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Tube Crackle?

Discussion in 'Amps and Cabs [BG]' started by carbonfold, Jun 9, 2012.


  1. carbonfold

    carbonfold Supporting Member

    Oct 13, 2006
    Dallas, Texas
    Ok, I have swapped out all the tubes in this amp, cables and bass. Still get the same results. Any idea what this crackling is?

    This is an all tube head with quad of 6550 in power, 5751 in PI, 12AX7 in V2 and V1. Only happens during the first few minutes of warm-up and disappears.

    I can turn the amp off and turn it back on without crackling. However, if I let the amp sit for 20mins, the crackle will come back during warm-up.

    Crackling does change if I mess with the Master volume slightly but nothing additionaly major over what is already there. Again, the crackling does go away after the amp warms up.

    So whatcha think? Cap or resistor bad? Tubes have been completely swapped out (back and forth) and issue continued. Check out the sound clip below.




    http://soundcloud.com/millhasbrook/tube-crackle
     
  2. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    Bad solder joint? Cracked resistor? Both of these could crackle as they warm up and expand.

    What's the history of the amp? Was it in regular use and working fine, then this started? Or is it an old, disused amp you're trying to return to service? I've heard crackling or popping from old tube amps... shortly before the old capacitors finally shorted completely.

    Obligatory warning: tube amps can store lethal electrical charges even when they are powered off and unplugged. If you don't already know how to be safe in there, find out before you open it up!
     
  3. What is the make and model of the amp? You say you swapped out all the tubes, does that mean you changed all the tubes? If the master affects the sound level that means it is in the preamp. Has the sound of 1) A carbon comp plate resistor. 2) A defective tube.
     
  4. Jeff Scott

    Jeff Scott Rickenbacker guru..........

    Apr 11, 2006
    Crackling tubes were a common "feature" of Eden WT amps as they warmed up.
     
  5. carbonfold

    carbonfold Supporting Member

    Oct 13, 2006
    Dallas, Texas
    I put a new quad/set of 6550's, biased and a new set of Preamp/PI 's in there and got this crackle.

    So, I swapped out all the 6550's (re-biased) with another set and the Preamp/PI 's.... same issue.

    I recently had all the switches and jacks upgraded. However, there was a TON of solder laying around in the chassis. So, blew it out and thought that the problem was gone.

    Next day, problem repeats as you can hear from the clip. I have no problem returning it to the tech and having it looked over but crackle comes and goes during warm-up.


     
  6. This is a sign of poor workmanship and an invitation to a future failure!

    Still no make or model so I have no clue if carbon composition plate resistors were typically used in your amp. IF it has carbon comp plate resistors (typically do cause hiss, crackles and pops) in the preamp stages, have them all changed for metal or carbon film. (Maybe by a different tech.)
     
  7. carbonfold

    carbonfold Supporting Member

    Oct 13, 2006
    Dallas, Texas
    Sorry bout that. Sovtek Bassov Mig 100



     
  8. Ah okay I have worked on only one Sovtek, don't remember if it used carbon comps. But a few 100K 1/2 watt resistors are darn cheap to replace. The one I repaired used 6L6 outputs and was a guitar head.
    I do remember it used really high plate voltages (on the outputs at least).
     
  9. LeDuck

    LeDuck

    May 26, 2012
    You probably have too much voltage on your 6550 plates as your B+ is rising to a high level before the cathodes heat up and the tubes begin passing current. B+ will decrease after the cathodes heat up and tubes start passing current.

    You can hook a voltmeter (rated to 600V minimum) to your B+ and see what the plate voltage is doing as the amp warms up. With hot cathodes, the tubes will begin conducting current immediately and the crackling sound will not occur.

    Make sure you have a proper 5AR4/GZ34 (not 5U4G) rectifier tube that warms up slowly and does not allow B+ voltage to your plates before the output tubes heat up. If you are using solid state rectifiers, then you are abusing your output tubes unless you allow the cathodes to fully heat up before applying B+ to the output tube plates.

    I had this trouble with a Heath W5M that came with 5U4G rectifiers that was applying B+ to my plates before the output tubes could conduct. B+ would rise to around 550V before the tubes started conducting!!! The tubes would start to conduct, and the B+ voltage would drop to around 450V. Switching to Sovtek 5AR4 rectifiers solved my problem. The previous owner was ignorant but willing to experiment, and he had made several "mods" that I had to set straight. If you buy used gear from musicians and audiophiles that like to fool around and experiment more than reading technical books, then you had better go through the gear and look for any mods they may have done.

    If your amp has solid state rectifiers, then you had better have a delay circuit that waits for cathode heat to come up before applying B+ or learn how to turn on with amp in standby and wait. You can always install a standby switch and allow the tubes to heat for at least one minute before switching on the B+ supply. Any tech can make this mod if the amp didn't come with it.

    As I did with my own preamp design, you can use a 555 timer IC chip and a relay switch to apply B+ after a reasonable length of time, and I went further and had an output delay that switches on the output after the circuit has settled down after about 45 seconds. This is necessary to prevent damaging anything that is powered through a direct coupled solid state amp with output down to zero or near zero Hz.

    Transformer coupled amps do not have response down to to DC, so that would not be necessary in your case. But many a good audiophile solid state amp and speakers have been damaged by hooking up a vintage tube preamp that slews voltage for a very long time as it warms up. It takes time for the preamp output coupling cap to charge and stabilize. The input side charges to 180V and the output side is tied to ground by a high value resistor on my preamp. That's a big slew, when it only takes about 2V at the input to send a power amp to full output voltage.

    Since electrical power can fail and come back on after a few seconds or minutes, I rather let an automatic circuit power up my tube gear properly that have to operate a standby switch to start up and shut things down properly.....which I can't do if I step out of the room when power fails.

    My tubed Conrad-Johnson control preamp and phono preamp both have these startup circuits from the factory, and all top quality modern audiophile tube amps should have them as it protects what are often very rare and expensive tubes and speakers....and DC coupled solid state amps that are not protected against huge input offsets. Crackling tubes are telling you they are being highly stressed and you will suffer premature failure at some point in time. There are plenty of schematics available online for installing auto startup circuits in tube amps. When tubed guitar power amps went from 5AR4/GZ34 rectifiers to solid state rectifiers, the delay circuitry and output muting circuitry failed to show up at the same time. That costs more money, and guitarists rarely are willing to pay what a C-J or Audio Research owner has to pay for a tube amp.

    Hope you have good quality fast blow fuses installed in your 6550 B+ supply rails, as you will be in need of them once a crackling tube decides its time to short out internally during startup.
     
  10. carbonfold

    carbonfold Supporting Member

    Oct 13, 2006
    Dallas, Texas
    Yep usually loaded with 6L6's, I went ahead a risked throwing in 6550's with a plate voltage of 550v. I know it's a crap shoot it the PT can handle the heater current draw.

    I know the tech did work on placing the NFB cable to 8ohm ( by my request vs 4ohm ), updates all three plastic switches to metal, output jacks to metal, update both input jacks and put in "I think" ceramic 1k grid resistors for the power tubes.

    When I was in the shop I noticed he was also working on I in the biasing area due to it not biasing enough for 6550's.

    I biased it at 35MA @ 549V with 6550.

     
  11. I hope you mean 1K ohm 5 watt output tube screen resistors?
    Has the filament voltage been checked after the 6550 mod for excessive voltage sag?
     
  12. Sounds to me like some ill advised modifications have been made and the amp is telling you it doesn't like them.

    Why change the NFB loop??
     
  13. carbonfold

    carbonfold Supporting Member

    Oct 13, 2006
    Dallas, Texas
    Heard that the NFB on the 4ohm tap will be more aggressive and 8ohm will give cleaner. From my understanding Marshall uses either Tap depending on what they where going for.

    Ill advised? As in the 6550's? Then I would assume that you're suggesting the PT is failing or are you taking about some other mod?

    Honestly the only mod I went with was 6550's. After placing 1k grid resistors, he said that it would run either 6l6 or 6550. Just bias the difference.



     
  14. carbonfold

    carbonfold Supporting Member

    Oct 13, 2006
    Dallas, Texas
    Ok update. So if I leave it on standby for approx 3-5 mins before firing it off, don't get any crackling. normal?
     
  15. No crackling is normal.

    Changing the NFB to the 4Ω tap results in less voltage being fed back through the loop, unless you adjusted the component values to suit the change. The amp has more gain and will be louder. Stability will be somewhat affected.
     
  16. Normal ?, work around maybe. You still need to find out if the filament supply is up to full voltage.
     
  17. carbonfold

    carbonfold Supporting Member

    Oct 13, 2006
    Dallas, Texas
    Ok, did a few more tests.

    Heater filament was a solid 6.4v at idle and a solid 6.4v when fired up.

    Checked biasing and plate voltage:

    550v@ 35mA/32mA/35.4mA/34mA

    Crackle still there with both V1 and V2 preamps pulled during warmup.
     
  18. carbonfold

    carbonfold Supporting Member

    Oct 13, 2006
    Dallas, Texas
    If anyone is following this :)

    Took it to another tech and he found that one of the large power supply caps were leaking. He informed me that I could live with the crackling and the amp would be fine. However, the caps would completely give at some point and would be best to go ahead and replace them to prevent major failure down the road.

    So, pulled the trigger in having all four replaced and lets see if that solves my problem.
     
  19. Jeff Scott

    Jeff Scott Rickenbacker guru..........

    Apr 11, 2006
    I doubt that will solve the problem. :meh:
     
  20. carbonfold

    carbonfold Supporting Member

    Oct 13, 2006
    Dallas, Texas
    You are correct. $43 dollars later after cap replacements and still cracking. I'm getting very irritated at our "Austin" techs.

    Second strike and my bill is getting larger.

    Sigh....


     

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