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Thread: Peavey Valve King 100 head

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    Quote Originally Posted by g1 View Post
    The user in that video makes a critical error which means the video is useless. He does not plug anything in to the amp and play it.
    We have no idea what kind of volume he is setting the amp to. Many amps will hum if you don't plug anything in and turn everything full up. For all we know, when we hear the hum in the video, he has the volume set so loud you could not sit in the room if you played guitar at that volume setting.

    Plug a guitar into your amp. Turn it up to a fairly loud volume that you actually play it at. Slowly bring up the reverb level. Does the hum seem loud compared to the guitar sound?
    Sorry to go through all this, but it seems you may not be familiar with the 'usual' level of hum from reverb.
    We can't make any assumptions from a video that has no reference to the guitar sound at playing levels.
    Please don't be sorry about going through it, I have much to learn and really appreciate the feedback. I've not been using a guitar for testing, I've been using a 400hz test signal when tracing the audio and a cheap scope to look at the waveform, I've also been using a ticking clock sample to get an idea on how well the delay on the reverb is, a little unorthodox . The hum relative to the test signal has always been considerably louder when the volume of the amp is low, which is why I thought it was a problem, but I do lack experience so it might be a case of me being a worry guts.

    Quote Originally Posted by nickb
    ...and it might help to do this:

    Put your meter on ACV and connect across the speaker. Unplug your guitar. Report the readings for low volume w/o revverb, high volume w/o reverb and high volume with reverb.
    .

    I'll get on that later and report back, I take it that's a reading without any form of signal?

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  2. #37
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    Quote Originally Posted by jondoe View Post
    I'll get on that later and report back, I take it that's a reading without any form of signal?
    No signal, just hum (and noise).

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    Quote Originally Posted by nickb View Post
    No signal, just hum (and noise).
    It read as 0.10 ACV at all positions of the volume/reverb as you requested.

    Afterwards I feed a metronome signal through the amp after and listened to the reverb output on pin 7 of the opamp, I've realised I can't actually hear the reverb under the humming noise, although the source audio signal is there. If I tap the reverb tank I get a strong and clear spring clanging sound.

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    Quote Originally Posted by jondoe View Post
    It read as 0.10 ACV at all positions of the volume/reverb as you requested.

    Afterwards I feed a metronome signal through the amp after and listened to the reverb output on pin 7 of the opamp, I've realised I can't actually hear the reverb under the humming noise, although the source audio signal is there. If I tap the reverb tank I get a strong and clear spring clanging sound.
    With just 0.1V hum and noise with all turned up I don't think you have a hum problem at all.

    So, the only question now is does the reverb sound normal otherwise?

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    Quote Originally Posted by nickb View Post
    With just 0.1V hum and noise with all turned up I don't think you have a hum problem at all.

    So, the only question now is does the reverb sound normal otherwise?
    Hmmm, well there is no reverb effect on the output signal that I can make out as cranking the reverb past 5 way causes the humming to drown out the input signal entirely, so if there was a slight reverb I can't hear it. I can't hear any reverb effect on pin 7 of the opamp and the only reverb I can hear clearly is the tank springs if I give it a whack, they sound loud and clear! I've had the tank off again and checked the contacts and resistance readings and they all appear good.

    I really don't think this is operating as it should.

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  6. #41
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    Looking over the responses in this thread it seems that it's all got very confusing. For example the hum with reverb up was just 0.1V but it then stated that it swamps the input signal, which at 100W, is tens of volts, even just 1 watt is almost 3 volts. Certainly there is a complication that this is intermittent and that means pain.

    What I would do is apply a 1khz 100mV signal to the input, plug something into the return to kill the main output (or use a dummy load) and look at the signal on the cable on the input to the tank. If that was missing I'd check U101 pin 3 and then U101 pin 1.


    Are you within driving distance of West Devon? I'd be happy to put it on the bench and take a look.

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    Last edited by nickb; 02-03-2019 at 06:14 PM.
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    Quote Originally Posted by nickb View Post
    Looking over the responses in this thread it seems that it's all got very confusing. For example the hum with reverb up was just 0.1V but it then stated that it swamps the input signal, which at 100W, is tens of volts, even just 1 watt is almost 3 volts. Certainly there is a complication that this is intermittent and that means pain.

    What I would do is apply a 1khz 100mV signal to the input, plug something into the return to kill the main output (or use a dummy load) and look at the signal on the cable on the input to the tank. f that was missing I'd check U101 pin 3 and then U101 pin 1.


    Are you within driving distance of West Devon? I'd be happy to put it on the bench and take a look.
    I think I'm confusing myself at this point I think it's best I start from the beginning, I'm going to order up some new 12AX7's as I'm using my known good spares, but I shouldn't trust them 100%. I do have a home made 50w dummy load clamped to my box steel workbench frame for a heat sink, I'll feed in a signal and see how it looks.

    Unfortunately I'm several hours away from West Devon so I can't take you up on that generous offer, but thank you

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  8. #43
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    Quote Originally Posted by jondoe View Post
    I think I'm confusing myself at this point I think it's best I start from the beginning, I'm going to order up some new 12AX7's as I'm using my known good spares, but I shouldn't trust them 100%. I do have a home made 50w dummy load clamped to my box steel workbench frame for a heat sink, I'll feed in a signal and see how it looks.

    Unfortunately I'm several hours away from West Devon so I can't take you up on that generous offer, but thank you
    I doubt you need more valves as there is no valve section that is unique to the reverb and further it just complicates things with more unknown changes. Please try the suggested tests and see what we get. It's important to realise that the signal level you get at those points does depend on the volume setting so make sure they are not at zero.

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    Quote Originally Posted by nickb View Post
    I doubt you need more valves as there is no valve section that is unique to the reverb and further it just complicates things with more unknown changes. Please try the suggested tests and see what we get. It's important to realise that the signal level you get at those points does depend on the volume setting so make sure they are not at zero.
    Understood, I'll hold off valve swapping. I've also put some new batteries in my multi-meter, they died earlier on today unexpectedly, that probably doesn't help accuracy. I fed in a 1khz 109mV (closest i can get to 100) and used my DIY tracer to follow the tone all the way to pin 3 on U101, but it's not on pin 1. Should I be able to hear the tone on pin 1?

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    Yes it should be there on pin 1. Measure the DCV on pin 1,2,3,4 and 8 wrt ground please.

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    Quote Originally Posted by nickb View Post
    Yes it should be there on pin 1. Measure the DCV on pin 1,2,3,4 and 8 wrt ground please.
    I did all of them while I was at it

    Pin;
    1= 0.71
    2= 0.00
    3= 8.55
    4= 0.00
    5= 9.32
    6= 9.42
    7= 9.65
    8= 18.99

    (made a typo above, edited.)

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    Last edited by jondoe; 02-03-2019 at 08:54 PM. Reason: typo.

  12. #47
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    Pins 1, 2 and 3 should all be about same i.e. 9V or so. Either the NJM4560 is bad or the ground side of the reverb tank is shorted to ground.

    Unplug the cable to the IN of the tank i.e. that goes to U101 pin 1 via P108/P105 and redo the DCV measurements.

    If it's now OK then check the DC resistance from the shield side of the on the cable that goes to the tank to ground. The out lead must remain plugged in to the tank and PCB. If this is just a few ohms then remove the tank and find out why the input shield is shorted.

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    Unplugging the IN side of the tank makes pin 2 rise to 0.72 volts, every other pin is the same as before. I swapped out the NJM4560 for a brand new one made and it made no difference. The existing one doesn't show any shorts on any of it's pins out of the board.

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  14. #49
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    The tank bringing pin 2 down to 0v suggests there is a short from ground to the shield side of input the tank.

    Lift one end of D118, unplug the input cable to the tank and retest the DCV.

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    I made a slight mistake on the pin 2 reading, it's always 0.72 VDC, not 0.00. I lifted a leg of the D118 the readings remain the same

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  16. #51
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    Then look for pin 1 being shorted to something. Wasn't the PCB resoldered at one time? Maybe there is a solder bridge.

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    Quote Originally Posted by nickb View Post
    Then look for pin 1 being shorted to something. Wasn't the PCB resoldered at one time? Maybe there is a solder bridge.
    I did put a socket in for the opamp, I tested for shorts on the sockets earlier and double checked those solder joints, they are pretty neat and tidy and no shorts. That leaves C125 I guess? I think I took that out already and tested it on a component testers, but I can always pop it out again. R129 and R34 both read as they should.

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    C125 being shorted would cause pin 1 to be high, not low. With the amp off remove U101 and measure the DC resistance on the socket from pin 1 to ground. Try both ways around and then repeat on the diode test range. Measure the DC resistance from pin 1 to pin 2 on the socket.

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    Quote Originally Posted by nickb View Post
    C125 being shorted would cause pin 1 to be high, not low. With the amp off remove U101 and measure the DC resistance on the socket from pin 1 to ground. Try both ways around and then repeat on the diode test range. Measure the DC resistance from pin 1 to pin 2 on the socket.

    Between pins 1 & 2 I have 22k resistance, in diode mode I get nothing in either direction. I put the opamp back in before testing resistance from pin 1 to ground and now I have higher voltages on Pins 1 and 2 and the reverb output on pin 7 is now carrying a signal....

    I'm going to knock on the head for the night and start fresh tomorrow, somethings fishy here!

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  20. #55
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    Didn't I say intermittent == pain! At least it's narrowed to a very small area. I think at this point it's up to you to find the connection issue.

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    Quote Originally Posted by nickb View Post
    Then look for pin 1 being shorted to something.
    I'd think D118 could be disconnected for test purposes?

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  22. #57
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    Quote Originally Posted by g1 View Post
    I'd think D118 could be disconnected for test purposes?
    Already done see #49

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    I've resisted the temptation to take this out into the street and thrash it with a tree branch, but I'm holding the option open.

    I've had the board out again this morning, I've been over every solder joint with a magnifying glass and anything that looked at me side ways has been touched up. I've had the opamp in and out a few times to ensure the connections are good, I've put the board back in and gone in double chopsticks prodding every exposed part, so far, so good, I still have an output on the reverb tank.

    I *think* the humming on the reverb has reduced a little, I've also taken the AC reading on the speaker terminals again;

    reverb 0

    vol down 50 mv
    vol up 53 mv

    reverb 10

    vol down 67 mv
    vol up 72 mv

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  24. #59
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    Quote Originally Posted by jondoe View Post
    I've resisted the temptation to take this out into the street and thrash it with a tree branch, but I'm holding the option open.

    I've had the board out again this morning, I've been over every solder joint with a magnifying glass and anything that looked at me side ways has been touched up. I've had the opamp in and out a few times to ensure the connections are good, I've put the board back in and gone in double chopsticks prodding every exposed part, so far, so good, I still have an output on the reverb tank.

    I *think* the humming on the reverb has reduced a little, I've also taken the AC reading on the speaker terminals again;

    reverb 0

    vol down 50 mv
    vol up 53 mv

    reverb 10

    vol down 67 mv
    vol up 72 mv
    You do not have a hum issue. Those are well into the normal range. You do have a bad connection on one or more pins of the opamp though

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    Quote Originally Posted by nickb View Post
    You do not have a hum issue. Those are well into the normal range. You do have a bad connection on one or more pins of the opamp though
    Huh, well in that case I'll stop being concerned about the hum and try and go for improving reliability. I'll take a few trips out with it in the boot of the car, take up and down the stairs a few times and back to the shed to see if something fails. If it passes a week of rough handling I'll tick it off the completed list, I can only do so much.

    Thanks again for your input nickb and others, I'm learning, slowly

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