Amp volume gradually lowers when playing??

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  • NerineNerine Frets: 1971
    Where are you Nerine?
    Arse end of nowhere in Lincolnshire. Near Spalding. 
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  • Nerine said:
    Where are you Nerine?
    Arse end of nowhere in Lincolnshire. Near Spalding. 
    Darn. If twas Lundun would point you at my amp tech.

    Bye!

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  • ICBM said:

    But if it's a smooth fade down and fade up, then I think it's likely to be a tube.
    A smooth fade-down could be a valve filament problem, but coming instantly back to full volume after engaging the standby rules that out.
    Just coming back to this, that's exactly the behaviour I was having with my VHT in the studio last year. Slow and deliberate sounding volume ramp downwards, with no "spikiness" to speak of. Turning the amp to stand-by and waiting a few minutes, and then turning it back on again, cured it. I couldn't figure it out, so I just took the head out of the headshell, and it stopped happening. Went completely away. Carried on with the recording session.

    Got the amp home a week later, in it's head-shell of course, and it was doing it at home as well. So I tried the same thing, and it went away again.


    Replaced the rectifier tube, and it went away full stop.

    Weird huh???


    The reason I suspected the recto tube in the first place was because back in the day if you remember my first SigX amp, and the famous 12AY7 tube experiment incident .... it actually turned out to be a dodgy recto tube back then too!!! The old one sounded like a bloody rainstick!! Except I'd never had an amp with a rectifier tube until that point, so I didn't know it was broken!! :lol:



    Anyway. If it were me, I'd be buying a preamp tube from some good stock, and test it out in every position in the amp first. If that doesn't make the issue disappear, I'd do similar with a set of poweramp tubes (always good to have a backup replacement of those anyway!) and if that doesn't cure it, then I'd be opening the amp up to check for dodgy joints.



    The thing that makes me think it's not a bad joint is the fact that Nerine said it was a smooth volume drop - I don't think a bad joint would result in that. I think it would result in way more erratic sound output, with the volume jumping all over the place in a very random and inconsistent manner.

    I'm no expert though :)

    Bye!

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  • ICBMICBM Frets: 71960

    The thing that makes me think it's not a bad joint is the fact that Nerine said it was a smooth volume drop - I don't think a bad joint would result in that.
    It does if it’s in the voltage supply and not the signal path. The filter cap downstream of the fault slowly discharges and the volume drops along with it.

    Your input is appreciated though, it’s entirely possible there is something I haven’t thought of. Quite a few times I’ve even put the amp back together thinking I’ve fixed the ‘obvious’ cause of the problem, only for it to come back! I’m sure most techs have :).

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • ecc83ecc83 Frets: 1589
    I am going to resurrect my theory about loss of filament power. The "clunk"  of the standby switch restores it and during the wait the valves power back up and are instantly working out of standby. Then the joint heats up and goes O/C again.

    Dave
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  • ICBMICBM Frets: 71960
    ecc83 said:
    I am going to resurrect my theory about loss of filament power. The "clunk"  of the standby switch restores it and during the wait the valves power back up and are instantly working out of standby. Then the joint heats up and goes O/C again.
    You could test that by only switching to standby for a second or two - if it comes back on straight away it’s not a filament issue. If it fades up slowly it is.

    Standby switches are useful ;).

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • ecc83ecc83 Frets: 1589
    ICBM said:
    ecc83 said:
    I am going to resurrect my theory about loss of filament power. The "clunk"  of the standby switch restores it and during the wait the valves power back up and are instantly working out of standby. Then the joint heats up and goes O/C again.
    You could test that by only switching to standby for a second or two - if it comes back on straight away it’s not a filament issue. If it fades up slowly it is.

    Standby switches are useful ;).

    ICBM said:
    ecc83 said:
    I am going to resurrect my theory about loss of filament power. The "clunk"  of the standby switch restores it and during the wait the valves power back up and are instantly working out of standby. Then the joint heats up and goes O/C again.
    You could test that by only switching to standby for a second or two - if it comes back on straight away it’s not a filament issue. If it fades up slowly it is.

    Standby switches are useful ;).

    Or, an added complication to diagnosis!


    Dave.
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  • NerineNerine Frets: 1971
    Well, the amp will be on for a good two/three hours tomorrow whilst I’m practicing, so I’ll report back with what I learn. 
    I’ll try a quick standby as well to see if that does anything. 
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  • NerineNerine Frets: 1971
    It probably won’t do it, now... 

    The amp should get plenty warm enough though. It’ll be on like 2-3 on the master for the duration which is pretty loud on an 800 (when not attenuated). 
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  • So how was it?

    Bye!

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  • NerineNerine Frets: 1971
    So how was it?
    Was actually just coming in to update as it happens, mate. 

    Errr. Yeah. It was fine... FFS. 

    I bloody knew it would be. Typical isn’t it. 

    It’s a mysterious amp, this one. The lowly JCM 800 reissue of unknown age, origin and provenance that sounds more angry than Thor gargling his own hammer, is louder than the birth of a star, yet occasionally gets pissy about missing the start of its favourite garden makeover show that’s available on catch up anyway... 

    It’s a head scratcher, this one... 


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  • Was the amp moved at all??

    Bye!

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  • digitalscreamdigitalscream Frets: 26458
    For what it's worth, this is exactly the behaviour I saw with my original Jet City JCA50H. It was even OK for a couple of rehearsals, then started doing it again. In the end, though, after coping with it for a while...it died completely, with a power transformer failure. Entirely my fault for just trying to power through instead of getting it checked out.

    Whether the PT was the cause or it whether it failed due to some other component dying, I don't know...because I can't get hold of replacement transformer to test it.
    <space for hire>
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  • NerineNerine Frets: 1971
    Was the amp moved at all??
    No. It wasn’t to be fair. 

    Although it was moved prior to me using the other day before I experienced the volume drops. 

    That said, I only moved it from atop one cab to another. I don’t think I caught it on anything. Mind you, we’re probably talking about pretty fine margins here, so it’s entirely plausible moving it interrupted the connection or slightly vibrated the solders or something. I think it was running for probably 10-15 mins before it started to happen if I remember correctly. 
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  • Hmmmmmm. Bloody sods law this.

    Bye!

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  • NerineNerine Frets: 1971

    For what it's worth, this is exactly the behaviour I saw with my original Jet City JCA50H. It was even OK for a couple of rehearsals, then started doing it again. In the end, though, after coping with it for a while...it died completely, with a power transformer failure. Entirely my fault for just trying to power through instead of getting it checked out.

    Whether the PT was the cause or it whether it failed due to some other component dying, I don't know...because I can't get hold of replacement transformer to test it.
    Yeah. I’ve kinda resided myself to the fact that it’s likely something a bit more serious that could possibly take down the entire amp. 

    In any case, I’ve plugged my Friedman back in now. I’ll have to get the Marshall looked at when lockdown has ended. That said, I’m willing to travel to take it somewhere if anyone knows any techs that are still working? It’s not like I’ve got much else to do at the weekends at the min. Hehe. 
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  • Just to be a bit dumb about it - are you sure it wasn't a dicky cable or something like that???

    Bye!

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  • ICBMICBM Frets: 71960
    Just to be a bit dumb about it - are you sure it wasn't a dicky cable or something like that???
    It won't be.

    The slow power-down over 10-20 seconds or so can only be one of two things:

    1. valve(s) cooling down due to a filament problem. Either a faulty valve, a bad contact between the valve and the socket, or a filament supply circuit failure which could include the power transformer.

    2. high-voltage supply dropping as caps(s) discharge due to the supply being interrupted. If it's this it must be downstream of the power valves or it would fade out a lot quicker than over 10-20 seconds.

    The question is which, and where.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • ecc83ecc83 Frets: 1589
    It is very, very rare but I have had an intermittent O/C heater on a pre amp valve. Actually a few weeks ago I was building a project and had an intermittent valve HOLDER heater connection. Bloody brand new ceramic HQ jobby as well!

    ***T'APPEN!

    Dave.
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  • NerineNerine Frets: 1971
    Well, so small update...

    I ended up speaking to Marshall. 

    Went on their website. 
    Called reception. 
    Got put through to service. 
    Spoke to service. 
    Emailed them some details. 

    Amp is being collected on Tuesday for repair. 

    No sooner had I received the email back from their service department, I also received a text straight away saying it’s being collected on Tuesday. 

    Cannot argue with their customer service AT ALL. The whole process was very easy and very quick. Wasn’t waiting on the phone, didn’t have to call back etc. Great customer service. 

    Anyway, I decided to go the Marshall route rather than a local tech because I’ve had to send a couple of amps to them before. One a JTM 45 which just didn’t seem to have much welly, so they happily investigated that. There was nothing wrong with that. It just isn’t as loud an amp as I was expecting. I used to run it into a 2x12 cab unmic’d and it used to run out of headroom a lot. 

    Then I had an issue with a VintageModern years ago. They replaced the board no questions asked and sent it back to me. 
    Both times they have been absolutely great. 

    Whether they replace the board in my 800 or not, who knows. It’s probably quicker for them to do that rather than futz about trying to figure out what’s going on. Will see. 

    In any case, seems like the easiest thing to do. 



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