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Thread: Help! Sick valve amp.

  1. #1
    Join Date: Jul 2016

    Location: Welsh Borders

    Posts: 283
    I'm Gary.

    Default Help! Sick valve amp.

    Hello, all. I need some suggestions about tracing a fault in my amp.
    It's an Audio Note single ended jobbie with 300B power valves. I built it myself from a kit donkey's years ago and it has served very well. Trouble is, though I built it I have no idea how it works - I just followed the instructions. Anyway, it was working fine and I took it out of my system a few months ago, since which time it has just been sitting on a shelf doing nothing. I put it back into the system today, thinking it would be nice to hear it again. But what do I find? One channel has nothing but a rustling sound coming out of it - fairly gentle, not a cone-splitting sound. It takes twenty seconds or so to become apparent after switch-on, and it does not vary with the volume pot position. I have of course swapped valves out, swapped them round and generally faffed around with the possible easy fixes, but no joy. Any ideas where I ought to start looking inside for my problem?
    IB

  2. #2
    Join Date: Apr 2012

    Location: London

    Posts: 685
    I'm James.

    Default

    Quote Originally Posted by Infinitely Baffled View Post
    Hello, all. I need some suggestions about tracing a fault in my amp.
    It's an Audio Note single ended jobbie with 300B power valves. I built it myself from a kit donkey's years ago and it has served very well. Trouble is, though I built it I have no idea how it works - I just followed the instructions. Anyway, it was working fine and I took it out of my system a few months ago, since which time it has just been sitting on a shelf doing nothing. I put it back into the system today, thinking it would be nice to hear it again. But what do I find? One channel has nothing but a rustling sound coming out of it - fairly gentle, not a cone-splitting sound. It takes twenty seconds or so to become apparent after switch-on, and it does not vary with the volume pot position. I have of course swapped valves out, swapped them round and generally faffed around with the possible easy fixes, but no joy. Any ideas where I ought to start looking inside for my problem?
    IB
    Hi, is it both channels or just one? This may help narrow the problem to PSU or signal section. Also have you swapped out the valve rectifier to make sure that is behaving properly? My instinct is dodgy solder joint, dry capacitor, that kind of thing.

    Sent from my GT-I9195 using Tapatalk

  3. #3
    RothwellAudio Guest

    Default

    Rustling sounds are often caused by an anode load resistor play up. However, you usually still get sound, just with added rustling.
    Anyway, I'd check the DC voltages around each valve just to narrow things down a bit. Have you still got the circuit diagram?

  4. #4
    Join Date: Jul 2016

    Location: Welsh Borders

    Posts: 283
    I'm Gary.

    Default

    Thanks for this. In answer to the first question, yes, it is just one channel so I interpret that as an indication that it is a signal section issue, not power supply (but please correct if the assumption is wrong).

    Yes I have still got all the assembly instructions, but if I recall correctly, they were in the form of clear, but rather unscientific drawings of different parts of the overall unit, rather than a comprehensive circuit diagram that a real EE would want to see. It was definitely a hobbyist's kit. I'll dig out what I have got, but also in the meantime I'll do some current measurements. Thanks for the suggestions so far!
    IB

  5. #5
    Join Date: Mar 2008

    Location: Galashiels

    Posts: 13,669
    I'm inthescottishmafia.

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    There should be test points detailed in the instructions, check these with a meter.
    “Music has always been a matter of energy to me, a question of fuel. Sentimental people call it inspiration, but what they really mean is fuel. I have always needed fuel. I am a serious consumer. On some nights I still believe that a car with the gas needle on empty can run about fifty more miles if you have the right music very loud on the radio”

    Hunter S Thompson

  6. #6
    Join Date: Feb 2008

    Location: South Wales

    Posts: 9,151
    I'm NotTakingLifeTooSeriouslyTheseDays.

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    Turn it off, wait for it to cool down, and pull each valve out of its holder one by one, and push them in and out of their socket a few times, if it was working when you last used it, it may just be slight contamination on the valve pins, try this first, it often works. also, sometimes if this happens after the amp has been on a while, the valve pins in the sockets may have lost their tightness agains the valve pins, and if the valve socket pin expands more than the valve pin, it can make an intermittent connection, and cause the noise your experiencing.
    Hope this helps.
    A...
    "Today scientists have substituted mathematics for experiments, and they wander off through equation after equation, and eventually build a structure which has no relation to reality"
    Nikola Tesla



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  7. #7
    Join Date: Jul 2016

    Location: Welsh Borders

    Posts: 283
    I'm Gary.

    Default

    Hurrah for De-Oxit!
    It looks like the problem must have been crusty valve pins. I now have both channels back nice and clear. This is a good thing because otherwise the job looks like it would very quickly have got beyond me. I took some measurements from the innards of the amplifier. Referring to the build instructions, there is a section on tests to carry out once you have completed the construction, and before using it. These tests are to be done without the valves in place and consist of a number of voltage measurements - some AC and some DC - between various points within the works. I fared reasonably well on these - the amp measuring up to spec in all cases except one, which we won't go into here given the happy outcome. Then, in the back page of the build booklet, there was as Ali suggested a list of check points with prescribed values. These were all expressed in terms of DC with respect to ground. I tried them all and didn't get a single millivolt's worth of reading out of my meter. Mind you, I had not put the valves back in to do these tests. The checklist did not say whether it was meant to be valves-in or valves-out, and frankly I am not keen on fiddling around up an amplifier's bottom with valves in, current running and only a pair of Marigolds between me and eternity. Don't know why, but it struck me as a far riskier thing to do than the valve-out tests. Can anyone tell me, should those tests have been conducted with the valves in? Because if not, I appear to have built the first amplifier that runs on fresh air.

    Anyway, many thanks to all. This was a great example of AoS community spirit and helped me a lot. Cheers.
    IB

  8. #8
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 6,089
    I'm Alan.

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    If you run into problems in the future that you can't overcome I'd be happy to look at it for you.


  9. #9
    Join Date: Jul 2016

    Location: Welsh Borders

    Posts: 283
    I'm Gary.

    Default

    Thanks, Alan. I already had you in mind if this turned nasty. Would be nice to catch up.
    Cheers
    Gary

  10. #10
    Join Date: Mar 2013

    Location: nottingham

    Posts: 328
    I'm nigel.

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    I had a similar problem a few weeks back with my Little Dot valve preamp.
    A low motor boating on just the left channel. I tried swapping over the tubes, speaker leads, phone leads and even took it out of curcuit
    Tried replacing my better tubes with the supplied junky Chinese tubes and still the chugging noise stayed.

    Then, in a moment of ahem...clear lucidity....I remembered i'd recently moved my virgin modem closer to my kit to tidy the frkkin rats nest of Ethernet cables behind.......

    Moved it away and all was well.......I used ALL my extensive library of swear words before the night was over.

    I'm still puzzled by the sound being on only one channel though.

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