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    Distortion and Hum ???

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    jimmeq

    Posts : 13
    Join date : 2017-02-24

    Distortion and Hum ???

    Post by jimmeq on Tue May 29, 2018 11:44 am

    I'd been having distorted sound issues with my ST-120 for a while now and finally got the chance to try and figure out the cause and am not sure why it started and have I really fixed it.

    Succinct:
    Can a tube cause distorted sound? Can a tube cause hum?


    Quick read:
    At one point I began to hear my ST-120 sound was distorting at times. (??) Thought I noticed some distortion even at lower volume levels between 10:00 to 2:00 on the attenuator. The sound seemed to have rougher edges on the vocals; more so with a four or more member band. It sounded OK with acoustic guitar and vocals, yet seemed overwhelmed with complex passages and I could hear distortion.  Electric guitar, drums...

    Then while troubleshooting the distortion issue a hum developed in both channels even with volume down and no input whatsoever. Going back to three RCA clear top's and a Sovtek 5AR4 seem to have resolved the issue of hum and distortion.

    Here are the details of my "troubleshooting". I am an absolute beginner with electronics. So I now wonder if the combination of replacing tubes with the Sovtek GZ34 and RCA clear top's resolved the hum problem. If you are brave, read my detailed best memory of what I did to decide if it was a tube issue or something more serious.


    The long of it:
    In my ST-120 I had KT-120's, RCA 12AU7 Clear top's and Mullard GZ34.

    To troubleshoot I by passed the line stage preamplifier and used just a Schiit Mani phonostage.

    I reverted back to the tubes that came with the kit except for the Mullard GZ34 - 6550 power tubes and the GE 12AU7's.

    Still thought I heard distortion even at moderate levels.

    Went through a shell-game swapping between the driver board 12AU7's. First a used GE 12AU7 by Mullard, then a new 12AU7 Mullard in the center slot, Sometimes it seemed better, other times I thought I still heard distortion.

    Swapped the Mullard GZ34 rectifier for the Chinese 5AR4; it arced, and the left side new Mullard 12AU7 driver board tube flashed. I powered off. Fuse survived. Tossed out the left 12AU7. Put the Mullard GZ34 back in.

    Again I reverted to GE's that came with the amp. Seemed OK, although now I began to hear a hum. The hum occurred with the volume down all the way and with no input.

    So now I'm troubleshooting hum and no longer the distortion issue.

    I went through another round of tube swapping, and still hum.

    I got three new Sovtek 12AU7W tubes. Still hum.

    Swapped in Tung-Sol KT120's. The  Mullard GZ34 arced pretty badly, and the fuse blew. (I think I did not give the amp enough time before I power cycled. ?)

    I replaced the Mullard GZ34 with a Sovtek 5AR4. The 6550's went back in as well. Waited 10 minutes and it powered on. Still hum.

    Replaced the GE 12AU7's with the Sovtek 12AU7W's. Still hum.

    Replaced the Sovtek's back to the RCA Clear Tops. Hum barely audible now.   (Normal?)

    Tubes now used are the 6550 / RCA Clear tops and Sovtek 5AR4.

    WHEW!


    Last edited by jimmeq on Tue May 29, 2018 12:17 pm; edited 2 times in total
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    Dave_in_Va

    Posts : 230
    Join date : 2013-04-02

    Re: Distortion and Hum ???

    Post by Dave_in_Va on Tue May 29, 2018 12:12 pm

    I'm not a tech like some of the people here but I've found that most tube amp problems are tube related and when you start messing with rolling tubes you must clean and re-tension your tube sockets.

    I'm sure that early on in my VTA ST70 ownership I lost an NOS Mullard 5AR4 and one of my NOS Mullard EL 34's because I was messing around swapping tubes in and out without cleaning and especially, re-tensioning the sockets.
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    Peter W.

    Posts : 750
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Distortion and Hum ???

    Post by Peter W. on Tue May 29, 2018 1:17 pm

    OK... one does what one can with what one has. And you seem to have gotten things down to a dull roar. But, a few questions:

    a) Is/was the hum 60 Hz or 120 Hz? 60Hz is typical of incipient rectifier failure as AC is getting past it. 120 Hz suggests inadequate filter caps, or, again, incipient failure of same.

    b) Distortion may be due to many issues, including poor rectification or poor filtration. Various small-signal tubes will amplify sections of frequencies in various ways, so distortion, hum presence, or not, and so forth will vary.

    c) I expect that if my rampant speculation is correct, and you used 'the very best' tubes (but for the rectifier), that would give the worst results in terms of distortion or hum. From the other angle, I think the tubes in place are slightly deaf, so you are not less distortion and little hum.

    I do not know how skilled you are at diagnostics, nor do I know what tools you have or how comfortable you might be around B+ voltages. But, the first thing I would test for is residual AC off the filter caps. If you measure more than a very few fractions of a VAC, you have a problem. And as you are well over 500 VDC on that line, you need to be exceedingly careful on how you measure, with what you measure and that you do not 'cross' anything.

    If you live anywhere near 19027, I would be glad to have a look - same to HMMmmmmmm - gratis.


    jimmeq

    Posts : 13
    Join date : 2017-02-24

    Re: Distortion and Hum ???

    Post by jimmeq on Tue May 29, 2018 2:46 pm

    Peter W. wrote:OK... one does what one can with what one has. And you seem to have gotten things down to a dull roar. But, a few questions:

    a) Is/was the hum 60 Hz or 120 Hz?  60Hz is typical of incipient rectifier failure as AC is getting past it. 120 Hz suggests inadequate filter caps, or, again, incipient failure of same.

    b) Distortion may be due to many issues, including poor rectification or poor filtration. Various small-signal tubes will amplify sections of frequencies in various ways, so distortion, hum presence, or not, and so forth will vary.

    c) I expect that if my rampant speculation is correct, and you used 'the very best' tubes (but for the rectifier), that would give the worst results in terms of distortion or hum. From the other angle, I think the tubes in place are slightly deaf, so you are not less distortion and little hum.

    I do not know how skilled you are at diagnostics, nor do I know what tools you have or how comfortable you might be around B+ voltages. But, the first thing I would test for is residual AC off the filter caps. If you measure more than a very few fractions of a VAC, you have a problem. And as you are well over 500 VDC on that line, you need to be exceedingly careful on how you measure, with what you measure and that you do not 'cross' anything.

    If you live anywhere near 19027, I would be glad to have a look - same to HMMmmmmmm - gratis.


    A) Because I heard the hum mostly from the mid range driver I will guess it was 120Hz

    C) Do you mean a so-so rectifier will cause hum? I don't understand what is meant that the tubes in place are "deaf".

    I consider myself an "assembler" and not a technician. I do have a multi-meter and am willing test around high voltage. I just need directions as to what, and where to test. Shoot...I'm 4+ hours from your zip.

    Thanks!

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    Peter W.

    Posts : 750
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Distortion and Hum ???

    Post by Peter W. on Tue May 29, 2018 3:16 pm

    Please note the interpolations.

    A) Because I heard the hum mostly from the mid range driver I will guess it was 120Hz

    That is a good guess. 60 Hz hum is quite distinct and not typical of audio stuff - mostly SS and mostly solid-state rectifiers.

    C) Do you mean a so-so rectifier will cause hum? I don't understand what is meant that the tubes in place are "deaf".

      /\/\/\/\/\/\/\/\ becomes /\/\/\/\/\/\  In the former case, you hear 60 Hz, in the latter case 120 Hz. So, knowing the frequency of the hum helps with diagnosis. And here is where a frequency counter can remove all doubt.  

    I consider myself an "assembler" and not a technician. I do have a multi-meter and am willing test around high voltage. I just need directions as to what, and where to test. Shoot...I'm 4+ hours from your zip.

    The first thing you want to test is for AC-over-DC on the B+ side. So, if you have a capable auto-ranging VOM (such as a Fluke, B&K or similar) put it on the ACV Setting and test all the DCV test points. If you get anything over a very small couple of fractions of a volt read-out on AC, your filter caps are on their way out. IF you get significant ACV on the rectifier output, then your rectifier is on its way out. And, of course, it could be both.

    This is the time for a very good tube checker. This could give you a quality test rather than just an emissions test.

    On tube 'deafness' - An amplifier handles all frequencies fed to it irrespective of source. However the response curve of any given small-signal tube varies considerably from tube-to-tube both new and as they age. "Considerably" is a weasel-word in this case, as the variations are not usually sufficient to matter *at all* in most cases. But in some cases, older tubes may be a little deaf at certain parts of the spectrum relative to other parts - tubes may also favor as well as be deaf. Hence "bright" or "neutral" or "dull" or "better at the low/high/whatever end". In other words, they vary.

    Hope this helps. But the bottom line is that hum is seldom "typical".  


    Thanks!

    [/quote]
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    deepee99

    Posts : 1965
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Distortion and Hum ???

    Post by deepee99 on Tue May 29, 2018 4:39 pm

    From a rank amateur's POV:
    Output tubes rarely induce hum or distortion, in and of themselves. They either work or they don't, although before failure they will sound a bit dodgy. They're basically a GIGO device.
    Signal tubes can drive you crazy.
    I'd follow PeterW's advice and be sure to check for AC leaks, if you know what you're doing. Wear lead gloves around that 500-volt stuff and do your prodding with a wooden or plastic probe.
    A properly functioning VTA amp should be quiet as a mouse fart, even with your ear right up next to the speakers.

    jimmeq

    Posts : 13
    Join date : 2017-02-24

    Re: Distortion and Hum ???

    Post by jimmeq on Wed May 30, 2018 2:38 pm

    Thanks for the insight on tube deafness! I expect to be able to test for voltage leakage sometime over the next week and will post my findings.

    A big issue for me is I tend to "permanently" install my gear with careful cabling that is hidden. Then it becomes a small project undoing the installation for repairs. This week I'm getting a new cabinet for my gear that will make it much easier to move components. Ahhh....
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    deepee99

    Posts : 1965
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Distortion and Hum ???

    Post by deepee99 on Thu May 31, 2018 12:57 pm

    jimmeq wrote:Thanks for the insight on tube deafness! I expect to be able to test for voltage leakage sometime over the next week and will post my findings.

    A big issue for me is I tend to "permanently" install my gear with careful cabling that is hidden. Then it becomes a small project undoing the installation for repairs. This week I'm getting a new cabinet for my gear that will make it much easier to move components. Ahhh....
    I have the same problem, jimmeq. Stereo's in a beautiful walnut cabinet but every time I move it away from the wall to get at something in the back, plugs fall out and I end up collecting dust-bunnies having to deal with lying on the floor until wife gets home and pulls me upright. Smile
    But it's *such* a pretty cabinet!

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