amplifier input impedance

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers T' started by hustist1, May 31, 2005.

  1. hustist1

    hustist1 Member

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    I've been reading all I can find about tube amps for the last 6 months (and there is a huge amount out there) and am about to attempt my first project.

    What I want to build is a 1-tube practice amp, with an ecl86.
    I wanted to use an ecl80 (lower output) but apparently the shared cathode in that tube causes problems.

    My problem is that I don't see how I determine what to put in between the input jack and the grid of the triode. All the schematics I found (and there aren't many) use the tube for a small hi-fi amplifier, but the impedance of (for example) a CD player or tuner is obviously very different than that of a guitar pickup. I've been looking at the Champ schematic, but I'm not clear how the differences between a 12a?7 and the triode of the ecl86 affect the input.

    If someone could enlighten me, it would be helpful. Thanks.
     
  2. John Phillips

    John Phillips Member

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    It shouldn't matter. AFAIK the triode section is the same as a 12AX7, but if not it won't make any difference to the input impedance requirement.

    The classic tube amp input section is a 1M-ohm resistor between the jack and ground, and 34K (the two 68K resistors of the #1 and #2 jacks in parallel when you're using #1) between the jack and the tube grid - a 33K resistor is fine if you're just using one jack.

    I think an ELC82 is the same pin configuration as the ECL86, with lower power output, but I could be wrong.

    It won't have a lot of gain without the second triode stage, BTW.
     
  3. hustist1

    hustist1 Member

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    Thanks; I thought it might be that it didn't make any difference. I wanted to hear it from someone, though.

    The gain doesn't matter much to me, I want a clean tone and don't play music that requires high gain. As long as I hear something and it sounds good enough, I'll be happy. The idea started when I considered pitching my pignose in the trash so I wouldn't be tempted to use it, but I think even so the transformers will be too big to fit in that little box.
     
  4. Ayan

    Ayan Supporting Member

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    It NOT that it doesn't matter. What happens is that the input impedance will be basically 1 Meg (provided you put a 1 Meg resistor between the input and ground), because the grid stopper resistor, which is what you were referring to, will be in series with the input impedance of the tube, and that will be a large number when compared to the 1 Meg resistor connected between the input and ground. If you wonder why most people use a 1 Meg, it's because lower values may load down your guitar pickups, and going the other way, numbers much higher than that will make the input noisy because it will turn it into a very high impedance network.

    Now, as for the grid stopper resistor itself, you know Fender's value, Boogie used none in most of their "Mark" amps, etc. For a cleaner and slighlty darker sound, use a larger value (you can try anything from 0 to as high as you want). For maximum brightness and dynamics, don't use one at all. However, note that the grid stopper does serve a purpose when mounted right at the tube's grid: it helps prevent stray signals from getting into the stage. The way Fender mounted theirs, at the input jack, probably did nothing to help in this area.

    Hope this clarifies things a little more for you. For what it's worth, I use 33K in my amps' first stage; some people can hear a significant difference between say 33K and 22K, while others can't.

    Cheers,

    Gil
     
  5. TheAmpNerd

    TheAmpNerd Member

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    Everyone seem to say it ends up 34K, but it doesn't.

    I've measure plenty of amps and it stays 68K.
    Why is that?
     
  6. Ayan

    Ayan Supporting Member

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    If you plug "something" into Input 1 in a Fender amp, the value of the grid stopper will be 34K, measured from the grid of V1A to the "+" of the input 1 jack, unless the contact in the input #2 switch is not closing properly (a seldom -- but not so seldom -- thing with all switchcraft type switching jacks).

    Gil
     
  7. hustist1

    hustist1 Member

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    Hmm, interesting. I should be ready to prototype this soon; looks like I have some more values to play with I wasn't expecting. Thanks for the info.
     
  8. TheAmpNerd

    TheAmpNerd Member

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    Checked again....

    ...shame on me.

    ...I've done this about a dozen times
    and it isn't brain surgery...I don't know
    how I was screwing it up.

    Ciao
     

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