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    Popping/Crackling coming from VTA 120 (Not the speakers)

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    Dneu2011

    Posts : 8
    Join date : 2018-04-13

    Popping/Crackling coming from VTA 120 (Not the speakers)

    Post by Dneu2011 on Sun Jul 01, 2018 9:36 pm

    I built a VTA 120 about 6 months ago and used my own tubes. I ended up having multiple issues with tube reliability in which I lost a KT120 and GZ34 GL Reissue, so ended up replacing/testing until I settled on a Weber WZ68 and 4 KT120's (purchased a new pair). The amp had been working great over the last three months without issues until a few days ago. I hooked it up to some newly recapped Dynaco A25's and let the amp play at low volumes to burn in the new caps. I check on the setup every 20 or 30 minutes to listen and after an hour and I half I realized the amp had turned off. I noticed the variac fuse was blown, so I replaced it and turned the amp on. I got a very loud crackling/popping coming from inside the amp, so I shut it off and realized it took out my last fuse. This had happened to me in the past (popping/crackling) and I had suspected it to be a bad driver tube and thought i fixed the issue.

    Today I bought some more fuses and went home to try again. I first tried cleaning all the pins of my driver tubes as well as my power tubes. I only inserted the left channel, turned on the amp, and the crackle came back and blew the variac fuse again. I decided to try a spare set of 6550s (only left channel) and a Sovtek 5ar4 rectifier. Immediately the variac fuse blew (no popping or cracking).

    I opened the amp, checked all the 10k bias resistors and they were fine. Everything else looks good in the amp. I haven't hooked it back up in fear of causing more damage. I was hoping to see if I could get some help troubleshooting. It seems when I use the Weber it pops and crackles for a few seconds before taking out a fuse and the Sovtek 5ar4 automatically takes it out.

    Thanks.

    Dustin
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    sKiZo

    Posts : 1496
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Popping/Crackling coming from VTA 120 (Not the speakers)

    Post by sKiZo on Sun Jul 01, 2018 10:21 pm

    Tried switching back to the original speakers?

    Bypass the variac?

    Check your speaker connections for shorts? Not uncommon to have a strand or two get away from you and short to ground ...

    If changing back to your original speakers fixes the issue, next step is to dig into the Dynaco boxes. Check all your new caps for values, polarity, shorts, etc.

    Dneu2011

    Posts : 8
    Join date : 2018-04-13

    Re: Popping/Crackling coming from VTA 120 (Not the speakers)

    Post by Dneu2011 on Sun Jul 01, 2018 10:48 pm

    Thanks for the recommendations.

    -Just switched back to original speakers and tried without the variac. I blew the amp fuse right away. Speaker connections look good. I even tried new driver tubes just in case those could be the culprit.

    Anythink I should try next?
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    j beede

    Posts : 467
    Join date : 2011-02-07
    Location : California

    Re: Popping/Crackling coming from VTA 120 (Not the speakers)

    Post by j beede on Mon Jul 02, 2018 12:00 am

    No need to damage anything during troubleshooting.

    1) Power down the amp and remove all tubes and "Weber devices"
    2) Insert a 60W (75W or 90W will do) incandescent bulb in series with your mains supply
    3) Power up the amp and watch the bulb. It may glow for a moment at initial turn-on then quickly dim down to a steady state.
    4) If the bulb dimmed or went dark (favorable) move on to step 5. If the bulb did not dim or lit up brightly you have a short circuit to locate.
    5) Power down, then insert all tubes except the 5AR4 or Weber device
    6) Power up and observe the bulb. It should glow brightly for a few seconds then begin to dim noticeably as the filaments heat up. It should not dim down as much as it did in step 3.
    7) If the bulb dimmed move on to step 8. If the bulb did not dim or lit up brightly you likely have a short circuit to locate or failed tube(s) to isolate.
    8) Power down and insert a relatively expendable rectifier (not the Mullard GZ-34 that your great uncle  hand carried home from the war).
    9) Power up and observe the bulb. It should glow brightly for a few seconds then dim noticeably as the filaments heat up. It should not dim down as much as it did in step 6.
    10) If the bulb dimmed your amp may actually play music at this point and you can move on to step eleven. If the bulb did not dim or lit up brightly, or blew a supply fuse you may have a bad segment(s) in your high voltage filtering. All tube audio hobbyists should keep a supply of 450V, 22µF electrolytic caps to swap in for just these sorts of circumstances. Start swapping 22µF in for the segments in your quad cap. DANGER HIGH VOLTAGE even when the amp is unplugged.
    11) Power down and remove the dim bulb setup. Put on your safety glasses and power up the amp. Voila! Or not... in which case come back and give an update.

    ...j
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    Peter W.

    Posts : 897
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Popping/Crackling coming from VTA 120 (Not the speakers)

    Post by Peter W. on Mon Jul 02, 2018 8:48 am

    Some interpolations to this excellent diagnostic process:

    j beede wrote:No need to damage anything during troubleshooting.

    1) Power down the amp and remove all tubes and "Weber devices"
    2) Insert a 60W (75W or 90W will do) incandescent bulb in series with your mains supply

    This lamp will light in proportion to the current running through it - and why it MUST be a conventional incandescent lamp. By the way, a 90 or 100-watt lamp would be preferred as it should be near full brightness ONLY when the Amp is operating at peak load.

    3) Power up the amp and watch the bulb. It may glow for a moment at initial turn-on then quickly dim down to a steady state.

    The only current on this lamp with all tubes removed should be the power-transformer quiescent current. The bulb should be dim, indeed. And there should be no turn-on surge as there are no cold filaments involved.

    4) If the bulb dimmed or went dark (favorable) move on to step 5. If the bulb did not dim or lit up brightly you have a short circuit to locate.

    This short will be in any of four common locations:
    a) The line-cord shorting to the chassis - easy to locate and fix.
    b) One of the transformer input leads shorting to the chassis  - easy to locate and fix.
    c) One of the transformer output leads shorting to the chassis - not as easy, but not hard.
    d) An internal winding short on the main power-transformer. - very nearly always fatal.

    ARE THERE GROMMETS on the chassis penetrations for the transformers? If none, start looking for shorts there.


    5) Power down, then insert all tubes except the 5AR4 or Weber device
    6) Power up and observe the bulb. It should glow brightly for a few seconds then begin to dim noticeably as the filaments heat up. It should not dim down as much as it did in step 3.

    Cold filaments pass a lot of current (bulb is bright). As they heat, their internal resistance goes up - bulb dims. Without the rectifier in place, there will be no B+, so you are seeing only filament and quiescent transformer loads - about 25 watts or so.

    7) If the bulb dimmed move on to step 8. If the bulb did not dim or lit up brightly you likely have a short circuit to locate or failed tube(s) to isolate.

    With the amp in this condition, it is acceptable to remove one tube at at a time. When/if the lamp dims suddenly, and re-brightens, that tube is the (a) culprit. Always keep going with any diagnostic test even if a cause is identified. It may not be the only issue.


    Cool Power down and insert a relatively expendable rectifier (not the Mullard GZ-34 that your great uncle  hand carried home from the war).
    9) Power up and observe the bulb. It should glow brightly for a few seconds then dim noticeably as the filaments heat up. It should not dim down as much as it did in step 6.

    Yes. You are now seeing B+, and the Amp will be pulling about 90% of full load (which will still be below the nameplate rating).

    10) If the bulb dimmed your amp may actually play music at this point and you can move on to step eleven. If the bulb did not dim or lit up brightly, or blew a supply fuse you may have a bad segment(s) in your high voltage filtering. All tube audio hobbyists should keep a supply of 450V, 22µF electrolytic caps to swap in for just these sorts of circumstances. Start swapping 22µF in for the segments in your quad cap. DANGER HIGH VOLTAGE even when the amp is unplugged.

    Or, some sort of mechanical short in the rectifier path, or a bad section in the rectifier itself.  BUT - when swapping, each section, the OEM rectifier section must be OUT-OF-CIRCUIT as if it is shorting, it will remain short. One issue NOT spoken to here if the amp shorts out on this last step is a failed output transformer - only when the rectifier is in place will it see B+, and a short to the chassis will pop the fuse.

    11) Power down and remove the dim bulb setup. Put on your safety glasses and power up the amp. Voila! Or not... in which case come back and give an update.

    ...j

    Now, a few things I would do first, and I do assume you have a decent VOM and perhaps alligator clips for one lead.

    a) Get pin-out diagrams for all tubes.
    b) Connect the black lead from the VOM (on OHMS) to the first filament pin. This is a good place for that alligator lead. Touch each other pin with the read lead - playing attention to the pin-out diagram. You should be getting a reading only on filament or filament center-tap connections. Nowhere else. And you should be able to see why  on the diagram. If you do, the tube is toast.
    c) Do a complete visual exam of the amp, top, bottom and sides. Use a dental pick to check every connection and look for cold-solders, loose wires and so forth. Then:
    d) Use the VOM (on OHMS) to look for shorts between any of the transformer leads to the chassis - use the schematic to determine which ones are relevant - and any of the filter cap elements to the chassis - as some of th ese leads are connected via a resistor - keep that in mind. The schematic is your best friend.


    Another "by the way" - sounds like eggs frying from an AMP and not at the speakers, is usually because something is cooking. As the main filter cap is surrounded by semi-liquid salts, that could be one source. And as the main power-transformer is a bunch of enameled wire tightly wound together, that could be another. And lastly, same for the output transformers when carrying B+.

    Lastly, there is a Kill-Or-Cure for OPT or Power Transformer winding shorts: Obtain a medium-voltage (100V or higher) high-uF capacitor (1,000 uF or better). Charge it fully - requires a DC power-supply or similar meants - and discharge it across the shorted winding. Works about 1/3 of the time.

    nmchiefsfan

    Posts : 46
    Join date : 2012-03-21

    Re: Popping/Crackling coming from VTA 120 (Not the speakers)

    Post by nmchiefsfan on Mon Jul 02, 2018 7:02 pm

    About 5 years ago I had a weber copper cap go bad on me with pretty much the same symptoms. They used to run pretty hot and mine couldn't take it. I don't know if that is the case any more as I haven't used one since. I have had great luck with the newer Tung-Sol rectifiers.

    Does it power up with no tubes/rectifier installed?

    Dneu2011

    Posts : 8
    Join date : 2018-04-13

    Re: Popping/Crackling coming from VTA 120 (Not the speakers)

    Post by Dneu2011 on Tue Jul 03, 2018 9:55 pm

    Hey. I have a very busy month for work, so I ended up bringing it to a local tech to look over and identify the problem section for the amp. He only charges $30 to do this so I figured I would ask for the help. At least he could test all my tubes to determine if some are bad. A few points to include before leaving it:

    -As for powering on the amp, I did test it with ONLY driver tubes and they lit up fine without blowing fuses.
    -I considered the Weber to be bad, but I also tested the amp with a spare Sovtek 5ar4.
    -One possible issue I could have had is I moved my variac to a new location (off the floor). There's a possibility that the voltage could have changed, causing the bias to change and the possibility of damaging some power tubes. Although, I did try a different set of 6550s and they did not work.

    Thanks.

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    Re: Popping/Crackling coming from VTA 120 (Not the speakers)

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