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Thread: what does this input resistor do?

  1. #1
    Supporting Member Randall's Avatar
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    what does this input resistor do?

    What is the purpose of this 1 Meg resistor at the input to ground? What would you expect would be the result if it was not there? I have this amp in my shop, and noticed this resistor is broken, but the customer loves the sound of it and is reluctant to let me replace it. I want to replace it.

    http://archive.gibson.com/Files/schematics/ga-25.gif
    It's weird, because it WAS working fine.....

  2. #2
    Senior Member Old Tele man's Avatar
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    Gridleak bias.
    ...and the Devil said: "...yes, but it's a DRY heat!"

  3. #3
    Supporting Member Jazz P Bass's Avatar
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    #1, the 1 M resistor sets the input impedance for the stage.
    It will not load down the guitar signal at 1M.

    #2, it helps drain off any residual grid voltage. (ie: grid leak resistor)

    Good info here: https://robrobinette.com/How_Amps_Work.htm#Signal_Input

  4. #4
    Supporting Member Chuck H's Avatar
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    To elaborate a little... That resistor maintains the input tubes bias by referencing the grid to 0V WRT the elevated cathode voltage. Since the DCR for a guitar circuit is relatively low the bias is maintained AS LONG AS A GUITAR IS PLUGGED IN. If you were to plug a cable into the high gain input with no guitar attached to the other end there is a good chance the tube will "run away". Meaning the bias would be hot enough to potentially fail the tube. There are some variable as to how that resistor (or lack of one) could affect tone. Using the "high gain" input I don't think you would notice any difference with a guitar turned up full, but at lower guitar volume settings there could well be some audible change between having the resistor there or not. If your guy uses one of the "instrument" inputs I don't think there would be any audible difference under any circumstances. So if he uses one of the instrument inputs I would just put the resistor in. If he uses the high gain input, but doesn't play with the guitars volume control much you may want to let him hear it both ways. Don't tell him which is which though. That should settle the matter.
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  5. #5
    Supporting Member Randall's Avatar
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    What I didn't mention is he just bought the amp, played it briefly, LOVED IT, and brought it to see what that tearing sound was. It was a torn and partially unglued speaker. I pulled it to get it reconed, and found that the other speaker was seized up. So he loved the sound of the amp with two blown speakers, so there's that. I don't think the resistor will make much difference in the face of these circumstances. I replaced it.

    But thank you for helping me to understand it better.
    It's weird, because it WAS working fine.....

  6. #6
    Senior Member Enzo's Avatar
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    The guitar is essentially in parallel with the resistor, so imagine putting a 1 meg resistor across your guitar - no audible effect. But it does provide a ground reference for the tube grid. A lot of the time you are not plugging a guitar into the amp. Often you are plugging an FX pedal into the amp. And as mentioned elsewhere, if that device has an output cap, that would leave your grid floating without the 1 meg.
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  7. #7
    Old Timer J M Fahey's Avatar
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    brought it to see what that tearing sound was. It was a torn and partially unglued speaker. I pulled it to get it reconed, and found that the other speaker was seized up. So he loved the sound of the amp with two blown speakers,
    So in his own words he's acknowledging heīs tone deaf ... or plain deaf, period.

    Your mistake having mentioned that resistor to him instead of plain replacing it, now he'll FOCUS on that and the moment he gets bored/tired about his amp heīll accuse you of having ruined it.

    Obviously he's "listening with his eyes", he *sees* itīs an old untouched tube Gibson and that's enough for him.
    Oh well.

    And replacing that resistor is not a matter of "taste" (of which he has none) but of basic functionality.

    That schematic shows 1.2 bias at the cathode .
    Why did Gibson bother to write that?
    Because it is IMPORTANT .

    Now what does it exactly mean?
    "Absolute" voltage does have no meaning, itīs always referred to some other point, in this case ground, and for that tube it means cathode is 1.2V positive respect to grid.

    And if we "float" the grid?
    Then cathode voltage will jump up in an uncontrolled way, plate voltage will go down, again uncontrolled, and grid voltage will go "anywhere" ,who knows?

    This will still work, sort of, because there is a 47k + 22k resistor going from grid to ground ... as long as nothing is plugged into the high gain input (actually the normal gain one, all others are attenuated).

    As long as that jack contact opens, grid floats unless a resistive path is connected (magnetic pickup or guitar volume control) , so it will NOT work with a perfectly functioning cap coupled pedal or preamp which does not have a resistor in parallel with output and even less with a straight Piezo pickup, go figure.

    Asking him to decide whether to replace that resistor or not carries a 50% chance (or more) of taking the wrong decision.
    Juan Manuel Fahey

  8. #8
    Supporting Member Randall's Avatar
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    Gotta defend my customer here Juan. He is a recording studio owner, who makes very good records, mostly on vinyl, knows his stuff, has a great sounding band, and a collection of high end studio mics, which he does know a lot about. So, can we cut him some slack here for this situation?

    He liked the distorted and perfectly awful dirty sound from this broken amp, does that make him clueless, or just adventurous in looking for new tone colors? He is a young man, BTW. I'm not sure how he will feel when I return the 1948 Gibson functioning properly, but that will be his issue to deal with. I will have done my job.
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    It's weird, because it WAS working fine.....

  9. #9
    Senior Member Enzo's Avatar
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    He would never know either way about the 1 meg resistor. So leave his blown speakers as is, and give him a working amp.

    He is apparently looking at the amp more as an effect than as a working useful guitar amp.
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  10. #10
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    Quote Originally Posted by Randall View Post
    Gotta defend my customer here Juan. He is a recording studio owner, who makes very good records, mostly on vinyl, knows his stuff, has a great sounding band, and a collection of high end studio mics, which he does know a lot about. So, can we cut him some slack here for this situation?

    He liked the distorted and perfectly awful dirty sound from this broken amp, does that make him clueless, or just adventurous in looking for new tone colors? He is a young man, BTW. I'm not sure how he will feel when I return the 1948 Gibson functioning properly, but that will be his issue to deal with. I will have done my job.
    This reminds me of visiting the Acropolis and seeing them doing a restoration on it: To what point of ruin are you going to restore it?
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  11. #11
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Randall View Post
    Gotta defend my customer here Juan. He is a recording studio owner, who makes very good records, mostly on vinyl, knows his stuff, has a great sounding band, and a collection of high end studio mics, which he does know a lot about. So, can we cut him some slack here for this situation
    Well, you are now adding a lot of data *after* the fact, donīt you?
    Is that fair?
    In any case: I am questioning his broken/nonfunctioning speaker preference. Period.
    I find it very hard to match that to your flying colours description.

    I also strongly doubt somebody who does not even notice one speaker (out of two) is not working at all, while the other is unglued buzzy (a mountain size problem) , and yet will reject your replacing a 1M resistor in parallel to a 69k one (if he uses one of the "Instrument" inputs as I guess) , which of course he would not even have known about or noticed if you hadnīt told him.

    In any case, thatīs a very minor matter, I think he will faint when he hears the amp, not only electronically repaired but with both speakers working properly.

    He is a young man, BTW.
    ??????? So ??????
    Is that even remotely related to what we are talking about?
    I'm not sure how he will feel when I return the 1948 Gibson functioning properly,
    If he loved the buzzy broken sound, then a fair guess is that he will not like the new one.
    but that will be his issue to deal with.
    That sounds kinda cold when applied to a good , paying Customer.
    I will have done my job.
    I applaud you for that .... hope *he* does not pop a vein or something.

    EDIT: this is a killer sounding one:

    and I would wholeheartedly agree with anybody who also likes it.
    By the way: both speakers were replaced by modern ones, ceramic to boot.

    Now, somebody who likes a malfunctioning one with just an unglued buzzy speaker and the other stalled? ...... gimme a break.
    Or at least donīt expect me to be awed by him .... at least not by his amp and (dead) speaker taste.
    Last edited by J M Fahey; 12-04-2017 at 07:12 AM.
    Juan Manuel Fahey

  12. #12
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    That one meg resistor does load the pickup some, and thus has some effect on the tone. The pickup is also loaded by the vol. and tone pots, which have more of an effect. Since the guitar pickup-cable circuit is resonant, the effect of the loading is to lower the Q (flatten the peak a bit) of an already low Q circuit. Believe me, this guy is not hearing the lack of loading by the 1 meg resistor!
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  13. #13
    g1
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    I'll agree with Enzo about leaving the speakers. Or completely replacing them so he can re-install the blown originals if he so desires, for that "special" tone.
    Certified Dotard

  14. #14
    Supporting Member Randall's Avatar
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    One is already re-coned, he did not like the sound of that one when it was pushed. The other has been sent to be re-coned, it was seized so he didn't notice it because he was barely hearing it.
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    It's weird, because it WAS working fine.....

  15. #15
    Supporting Member The Dude's Avatar
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    If it was siezed (shifted gap), it can't be reconed without demagnetizing, repairing the magnet/pole piece structure, regmagnetizing, and reconing. I hope it was an expensive speaker! It's not going to be cheap to repair. That, and if the actual magnet is cracked, it's near impossible to fix. Most guitar amp speakers aren't worth the effort in this case.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  16. #16
    Supporting Member Randall's Avatar
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    Ted Weber did that for me once for $10 to a speaker that didn't need to be reconed.
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    It's weird, because it WAS working fine.....

  17. #17
    Supporting Member The Dude's Avatar
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    Certainly possible, but rare. Usually, when the magnet shifts, it deforms the voice coil form and chews up the coil. I'm not sure I would trust the speaker without being reconed.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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