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Thread: Pro Jr Point-to-point hum issue

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    Pro Jr Point-to-point hum issue

    Hello, it's been a while ago since I posted, but it was time for new build project now. I have been converting a Pro Jr. Amp into a version with an eyelet board - see the pictures for the schematic and actual amp.
    I am having a serieus issue with it. I have followed the same design guideline as other builds and this one hums like crazy. Could you please help me with a structured way of trouble shooting this situation?
    Hums seems like 60Hz (or double?) noise... It goes away when v1 tube is pulled. Also the grid wire v1A in the schematic seems to pick up a lot of noise. When fiddling around with this wire it becomes stronger or less, but never seems to go away entirely. Seems to me that is not the source of the hum.
    Who can advice me with a troubleshooting "plan"?
    Help is greatly welcome.
    Regards from the Netherlands,
    Mark
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    Since it's changing with moving the wires around, I'd say it's being picked up by a signal wire somewhere at either grid of V1. My first suspect would be the second gain stage, as it's running right close to the heater wiring. Possibly that other long wire, even though it's going to "not-V1."

    I'd move those as far from the heater wiring as possible; also check that any other signal-carrying wires are well away from high correct AC sources. Looks like some of your PI wiring or parts may come close, too? Maybe even use shielded wire there also, if you must. Someone else may jump in with the "ground the grid" trick to possibly help isolate which stage it's coming from.

    Personally, and I know it's a little late now, when I've rebuilt amps, I'm now a proponent of laying the heater wiring along the chassis like Ampeg & Marshall; as I build True PTP it gives me more freedom to fly wires high and I've had good luck with it.

    Justin

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    Try grounding V1b grid, then V1a in turn - does the hum disappear?

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    Also, I'm having some trouble seeing it between the layout & photos; check your wiring, around the Volume pot & rhe far right side of the board. That resistor hanging off the middle terminal looks like it should be connected to a different resistor on the board? Also V1 to board.

    Justin

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    I suspect your hum is 50Hz or 100Hz rather than 60. But it is important to determine which.

    I see you used a Cliff type jack as input. Does it have a chassis connection at the jack?

    get your heater wires AWAY from any signal wires. That red wire going to one of the controls for example. And try when wires come near each other the signal wires sit at 90 degrees to the heater or other AC wires.

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    Quote Originally Posted by Mick Bailey View Post
    Try grounding V1b grid, then V1a in turn - does the hum disappear?
    Grounding v1B (first stage=first half of 12AX7) did not do anything - hum is still there. Grounding v1A (second stage=second half of 12AX7) is doing 'something': the hum is less but still there. Grounding either v2A or v2B is very effective = no hum at all.

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    Quote Originally Posted by Justin Thomas View Post
    Also, I'm having some trouble seeing it between the layout & photos; check your wiring, around the Volume pot & rhe far right side of the board. That resistor hanging off the middle terminal looks like it should be connected to a different resistor on the board? Also V1 to board.

    Justin
    Noted very well! I have been running the connection of v1A via the 100R on the board to the pot, but I have chosen to be a bit more 'flexible' and run a separate wire to play around a bit. Moving away from the heaters doesn't seem to affect the hum. Moving it to the board and off the board, left to right, etc. is changing the amount of hum, but never dead silent. I have been trying to do the v1A with a shielded wire as well, but did not change a lot: it still picks up hum/noise as I am moving it around?!

    (Note that I switched v1A and V1B compared to the schematic)

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    Quote Originally Posted by Enzo View Post
    I suspect your hum is 50Hz or 100Hz rather than 60. But it is important to determine which.
    Yes, I am in Europe so it should be 50 or 100. Thanks.

    Quote Originally Posted by Enzo View Post
    I see you used a Cliff type jack as input. Does it have a chassis connection at the jack?
    First I had the Switchcraft, but I swapped it to avoid touching the chassis to avoid a ground loop... But it did not do anything to the hum...

    Quote Originally Posted by Enzo View Post
    get your heater wires AWAY from any signal wires. That red wire going to one of the controls for example. And try when wires come near each other the signal wires sit at 90 degrees to the heater or other AC wires.
    Now, it's just for playing around I have put the red wires in different places, moving it around. More away from the heaters, closer to the heaters, but not change in hum... Only thing that is changing the level of hum is the v1A grid wire (100R to pot).

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    Quote Originally Posted by Justin Thomas View Post
    Since it's changing with moving the wires around, I'd say it's being picked up by a signal wire somewhere at either grid of V1. My first suspect would be the second gain stage, as it's running right close to the heater wiring. Possibly that other long wire, even though it's going to "not-V1."

    I'd move those as far from the heater wiring as possible; also check that any other signal-carrying wires are well away from high correct AC sources. Looks like some of your PI wiring or parts may come close, too? Maybe even use shielded wire there also, if you must. Someone else may jump in with the "ground the grid" trick to possibly help isolate which stage it's coming from.

    Justin
    I have been playing with the wires, but I don't get much improvement. I also think the problem might be around V2 or V1 "towards" V2, from V1 to the grid of V2B. When I touch the 0.01uF cap, it is 'microphonic'/highly sensitive. Any ideas how to improve? Is there any other way of 'investigating' next to the grid to ground methode?

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    Capacitor across R4

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    I have been running the connection of v1A via the 100R on the board to the pot
    The 100R resistor is a grid stopper and should be soldered directly to the tube socket.

    Is the inside surface of your chassis bare metal or coated?

    Do the pot cases connect to ground?

    PLease post the complete schematic showing the heater supply.

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    Last edited by Helmholtz; 03-30-2020 at 03:51 PM.
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    Quote Originally Posted by mozz View Post
    Capacitor across R4
    Then the hum will be gone?

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    Quote Originally Posted by Helmholtz View Post
    The 100R resistor is a grid stopper and should be soldered directly to the tube socket.
    OK - I will do that.

    Quote Originally Posted by Helmholtz View Post
    Is the inside surface of your chassis bare metal or coated?
    Good point! I thought it would be bare metal, but looking at it closer it could be coated...


    Quote Originally Posted by Helmholtz View Post
    Do the pot cases connect to ground?
    I do not use the pot cases as ground. But I have not isolated the speaker jack, yet. Everything else is not connected to ground. The only ground connection to the chassis (next to the speaker jack) is through the point close to the input jack (see picture of my layout).

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    I do not use the pot cases as ground.
    Yes, but the pot cases act as shields and should connect to ground. Does hum increase when you touch them?

    Please post the complete schematic showing the heater supply.

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    Quote Originally Posted by Helmholtz View Post
    Yes, but the pot cases act as shields and should connect to ground. Does hum increase when you touch them?

    Please post the complete schematic showing the heater supply.
    I put the 100R at the tube post. Ran the wire nicely on the bottom of the chassis. That improves it. Still hum is there.
    If I am close (with my hand) to the pots with volume open, then it is giving much more hum. Also when I tap on the 0.01uF caps I hear a 'microphonic' noise.
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    If I am close (with my hand) to the pots with volume open, then it is giving much more hum.
    So please make sure that the pot cases connect to the chassis.

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    Check heater balancing resistors (47R) and connections. Make sure that heater voltage is symmetrical wrt ground.

    It would really be helpful to know if your hum is 50Hz OR 100Hz.

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    Quote Originally Posted by Helmholtz View Post
    Check heater balancing resistors (47R) and connections. Make sure that heater voltage is symmetrical wrt ground.

    It would really be helpful to know if your hum is 50Hz OR 100Hz.
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    Quote Originally Posted by klooon View Post
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    As the hum signal harmonics are multiples of 50Hz, your problem doesn't seem to be related to power supply ripple.


    If your chassis is actually coated, make sure that all chassis connections make good contact.

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    Is that one of the Fenders that comes with ashielded back panel & is known to make lots of noise if the panel is off but it goes away with the panel on, normally?

    I didn't see you mention if it hums in the cab with the panel on & I know it's a long shot, but can't hurt to try... Given there's a precedent for that. Otherwise I'm iut of ideas.

    Justin

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    It seems to work fine now! I rewired and rewired it, put shielded wire to both grids of V1. Shaved some coating off. Put the 100R at the tube socket with shielded wire and put underneath the board.

    Was still a bit picking up noise, and the put the cover on like suggested above... And then: fine!!

    Thanks so much for the input!

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    A couple of suggestions.

    Does the setting of the volume control affect the hum level?

    I don't see a connection between signal ground and the chassis. On the original layout this was done at the input and speaker jacks. It looks like you have insulated jacks in both locations so try connecting the chassis to signal ground and see if it helps.

    I would be inclined to use a screened cable from the volume control to V1. I notice you did make some improvements to that wire. Connect the shield to signal ground at the volume pot only with the 100R at the V1 end ( as you now have).

    Your spectrum plot does have a fundamental of 50Hz but the 100Hz line is higher. Therefore I think you have both 50Hz (heater or other 50Hz AC source) and power supply 100Hz issues.

    PS: Oh I see kloom posted while I was writing this...oh well!

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    I am still not happy. I moved around the wires again and it still gives too much hum to my taste.

    Quote Originally Posted by nickb View Post
    A couple of suggestions.

    Does the setting of the volume control affect the hum level?

    Yes. Very clearly. Volume at 1 and the hum is very bad. Turning the volume past 9 and it is gone.


    Quote Originally Posted by nickb View Post
    I don't see a connection between signal ground and the chassis. On the original layout this was done at the input and speaker jacks. It looks like you have insulated jacks in both locations so try connecting the chassis to signal ground and see if it helps.
    I have one central ground point - close to the input, so it should ground "everything". Also what you mean by the signal ground?? Please, could you explain?


    Quote Originally Posted by nickb View Post
    I would be inclined to use a screened cable from the volume control to V1. I notice you did make some improvements to that wire. Connect the shield to signal ground at the volume pot only with the 100R at the V1 end ( as you now have).
    Yes, I have improved this now. Still moving this shielded wire is changing the "sensitivity" to noise a bit...

    Quote Originally Posted by nickb View Post
    Your spectrum plot does have a fundamental of 50Hz but the 100Hz line is higher. Therefore I think you have both 50Hz (heater or other 50Hz AC source) and power supply 100Hz issues.
    And that's related to the above?!

    mmm. what to do?

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    What happens if you break the connection between the two 47 ohm resistors to ground off your heater wires (puesdo ground), and take that connection directly to chassis at/near the power xfmr mtg screw? I'd also try using a 100 ohm/2W or greater linear pot as hum balance in place of the two resistors, and tweak the balance to minimize hum. We see in your frequency spectrum plot the third harmonic (150Hz) is highest, while also seeing high power supply harmonics, as nickb pointed out.

    In spite the problems encountered, I do love your rebuild form factor!

    What are the two rear panel switches? One looks to be a power output select switch (15W/3W), but don't see what the adjacent switch is/does.

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    Quote Originally Posted by klooon View Post
    I am still not happy. I moved around the wires again and it still gives too much hum to my taste.

    Yes. Very clearly. Volume at 1 and the hum is very bad. Turning the volume past 9 and it is gone.

    I have one central ground point - close to the input, so it should ground "everything". Also what you mean by the signal ground?? Please, could you explain?

    Yes, I have improved this now. Still moving this shielded wire is changing the "sensitivity" to noise a bit...

    And that's related to the above?!

    mmm. what to do?
    OK. I see the chassis connection now. Now I wonder how I managed to miss it before!

    Let's concentrate on the hum at volume on 1. I take it that "1" is zero volume and it's more of a buzz than hum. It might be that the wire from the common ground on the board to the volume pot is picking up noise. As a test disconnect the screened wire that goes to the wiper and connect it directly to the common ground. If the hum / buzz is gone or much reduced then try soldering pin 3 of the volume pot directly to the metal body instead of the wire that currently goes to pin 3 of the pot or use a short wire if need be and put the screened wire back on the volume pot.

    And do try what nevestlab suggested. Right now the mid point of the two 47 ohm resistors is connected to a rather noisy place and could inject noise everywhere via the heater wiring.

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    Thanks for your help!

    Quote Originally Posted by nevetslab View Post
    What happens if you break the connection between the two 47 ohm resistors to ground off your heater wires (puesdo ground), and take that connection directly to chassis at/near the power xfmr mtg screw?
    I did this and it does not change. The hums is just as much.
    Quote Originally Posted by nevetslab View Post
    I'd also try using a 100 ohm/2W or greater linear pot as hum balance in place of the two resistors, and tweak the balance to minimize hum. We see in your frequency spectrum plot the third harmonic (150Hz) is highest, while also seeing high power supply harmonics, as nickb pointed out.
    I tried this. The low frequency hum is not changed (at minimum volume), higher frequency buzz is less or more depending on the setting of the pot.


    Quote Originally Posted by nevetslab View Post
    In spite the problems encountered, I do love your rebuild form factor!
    Thanks!

    Quote Originally Posted by nevetslab View Post
    What are the two rear panel switches? One looks to be a power output select switch (15W/3W), but don't see what the adjacent switch is/does.
    Yes, one is the attenuator and the other one is a negative feedback switch (on/off)

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    Quote Originally Posted by klooon View Post
    Let's concentrate on the hum at volume on 1. I take it that "1" is zero volume and it's more of a buzz than hum.
    Yes, volume "1" is zero volume. No it's more a humming noise. Higher frequency buzz is also there at higher volume settings and changes with moving the grid wire to the volume pot around.

    Quote Originally Posted by nickb View Post
    It might be that the wire from the common ground on the board to the volume pot is picking up noise. As a test disconnect the screened wire that goes to the wiper and connect it directly to the common ground. If the hum / buzz is gone or much reduced then try soldering pin 3 of the volume pot directly to the metal body instead of the wire that currently goes to pin 3 of the pot or use a short wire if need be and put the screened wire back on the volume pot.
    I just tried this and no changes to the hum and buzz.

    Quote Originally Posted by nickb View Post
    And do try what nevestlab suggested. Right now the mid point of the two 47 ohm resistors is connected to a rather noisy place and could inject noise everywhere via the heater wiring.
    I just tried this and does not change the hum.


    What I just noticed, is that when I plug into a wall socket that has no ground, you feel the "electricity/voltage" on the chassis. Is this normal??? What could be the cause of this??

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    Quote Originally Posted by klooon View Post
    Yes, volume "1" is zero volume. No it's more a humming noise. Higher frequency buzz is also there at higher volume settings and changes with moving the grid wire to the volume pot around.


    I just tried this and no changes to the hum and buzz.


    I just tried this and does not change the hum.


    What I just noticed, is that when I plug into a wall socket that has no ground, you feel the "electricity/voltage" on the chassis. Is this normal??? What could be the cause of this??
    Turn the plug around. scratch that, I'd suggest you have a serious wiring error and need to find this with the amp unplugged. What you describe is common with a transformerless amp or a amp with a failed deathcap. Neither of which I see in your layout. Stop! and find the issue before continuing on
    nosaj

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    Quote Originally Posted by klooon View Post
    Yes, volume "1" is zero volume. No it's more a humming noise. Higher frequency buzz is also there at higher volume settings and changes with moving the grid wire to the volume pot around.


    I just tried this and no changes to the hum and buzz.


    I just tried this and does not change the hum.


    What I just noticed, is that when I plug into a wall socket that has no ground, you feel the "electricity/voltage" on the chassis. Is this normal??? What could be the cause of this??

    First, never, ever use an ungrounded socket. If it's just a slight sensation likely it's just stray capacitance between the windings. Using an ohm meter to check that the earth from the plug has a low resistance to the chassis. Also check that live goes to the power switch and fuse. Don't just look at wires. Assume that the wire colours can't be trusted so measure from the plug. Check the neutral does not connect to the chassis

    You hum issue is elusive. Disconnect the grid from the screened wire and 100 ohm grid stopper that goes to the volume control wiper and short the grid to the cathode using as direct a connection as you can manage. What happens to the hum?

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    Quote Originally Posted by nosaj View Post
    Turn the plug around. scratch that, I'd suggest you have a serious wiring error and need to find this with the amp unplugged. What you describe is common with a transformerless amp or a amp with a failed deathcap. Neither of which I see in your layout. Stop! and find the issue before continuing on
    nosaj
    Thanks. I will go more in detail through the layout and amp!

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    Quote Originally Posted by nickb View Post
    First, never, ever use an ungrounded socket. If it's just a slight sensation likely it's just stray capacitance between the windings. Using an ohm meter to check that the earth from the plug has a low resistance to the chassis. Also check that live goes to the power switch and fuse. Don't just look at wires. Assume that the wire colours can't be trusted so measure from the plug. Check the neutral does not connect to the chassis
    I swapped them around, but no change. Also in the Netherlands, you can plug it either way (180deg), so swapping L and N. I swapped it with the plug and also in the amp, but no difference...

    Quote Originally Posted by nickb View Post
    You hum issue is elusive. Disconnect the grid from the screened wire and 100 ohm grid stopper that goes to the volume control wiper and short the grid to the cathode using as direct a connection as you can manage. What happens to the hum?
    I just tried this. No hum at all....
    When I did not connect something to the grid, (so not jumpering grid and cathode, thus leaving the grid unsoldered), there was a very big oscillation... beeping noise that increased so much that I needed to power off the amp...

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    Quote Originally Posted by klooon View Post

    I just tried this. No hum at all....
    When I did not connect something to the grid, (so not jumpering grid and cathode, thus leaving the grid unsoldered), there was a very big oscillation... beeping noise that increased so much that I needed to power off the amp...
    Good. That narrows it to the grid circuit.

    Reconnect the screened wire to the grid pin 7 and connect the other end ( used to go to the wiper) with the the screen to the ground end of the 1.5k cathode resistor. How's the hum? Now try positioning that wire so that it's parallel to the 1.5k resistor and the wire to the cathode. What we are trying to do it minimize the area of the loop formed by grid->ground->cathode resistor->cathode. Better? If better then you are getting magnetic fields coupling into the grid circuit, a bit like the secondary of a transformer. You can reduce the coupling by keeping the loop area to a minimum, you can position it are right angles to the intrusive magnetic field which practically speaking may be very hard to to, or try to figure out what is creating the field. The usual culprit is heater wiring, so twist and reposition. The HT wiring can do it as can the power transformer itself.

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    Quote Originally Posted by nickb View Post
    Good. That narrows it to the grid circuit.

    Reconnect the screened wire to the grid pin 7 and connect the other end ( used to go to the wiper) with the the screen to the ground end of the 1.5k cathode resistor. How's the hum? Now try positioning that wire so that it's parallel to the 1.5k resistor and the wire to the cathode. What we are trying to do it minimize the area of the loop formed by grid->ground->cathode resistor->cathode. Better? If better then you are getting magnetic fields coupling into the grid circuit, a bit like the secondary of a transformer. You can reduce the coupling by keeping the loop area to a minimum, you can position it are right angles to the intrusive magnetic field which practically speaking may be very hard to to, or try to figure out what is creating the field. The usual culprit is heater wiring, so twist and reposition. The HT wiring can do it as can the power transformer itself.
    Thanks! First time I tried to connect the screened side to the ground end of the 1k5 and it was dead quiet. Then I started moving around the screened wire (including the stiff end with the resistor attached) and then it got worse as if the amp was going to explode. A big increasing noise/hum. But I can't get it back to the situation where it is quiet. Even worse: I removed the screened wire and tried jumpering the grid and cathode and it makes it noise.. Maybe I destroyed the tube feet? Maybe install a new one and resolder V1 all over?! Maybe lower the heater wires for V1? Any advice/good practice picture?

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    Oh dear! Well there are a number of possibilities so you will have to troubleshoot logically. But a quick visual and push and shove check that no solder joints have come loose, especially on grounds and filter caps. If that doesn't reveal anything pull all the tubes and try it out. Start replacing one EL84, then the other then the phase inverter V2 and finally V1 and see when the noises start.

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    Quote Originally Posted by nickb View Post
    Oh dear! Well there are a number of possibilities so you will have to troubleshoot logically. But a quick visual and push and shove check that no solder joints have come loose, especially on grounds and filter caps. If that doesn't reveal anything pull all the tubes and try it out. Start replacing one EL84, then the other then the phase inverter V2 and finally V1 and see when the noises start.
    I did the check and it's only noisy when V1 comes in again. When leaving grid bare (no connection with any wire), there is a huge hum... I then I put a small unshielded wire from the grid to the hot end of the cathode resistor 1k5. And it is silent. Then putting a shielded wire with a 100R at the grid lug and only connect the shield to the ground side of the 1k5 cathode resistor and it is very noisy; moving around with the wire and it increases in hum and noise very badly.

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