Amp Diagnosis Help

Discussion in 'Glowing Bottle Tube Amp Forum' started by SixStringSlinger, Oct 12, 2019.

  1. SixStringSlinger

    SixStringSlinger Friend of Leo's

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    I have an Egnater Tweaker combo, never had a problem before. Last night while playing I noticed a sort of whine coming from it. Almost like the ear-ringing from tinnitus. It was totally independent of the guitar, or any guitar settings. No pedals being used. It wasn't loud, but it was there.

    This morning it was there again, from the get-go this time. After maybe 20 or 30 minutes, the guitar sound all but cut out, and the whine got louder and more like feedback.

    I suspect this may have something to do with the tubes; some quick web-searching suggests a microphonic or otherwise "bad" preamp tube. But the fact is I know nothing about this. Any ideas what could be wrong, and how to fix it?
     
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    If you don't have any tech skills, swapping tubes is about all you can do. If you've tried that and it didn't fix the issue, it's tech time.
     
  3. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    Gain access to the valves, ensure the speaker is connected, switch on. While it warms up, play some tunes. May as well.

    When the problem starts, put down the guitar, turn the amp up and tap firmly on each valve in turn with the eraser tip of a pencil. Do -not- use a screwdriver blade!

    Any popping, clanking, rattling, feedback or extraneous noises made while doing this signal a valve fault. Make a note of which valve it is.

    If it turns out to be one of the larger power stage valves, replace them both. Combo amps, especially those played loud, can be torture chambers for valves. If you're not happy doing that, then it's tech time.

    If you're not happy doing that, and your amp has an FX loop, plug your guitar into the FX return. This bypasses most of the pre-amp stage. If the sound goes away, it's pre-amp. If it still makes it, it's either the phase inverter valve ( usually the one closest to the power valves ) or a power valve. Phase inverters work -very- hard and oddness can ensue when one starts to climb the glowing curtains to sing in the choir eternal.
     
  4. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    @VintageSG provides some good info, but if you don't know anything about amp tech work it's not going to help.

    As @corliss1 said, all you can do is swap tubes. There qre NO repairs youi can do, and you should not remove the chassis - you could kill yourself even if the amp os unplugged! Many amps store deadly voltage in power capacitors.

    A lesson, though you should own a spare one of each preamp tube and a set of spare power tubes. Not just for these situations - tubes are glass, fragile, and easily damaged just from bumping the amp around.
     
  5. Intubator

    Intubator Tele-Meister

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    But, don't come close to the tip of the pencil with any part of your hand. The graphite can actually be a conductor... A wooden chopstick is what i usually use.
     
  6. King Fan

    King Fan Friend of Leo's

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    Lots of good info here. For the record, it is safe to do the tests mentioned here, and although a pencil shouldn't be used to chopstick wires and connections in a live amp, clinking the outside of live tubes with a pencil cannot shock you.

    If it's a microphonic tube, and it's fairly likely to be, either the clink test or simple replacement may help. The clink test is quicker and easier to try first, especially if you don't have a known-good replacement tube of each type in your amp. If I'm right for a Tweaker you'd only need one good 12ax7 to try in each of the first 3 slots and then a good 6V6 (or two, as mentioned) to test the power slots.

    On average, the small preamp tubes are more likely to go than the power tubes.
     
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  7. Axis29

    Axis29 Poster Extraordinaire Ad Free Member

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    Tubes fail more often than other components in an amp. I hate to admit how many times I've gotten all worked up and panicky over something one of my amps does... I've even pulled chassis and started looking at all kinds of complicated things only to find the simplest solution was the correct one.

    Find some known good tubes and replace the ones you have, one at a time. If a good tube doesn't fix the problem, put the old one back in.
     
    Last edited: Oct 13, 2019
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  8. SixStringSlinger

    SixStringSlinger Friend of Leo's

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    Thanks for the replies. I pulled the amp open and removed the metal covers from the preamp tubes (for protection, I guess? They also have springs that I suppose keep the tube properly plugged in). I tried the chopstick-tap on all the tubes and heard nothing special. I tried this with nothing plugged in, with just a cable and with a guitar on the other end of the cable.

    I put the metal covers back on, and here the problem started up again when I tapped the covers. I removed the covers and tapped them all again, and this time one of the preamp tubes rang out, almost as though I had tapped on a spring. Then came the whine, followed by the feedback sound.

    So I'm guessing I have a bad preamp tube.
     
  9. SixStringSlinger

    SixStringSlinger Friend of Leo's

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    This manual...

    http://www.egnateramps.com/manuals/TweakerIM.pdf


    ...seems to suggest that one of the 12AX7's is for the effects loop. I don't use the loop, and that's not the problematic tube. Can I swap them for the time being, if I'm not going to use the effects loop?
     
  10. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yes, that would indicate a bad preamp tube.

    The metal covers are there for electronic shielding so there is less noise getting into the tube. The spring is there to help with microphonic problems, which is that tap test you're doing.
     
  11. 1300 E Valencia

    1300 E Valencia Friend of Leo's

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    Yes, swap those two pre-amp tubes (V3 swapped for V1), see if the problem (microphonic feedback) stops.
    Is the problem tube the one labeled "V1" in the manual? That's the very first tube the signal goes to, usually the tube closest to the input jack, and a very likely suspect even before testing. You may have just fixed your own amp. Congrats!
    But please let us know if it works.
     
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  12. SixStringSlinger

    SixStringSlinger Friend of Leo's

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    Yes, V1 was the problem one.

    So it kind of worked and it kind of didn't.

    I swapped V1 and V2 (you said V3, but V2 is the effects loop one I asked about, so I swapped 1 and 2). The microphony was mostly gone, but there was still a bit of extraneous noise, especially when flicking switches. So I then swapped V2 (the bad one now) and V3. No noise at all, so I rejoiced and put everything back together.

    When I plug in a guitar, though, the output is severely cut. I typically keep the master volume maxed and raise the gain as necessary, usually to about 9 o'clock playing at home. Now I have to take it to at least noon to get a similar volume, and even then it's missing some oomph. I double checked and confirmed that all my other settings are where I normally keep them.

    So the bad tube is now, as far as I can tell, in the power amp section. I don't know much about amps, but this seems to me to explain the output problem. My original idea was to have the bad tube in the effects loop section, but as I said, that was still noisy.
     
  13. Intubator

    Intubator Tele-Meister

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    Sounds like new tube time! Cool that it was a relatively minor issue and you were able to troubleshoot the problem. May want to buy an extra one in addition to the one you'll replace. If the other two have the same kinda mileage on them as the first, then one or both may go soon as well...
     
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  14. Axis29

    Axis29 Poster Extraordinaire Ad Free Member

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    I didn't look up the manual... Are all three preamp tubes 12ax7's? If one is a 12AT7 or some other tube, it'll have less gain. Might effect where it hits in the circuit?
     
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  15. Les Paul lover

    Les Paul lover Tele-Holic

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    My bet is that your problem is solely with the valve you've just stuck in V3 (I assume phase inverter?).


    You need to discard V3. It was causing issues in V1 and V2, and now it's in V3 the output drops.... you need to replace that one.

    As others have mentioned, if you have a valve amp, you need spare valves at hand. You might want to purchase a complete set including power valves anyway. But start by removing the current offending valve, and replacing it.with a good one. Once that's done and your amp is working, chuck the culprit away.

    If you have swapped ECC83 and ECC81 replace them in their correct positions too.
     
  16. SixStringSlinger

    SixStringSlinger Friend of Leo's

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    Hey all. My local Sam Ash had the right tubes in stock so I picked up a complete set and replaced all the 12AX7's in my amp. It sings like a bird now! I tossed the offending tube and I'm keeping the other two (which seem fine for now) as backups.

    Thanks for the advice!
     
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  17. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    I had no doubts :D
     
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  18. MuddyWolf

    MuddyWolf Tele-Meister

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    Good preamp tubes can and should last years but could die at any time. Keep spares on hand and hope you don't need them. Be ready to replace power tubes about once a year if you play every day. I've had power tubes last decades in old fenders. And had them last only months in old marshalls The point is a tube amp is like a pet not a plant. They need feeding and attention.
     
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