How do you diagnose possible tube problem?

Discussion in 'Amps and Cabs [BG]' started by Crackhead666, Nov 18, 2021.

  1. I have a ampeg SVT 3
    With i believe 5 tubes. This last week have noticed a ugly growl on E string from 3rd fret to 7th. Point is its using a lot power to reproduce sound there than g string 12 ft. I had a single preamp tube on last amp . which exhibited same type of problem so I replaced tube and it worked.
    Not sure if tube problem but besides cables basses ect. Now think time to go tubing. Truth I know nothing!! So I am looking for some simple tube inspection technics. You tube got a guy tapping on his SVT 2 with chopsticks . I got chop sticks. I know supposed to look for discolorations on tubes any thing else I should look for?
    Thank you.
  2. jeff62

    jeff62 Supporting Member

    Oct 24, 2005
    Central FL
    Preamp tubes are cheap. Buy a couple and methodically replace one at a time and then test.
    Ampslut, friend33 and Crackhead666 like this.
  3. beans-on-toast


    Aug 7, 2008
    Take this with a grain of salt but there is useful information.

    From an Ampeg user 's manual:

    Any time you notice a change in your amplifier's performance, check the tubes first. If it's been a while since the tubes were replaced and the sound from your amplifier lacks Punch, fades in and out, loses highs or lows or produces unusual sounds, the power tubes probably need to be replaced. If your amplifier squeals, makes noise [ed. including crackling, popping, or hissing, frying bacon], loses gain, starts to Hum, lacks "Sensitivity", or feels as if it is working against you, the preamplifier tubes may need to be replaced. The power tubes are subjected to considerably more stress than the preamplifier tubes. Consequently, they almost always fail/degrade first. If deteriorating power tubes aren't replaced they will ultimately fail. Depending on the failure mode, they may even cause severe damage to the audio Outputtransformer and/or other components in the amplifier. Replacing the tubes before they fail completely has the potential to save you time, money and unwanted trouble. Since power tubes work together in an amplifier, it is crucial that they (if there is more than one) be replaced by a matched set. If you're on the road a lot, we recommend that you carry a spare matched set of replacement power tubes and their associated driver tubes. After turning off the power and disconnecting the amplifier from the power source, carefully check the tubes (in Bright light) for cracks or white spots inside the glass or any other apparent damage. Then, with the power on, view the tubes in a Dark room. Look for preamplifier tubes that do not glow at all or power tubes that glow excessively red.

    According to Groove Tubes, the tubes should be changed if your amp is experiencing any of the following symptoms:

    • loss of tone, clarity, Sustain and harmonic richness
    • inconsistent output level
    • lack of mid-range Punch and Definition
    • rattling, whistling or humming
    • feedback or metallic sound on certain notes
    • Weak sound and power loss.
    GreaserMatt, MrKpawz, JimmyM and 7 others like this.
  4. Wasnex


    Dec 25, 2011
    A good shop would typically use a tube tester to check the health of the tubes. Tube tester can check for gas and measure factors like emissions and transconductance.

    Since you probably don't have a tube tester, you can roll good tubes through the amp. The SVT-3 Pro uses four 12AX7s and one 12AU7, so you need one of each tube type.

    Other common names for 12AX7: 7025, ECC83, ECC803. Also, 5751 is essentially a 12AX7 with slightly lower gain.

    Other common names for 12AU7: ECC82, ECC802.

    There are also a variety NOS (new old stock) designations for these tube types as well.

    When you are rolling tubes, I suggest only replacing one tube at a time. This is so you can actually identify which tube is bad. Get rid of bad tubes, and keep good tubes as spares. Or if the old tubes sound better than your new tubes, leave them in the amp.

    The chop stick test is good to identify a tube that is either microphonic are sensitive to mechanical shock. To perform this test, hook up the amp to a cab and set the gain and volume for medium output. Tap each tube. Each tube should produce a dull sounding, well damped transient. If a tube rings or produces an obnoxious mechanical rattle, it may actually work fine in low gain tube stages. I.E. a tube may be problematic in one circuit but work perfectly fine in another.

    Brand new tubes may be microphonic or sensitive to mechanical shock...try to get a replacement or refund. Short plate tubes are generally less likely to have these problems than long plate tubes.

    Good luck!
  5. After reading the according to grove tubes. I have noticed there's also been a loss of output but not all the time I just thought Im losing my mind. There's a lot of help . I guess ill be busy screwing with tubes.
    beans-on-toast likes this.
  6. Wasnex


    Dec 25, 2011

    You may just need to work them in and out of the tube sockets to clean up the connection.
    friend33 likes this.
  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Or, correctly identify the cause of the symptoms and repair what’s actually bad.
    AGCurry, salcott, DJ Bebop and 2 others like this.
  8. El Güero

    El Güero Inactive

    Oct 5, 2015
    In my limited experience, a tube can check out just swell on the tube tester, but still cause funny noises when used in an amp. Trial and error...the proof is in the pudding.
  9. themickster


    Oct 4, 2015
    Only mess with the inside of the head if you are trained and qualified. They run at a deadly 400 or 500 volts. New valves/tubes need biasing by someone that knows what they are doing. Wrong bias will either make the head underpowered or burn the valves out early. Also running the head with faulty valves will damage it.
    salcott and pcake like this.
  10. bobba66


    May 18, 2006
    Arlington, Texas
    When I was a kid my Dad would pull the tubes out of the tv set and take them up to Sun Rexall Drugs. They had a machine there that you could use to test the tubes. Surely a device like that still exists in our modern era?:woot:
    Crackhead666, pcake and bobyoung53 like this.
  11. Analogeezer


    Jul 29, 2021
    The SVT-3 Pro uses a tube based preamp (I believe the graphic EQ is IC based though) into a MOSFET power amp section.

    So no biasing of the tubes involved, BUT the MOSFET section can require biasing but not based on tube replacement. Biasing the MOSFET amp section is tricky and best left to a COMPETENT TECH (which are becoming more and more rare).

    The advice about retubing it was good, I would retube the entire thing one at a time and find the bad tube, IF that is the cause of the problem. Then keep the old tubes as spares.

    I fully retubed my SVP-Pro preamp (which is basically the preamp section of the SVT-3 Pro in a single rack space) with some high faluttin matched tubes shortly after purchase. I of course kept the original tubes as spares but have never needed them.

    Preamp tubes generally last a long, long time so it could be the OP has another problem with the amp that is not preamp tube related.

    Wasnex likes this.
  12. Bboopbennie


    Jun 16, 2019
    This is good info. The plate voltage is high 400 volts >. Caps are not huge, but it might feel like a Aligator bit off the end of your finger. If you are going to mess with this, look up " Chicken Stick " and how to use it. I have an amp that uses a 3-500 zg tube @ 3000 volts on the plate. It Will kill you if you are careless ?
    Last edited: Nov 19, 2021
  13. TheSpinDoctor


    Apr 28, 2021
    The Ampeg SVT-3 PRO has three pre-amp tubes (V1, V2, and V3) and two tubes in the power amp section (V1/JL1 and V2/VL2). V1 - V3 are all 12AX7 tubes. V1/JL1 is a 12AX7 and V2/JL2 is a 12AU7. Wasnex provided useful guidance for testing the tubes. If you have another power amplifier or a bass amp with a power amp input, you can bypass the tubes in the power amp section. If the problem exists using the pre-amp section of the SVT-3 PRO amp and the power amp section of another amp, then your tube problem likely is with the three 12AX7 tubes. If the problem exists only with using the power amp section of the SVT-3 PRO, then your tube problem is likely with the 12AX7 or 12AU7 tube. If you choose to open the SVT-3 PRO to swap tubes, you will need to drain the charge from the large capacitors in the power supply section. Turn off the SVT-3PRO. [Usually a screwdriver is used to quickly ground the side of each capacitor not connected to ground. A large spark may occur, so take care. This step is not absolutely necessary, but does increase you safety. If you are comfortable working inside a tube amplifier, then skip this step.] After replacing one tube, turn the SVT-3 PRO on and check to see if the sound has improved. Do this between testing each tube in the SVT-3 PRO. The power amplifier section of the SVT-3 PRO has a potentiometer (trim pot) to adjust the bias voltage for the eight output MOSFET transistors. It is necessary to measure the voltage drop across the flame-proof (large ceramic) resistors attached to the gate of each MOSFET. This voltage will vary for each MOSFET. The bias voltage should be adjusted to an average of 25 mVDC for the eight MOSFETs. The trimpot is extremely sensitive, so the smallest adjustment can cause a large change in the bias voltage. Some people have used nail polish to secure the trim pot so movement of the SVT-3 PRO does not cause the bias voltage to change. I opted to purchase a 10-turn potentiometer to replace the one used by Ampeg. Schematics of the Ampeg SVT-3 PRO pre-amp and power amp sections are available online.
  14. themickster


    Oct 4, 2015
    I'm a registered Portable Appliance Tester. I wouldn't like anyone to mess with mains voltage, let alone the inside of a valve head.
    Rich Fiscus and agedhorse like this.
  15. themickster


    Oct 4, 2015
    Fair enough. I know more about electrical safety than I do about valve heads. Thank you for teaching me something.
  16. 40Hz

    40Hz Supporting Member

    May 24, 2006
    Whenever I run into an issue with one of my tube amps beyond swapping out an obviously failed preamp tube, I just bring it to a tech who’s qualified to work on tube amps. Faster and cheaper in the long run. :)

    Luck! :thumbsup:
  17. Brich


    Nov 18, 2013

    I also have a 2007 svt 3 pro, with a strange and ugly distorting sound that's intermittent. I've had the bias adjust and the tech also used a dab of silicone* to secure the adjustment. 2 techs and several tubes swapped with no luck so far. Which is a bummer I love this head
  18. Sushi Box FX

    Sushi Box FX Supporting Member Commercial User

    May 23, 2020
    Chicagoland, USA
    Sushi Box FX, owner
    I have one in my basement, but I will echo the comment above that a tube can test good on simple testers and still have problems in an audio circuit. More expensive testers (my old work had one that was $10k, it was fancy as hell) can test a lot more parameters. If a tube is losing seal or has an internal short those are the kind of things a tester can absolutely pin-point, but not-sounding-as-good-as-it-used-to is harder to test for.
    Wasnex likes this.
  19. micguy


    May 17, 2011
    Those notes are, coincidentally? (not sure) in the ballpark of 50 to 60 Hz - the power frequency. That may or may not have something to do with your issue. Uf the amp has a bunch of hum, you may have a power supply issue.

    A tube tester can check certain things, but things like microphonics it won't catch. One note (if it's a preamp tube) - you can often swap tubes into different positions for things like a slightly microphonic tube - it'll cause a whole lot more problems in the first position, but in later positions, it can be pretty benign. In this case, though = an "ugly growl" - if it's a tube (it could be a lot of things), its probably not a slightly microphonic tube.

    In diagnosing what's up, the first thing I would suggest is to take the head off the cabinet (assuming it's on there when it's sounding ugly). If the growl goes away, then you probably have a vibration related problem - a microphonic tube, a cracked part or pc board trace, a bad solder joint, an input jack that's dirty - something like that.
    byacey and Wasnex like this.
  20. bassdude51

    bassdude51 "You never even called me by my name." Supporting Member

    Nov 1, 2008
    Central Ohio
    I've seen tube testers on Craigs List and I'm sure they are available on good ole eBay and elsewhere.

    Buy a tube tester. Or, perhaps there is an electronic repair shop in your area that will test your tubes.

    Thing is with old used tube testers is that the capacitors go bad in them. So, tube testers need to be tested and evaluated.

    Ahh, welcome to the wonderful world of tube amplifiers! Ain't it great?

    (I remember when Radio Shack used to carry tubes!)........ Back when I was a kid 50s, 60s, 70s, every hardware store in town had "tube testers" for DIY testing.................obviously, not anymore!

    Damn power tubes are expensive! Wonder why solid state?

    We hope that you figure out the problem.
    Wasnex likes this.