd PI grid, is it a simple as running the reverb signal in through a 100k resistor (matches the 100k feedback resistor)? I'm feeble.
     

Share This Page