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Thread: Safe to put V1a coupling capacitor on a switch for variety?

  1. #141
    Supporting Member eschertron's Avatar
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    If I was a betting man, I'd "bet a packzi to a donut" that the low-ish impedance at the cathode means less hum introduced there than you imagine.
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  2. #142
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    I broke 1 of the 9 loop and it brought the noise down to 19mV from 27mV..... That was the only one that was an easy break, but yeah, looks like I need to rewire these switches like I should have in the first place.

    Edit..Inverting all switches made it worse. But only by a tiny amount. However, it brought the hiss down so it's still a net gain in reduction.

    What's a good target level in mV at the tap with both gain and volume maxed on a high gain 4 stage like this?
    Last edited by Mr. Bill; 02-13-2018 at 08:54 PM.
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  3. #143
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Mr. Bill View Post
    However, it brought the hiss down so it's still a net gain in reduction.
    This seems odd. The impedance at that circuit is very low and hiss is typically a byproduct of gain relative to Johnson noise (thermal noise) which is a byproduct of circuit resistance. I can't think of anything about the cathode circuits that should change the hiss level noticeably. I fear something you've done has reduced gain or your testing was done in two different gain modes. That is, I don't think you've actually lowered hiss unless you've lowered gain because that's impossible (though there are exceptions that don't apply here).
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  4. #144
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Mr. Bill View Post
    What's a good target level in mV at the tap with both gain and volume maxed on a high gain 4 stage like this?
    They're all different due to grounding scheme limitations. And how can differentiate between hum and hiss just reading AC noise on the output? I can only tell you that with that many stages maxed there should be so much hiss that hum is a moot point That's not necessarily a bad thing, it's just the brakes. I have my own amp open on the bench right now, but it's not a high gain preamp type amp. It's an overdrive the piss out of the power tubes amp and it's only 17 watts. Still... Two stages into a third that boosts to OD the PI that is already ODing the power tubes, so, five stages with the last three clipping. I'll go take a measurement and we can adjust for wattage Be back in a bit.
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  5. #145
    Supporting Member Chuck H's Avatar
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    I have 46mV of hash on my output with the tone stack pots at 5, presence 0 and the volume dimed. Note the presence setting!!! And that 46mV is for a 17W amp. I consider this entirely acceptible. Of course, since I'm dimed and I have no no master volume this is measured with the power amp wide open. Not sure if you're testing like this. At these settings my amps gain is in pseudo metal territory. Probably not dissimilar from yours.
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  6. #146
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    With presence and all tone controls at noon with both gain and volume dimed I'm getting 134mV at 100 watts.

    Will draw schematic to help investigate how hiss was reduced. Total noise came down by about 10mV.

  7. #147
    Supporting Member Chuck H's Avatar
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    We're actually ham handing this. That mV indication is meaningless without knowing the load, so... I'm using an 8 ohm load. So that's .000265-ish watts of noise And by your numbers, unless you're using a very low load, I would think you've done better considering it's a 100W amp.
    Last edited by Chuck H; 02-14-2018 at 04:51 AM.
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  8. #148
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    8 ohms That rogue wire trick of yours shaved off another 10 or so!

    Here's how the switches were wired before.
    ground-switch.png


    Side note. Since elevating the filaments, the amp makes a couple quiet pops while warming up, then never again. Is this maybe something to do with the elevation? Perhaps my larger than needed cap value?
    Last edited by Mr. Bill; 02-14-2018 at 05:52 AM.

  9. #149
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Mr. Bill View Post
    Perhaps my larger than needed cap value?
    Perhaps. If noise goes up when you lower it to 22uf, just bump it back up to 100uf (it'll be as quiet at 330uf fo reals) and since there's no significant current through the circuit regularly it shouldn't matter at all when using the amp other than those little noises at startup.
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  10. #150
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    Popping was cathode follower slowly frying a tube that was good a few days ago. It was getting worse and I tracked it down. Hum with gain off / volume max is now down to 10mV with a gain max / vol max noise level of ~129mV. I kinda think I'm reaching an apex here of as good as it's going to get.

  11. #151
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    Quote Originally Posted by Mr. Bill View Post
    Popping was cathode follower slowly frying a tube that was good a few days ago.
    Some tubes (brands) are not happy in cathode follower positions due to heater to cathode voltages. You may have to get picky for that spot.
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  12. #152
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by g1 View Post
    Some tubes (brands) are not happy in cathode follower positions due to heater to cathode voltages. You may have to get picky for that spot.
    He means Russian tubes Not all of them, but many. I use Shuguangs (China) in CF's.
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