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Thread: Input decoupling capacitor - before V1 grid. Should I?

  1. #1
    Supporting Member jmaf's Avatar
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    Input decoupling capacitor - before V1 grid. Should I?

    Most amps connect the guitar input directly to the grid of V1. Some do it via a grid stopper, which is pretty common as well.

    My question is: what are your previous experiences with decoupling the guitar from V1 input via a capacitor? Such as the Orange Tiny Terror?

    Do you think it increases safety, by keeping the musician far away from a plate-grid short?

    Does it negatively affect tone if for example I used a large value as to attenuate as little as possible of the audio spectrum?

    When the musician touches the strings, does it still quiet the amplifier up as DC-coupled ones do?

    Thanks in advance for any insights.
    "Tell them I said something." - Pancho Villa's last words
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  2. #2
    Old Timer defaced's Avatar
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    The caps I have seen in circuits haven't been physically large enough for the values they are to be high voltage caps. This indicates to me that this is purely a sonic thing. A plate-grid short has the plate and grid resistor in series with it, so you'd get a nasty bite, but it's not likely to take you out. 325v B+ on the first stage (which is high), 100k plate + grid resistance (which is low), and you get 3.25 ma of current. Usually between 10 and 50 ma and with high voltage and you start having serious safety concerns.

    Ground is still DC connected, so you still get noise reduction from the guitar's string ground.

    IMO, it's purely option, and in my experience, doesn't do anything to the sound (with the size caps I've used - 0.22u ). Yes, that statement does conflict with my first statement that this is purely a sonic thing. That's because I honestly don't have a good reason why they are used.
    -Mike

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    Supporting Member jmaf's Avatar
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    Quote Originally Posted by defaced View Post
    The caps I have seen in circuits haven't been physically large enough for the values they are to be high voltage caps. This indicates to me that this is purely a sonic thing. A plate-grid short has the plate and grid resistor in series with it, so you'd get a nasty bite, but it's not likely to take you out. 325v B+ on the first stage (which is high), 100k plate + grid resistance (which is low), and you get 3.25 ma of current. Usually between 10 and 50 ma and with high voltage and you start having serious safety concerns.

    Ground is still DC connected, so you still get noise reduction from the guitar's string ground.

    IMO, it's purely option, and in my experience, doesn't do anything to the sound (with the size caps I've used - 0.22u ). Yes, that statement does conflict with my first statement that this is purely a sonic thing. That's because I honestly don't have a good reason why they are used.
    Thanks for your input. Yes, you pointed something interesting out: they're normally high voltage caps...which means they're probably made to withstand some form of high voltage DC. Could it be a safety thing only since they don't change tone much?
    "Tell them I said something." - Pancho Villa's last words
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  4. #4
    Old Timer Tom Phillips's Avatar
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    A bit of additional information.
    You will find series input caps in old designs that used a grid leak biased input stage. See the Fender 5C1 Champ and the 5B3 Deluxe for a couple of examples. The cap was in the circuit to prevent the pickup load from disturbing the bias of the input stage. The cap is not required when the input stage is cathode biased. If an input DC blocking cap was needed for safety, I'm sure the certification agencies would have required modern amp builders to add it.
    Cheers,
    Tom
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    He actually said that they're not high voltage caps.

    I can't say as I never investigated the rating on these caps. But I would think that a cap in this position can do a couple of different things. One thing it would do is decouple the giutar output load from the amps input load. So the volume control on the guitar would no longer be in parallel with the 1M V1 grid load. Logically this also creates a phase shift too for better or worse. It does isolate the V1 grid from any other voltage that could throw off bias. This seems to keep with logic used in all other preamp circuits and makes good electrical sense. It can be used as a high pass filter. Sometimes a tube itself will leak a little and cause a scratchy guitar volume. This won't happen with the input cap, again for better or worse since this is often the best indicator of the problem. There are other consideration too but I'm not able to come up with them right now. I have considered using them at times but haven't gotten around to it. I may never since the pros and cons all seem subjective.

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    Senior Member Enzo's Avatar
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    The incredibly rare plate to grid short in a tube is the last thing on anyone's mind.

    In a grid leak bias you need a cap so the outside world doesn't swamp your tender grid bias. But grid leak bias is also pretty darn rare.

    The input cap is there to keep DC off the tube grid. Imagine if you have some effect pedal that has a few volts of DC offset at the output. Now how will your amp react if the input stage now has +5v there? or -5v there? A largish value cap that passes signal to the grid but blocks DC, solves that problem.
    Education is what you're left with after you have forgotten what you have learned.

  7. #7
    Supporting Member jmaf's Avatar
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    Thanks Tom, Chuck and Enzo, all good points there. I will add a large cap to my next project, I liked the possibility of avoiding the scratchy pot and also effect pedals with a DC offset. I'll rearrange the 1M grid leak post capacitor, as now it is on my input jack. Appreciate your feedback, once again.
    "Tell them I said something." - Pancho Villa's last words
    For Portuguese speakers: Compreenda seu Amplificador Valvulado

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    Quote Originally Posted by jmaf View Post
    I'll rearrange the 1M grid leak post capacitor, as now it is on my input jack.
    Yupper. With an input cap ahead of the grid load the tube has no bias. Tubes perform very poorly with no bias (sarcastic understatement).

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    Well, keeping DC of the grid, keeping high voltages of your hands and keeping low freq from entering your amp.....

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    Quote Originally Posted by Enzo View Post

    The input cap is there to keep DC off the tube grid. Imagine if you have some effect pedal that has a few volts of DC offset at the output. Now how will your amp react if the input stage now has +5v there? or -5v there? A largish value cap that passes signal to the grid but blocks DC, solves that problem.
    Speaking to the designer of the TT, this is indeed the purpose of the DC blocking cap on the input.

  11. #11
    Noodle of Reality Steve Conner's Avatar
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    In an old thread, we theorized that an input blocking capacitor would limit the ability of drive pedals to overdrive the first stage. Any grid current would just charge up the cap and make the bias bounce around. I argued that this might make the amp seem less "pedal friendly".

    One of my old homebuilt amps does have an input coupling capacitor, but it's a high-gain master volume type thing. I don't use it with drive pedals.

    And, it is considered good practice in modern design to put DC block capacitors on the inputs of things, for the reason mentioned above.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Quote Originally Posted by Steve Conner View Post
    In an old thread, we theorized that an input blocking capacitor would limit the ability of drive pedals to overdrive the first stage. Any grid current would just charge up the cap...
    I find that hard to believe, considering all pedals have an output capacitor already! An extra cap at the input of the amp would simply reduce the effective coupling capacitance a tiny bit (assuming it has a large value).

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    Quote Originally Posted by Merlinb View Post
    I find that hard to believe, considering all pedals have an output capacitor already! An extra cap at the input of the amp would simply reduce the effective coupling capacitance a tiny bit (assuming it has a large value).
    You've never heard of the "Jedi Cap Trick" ?? no, I suppose not.... :[

    -g
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    The Jedi cap trick involves using the farce... Uh... Force. You have to believe in something that cannot be seen or proven.

    And a little green dude messes with your mind.
    "I should have been born sooner. Of course, if I had been, I might be dead now." trem

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    Actually, it was believed to be practiced by Eric Clapton on some of his rigs. :}

    -g
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    Quote Originally Posted by mooreamps View Post
    You've never heard of the "Jedi Cap Trick" ?? no, I suppose not.... :[
    AFAIK, the Jedi cap trick is the practice of placing a poly' cap in parallel with an electrolytic cap (usually in the PSU). I don't think it has anything to do with this discussion.

  17. #17
    Noodle of Reality Steve Conner's Avatar
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    These are not the diodes you're looking for...
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  18. #18
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    Quote Originally Posted by Merlinb View Post
    AFAIK, the Jedi cap trick is the practice of placing a poly' cap in parallel with an electrolytic cap (usually in the PSU). I don't think it has anything to do with this discussion.

    It was the input cap trick used by EC on some of his gear... Others have called "that" by this name.. How am I supposed to know better by what I read on some of these talk forums ?? :|

    Besides, TWNTAWWLF. :}


    -g

  19. #19
    Noodle of Reality Steve Conner's Avatar
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    Well, I spent a happy Sunday afternoon hacking tube circuits, and found something relevant to this thread.

    My Ninja Deluxe amp always had an annoying pop when switching channels, and today I was determined to track it down. The amp uses relays for channel switching. Right in front is a relay that connects the active channel's grid to the input jack, and shorts the unused grid to ground.

    To cut a long story short, the tube grids had a little DC on them. One channel had 33mV, the other had 5mV. Since the grid leak resistor is 1M ohm, this implies only a few tens of nanoamps of grid current, and I believe it's quite normal: wasn't this how grid leak bias worked? But anyway, when the channel input was shorted, it also shorted this DC voltage, and that was what caused the pop.

    Adding an input coupling capacitor to each channel cured it completely, because it took the DC off the relay contacts. So there's one more reason to have input capacitors, over and above the ones we've discussed.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    That interesting about the gird voltage, I had an amp that was hissing like crazy and by changing the valve there was a noticable difference in hiss and the grid voltage was significantly lower too. If you stuck in a higher grid stopper resistor the voltage went up and so did the hiss.

  21. #21
    Senior Member ThermionicScott's Avatar
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    I have a Strat-copy with a scratchy volume pot that I've been too lazy to clean or replace. Whenever I plug into my 5B3/5B5 build, that scratchiness is much reduced and it's nice. Still should clean it sometime, though...

    - Scott

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    I rebuilt an old Traynor Voice Master PA into a single channel Super Reverb once for a friend. He had some old tubes he wanted to use so I put them in. One was a smooth plate Telefunken 12ax7 that I stuck in the V1 position.

    When I fired it up all semed fine until I turned down my guitar and still had a small amount of volume.?. So I turned it quickly back and forth, no fix. I figured it for a bad pot until my friend told me that his guitar was doing it also.?. Since there's nothing to speak of between the guitar and V1 grid I replaced V1 and the problem went away. Not sure what could cause this but it seems an input cap would have made a difference, for better or worse.

    Since the amp sounded fine except for the wierd guitar volume thing I can't say if there were any bias issues with the Telefunken that may have been complicated by a cap. The replacement didn't sound any different than you might expect changing V1 for a different brand. Broke my heart to throw away his Telefunken smooth plate 12ax7. I don't personally like them for guitar amps but I know how coveted they are.
    "I should have been born sooner. Of course, if I had been, I might be dead now." trem

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