5e3 build hum/hiss troubleshooting

Discussion in 'Shock Brother's DIY Amps' started by choosebronze, Jul 28, 2020.

  1. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    I posted this over at EL34World first but that topic seemed to lose focus of the issue. I figured I'd try again here. I finished this 5e3 build last week. Standard layout without Standby switch (Robinette's drawing) with the addition of his 3-way feedback switch, rectifier protection diodes, and power tube screen resistors.

    The amp worked on first power-up, had to fix a couple things and now it sounds great. Except I've got a noise I can't chase down. I don't know if I should call this a hum or a hiss: https://www.dropbox.com/s/rary9vnla8j9mkl/5e3humhiss.m4a?dl=0 .. (There I've got the tone at 12, I turn the Normal Vol up from 0-12 and back, and then the Bright Vol up and down the same way). The best way I can explain it is it sounds like the crowd at a baseball game on TV when no action is happening. Just this mellow breathy static.

    It behaves as if V1 were microphonic and I think that's all part of this issue, but it must be coming from the lead dress or a component, not the tube itself. I've put 3 different tubes in V1 and a light tap on the tube pops loudly through the speaker. A tap on the chassis or chopstick of the wires going to V1 produces a quieter version of the same sound.

    It occurs when the volume is turned up on either the Bright or Normal channel, I assume it's something before V2. That narrows it down quite a bit. I also assume some component isn't properly grounded. But all of the components in that section look good to me. I've gone over the input jacks a dozen times and they look wired correctly to me. I can post a photo of the jacks if needed. It did trick me into thinking it was the Bright Cap at one point but I removed one leg of that cap and the problem persists. I swapped out the tone pot just in case, that didn't change anything.

    Solder joints look fine to me and I've re-flowed most of them just to be sure. Rather than rebuilding that whole section, is there a most likely culprit in there other than the jacks? Is there a "best" order to troubleshoot the components? I've never heard a sound like this before so I'm lost.
     
    Last edited: Jul 28, 2020
  2. BigDaddy23

    BigDaddy23 Tele-Holic

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    Hey mate - did you record that with a guitar plugged into the input or nothing at all in an input socket? It’s normal for some hiss to be there but I could hear a bit of AC buzz in the background which I think is more of a problem. Check your grounds, especially the input jack/preamp ground. That needs a star washer and done up tight to ensure a good ground.

    Some folks like to use metal film resistors for the input grid stoppers as they are apparently quieter hiss wise as opposed to carbon composition. I don’t have enough comparative experience in my builds to accurately say one is louder than the other but I’ve only used metal and carbon film and metal oxide as input grid stoppers. Putting them right on pin 2 rather than on the board >wire to pin 2 is also recommended to limit the chance of it behaving like an antenna, picking up (and therefore amplifying) erroneous noise.
     
  3. Paul-T

    Paul-T Tele-Meister

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    Was yours the build with the blue/white wires to the preamp valves?

    You'd be amazed how much noise can come from one simple loop of wire. You could write a thesis on it. It's definitely worth spending an afternoon on the lead dress to the preamps, redoing the soldering.

    Take a look at some of these builds and take note, especially, of the preamp lead dress. I coped d'tar and King fan's lead dress religiously and my 5E3 seems very quiet.
     
  4. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Please post some pictures of the wiring.
     
  5. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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  6. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    That audio is with no instrument connected. I think I hear the buzz you're talking about. I can't really tell what it is because I can't hear it clearly under the hum. I'll triple-check all the grounds. Input jacks all have lock washers and are on tight.

    I don't think so, I tend to use a lot of leftover wire leading to much uglier amps than the pretty color-coordinated ones. I'll take some pictures tonight and look at the topic you linked to. I've re-wired pretty much all the V1 leads at this point but I can do it again. I also found a topic where Rob Robinette was talking about how finicky and noisy his leads were when touching each other, so I've spaced out all the V1 wires.

    That's wild! I've always heard people talk about "noisy" carbon resistors before but I didn't know they meant this loud! I assumed they meant some kind of minor audiophile imperfection. Easy enough to pick up a couple metal film resistors and try them.
     
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  7. dan40

    dan40 Tele-Afflicted

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    If there was no cable plugged into the input jack when the noise was happening, that would indicate that the switch in the shorting jack is not closing completely and grounding the grid. Either that or the jack is not properly grounded to the chassis. Can you post some good closeups of the input jacks?
     
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  8. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    I rewired all the input jacks and V1 just to safe. Also replaced the 68K grid resistors with metal film. No change in symptoms. I know from this angle it looks like there's a connection to the grounded tab of the Bright 2 jack but there isn't, it's just old solder.

    I've seen guys with even shorter wires going to the tube sockets. Maybe a cm or two. Could that distance make this much difference? If I shorten the wires anymore I don't see how I'd get the board out without disconnecting everything.

    Sorry, I know the board is dirty. Haven't cleaned the flux residue yet.

    5e3 1.jpg 5e3 5.jpg 5e3 2.JPG 5e3 4.jpg
     
    Last edited: Jul 29, 2020
  9. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Sounds like it is initiated from the cap side or rectifier area and picked up by the preamp. More pictures please of filter caps and PT connections.

    Do you have a spare filter cap to run in parallel with the filter caps?

    EDIT: Listening more carefully it seems to be a 60Hz hum so I'll change my mind. I am thinking a ground loop or the poor lead dress especially near the heaters. Bad tube?

    What kind of heater center tap are you using and where is it terminated?
     
    Last edited: Jul 29, 2020
  10. dan40

    dan40 Tele-Afflicted

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    I was referring to the shunt switch located on each of the jacks. They should close and ground the grid of v1 with no cable inserted into the jack. Occasionally you will get a new one where the switch does not fully close and complete the ground. A gentle bending of the contacts will usually get the switch closing properly.

    Is this noise also occurring with a guitar plugged in and playing?
     
  11. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    Thanks for the replies guys. Here are some more pictures. Did some more testing: I've tried replacing all tubes and the issue hasn't changed and I think you're probably right with grounding, since V1 is still acting microphonic with all new tubes and all the new wiring I've done.

    Heater is center tapped from the PT, tied to a PT bolt along with the HT ground and power amp ground. I do have a couple spare caps I could run parallel with the filter caps just to be sure. Just gator clip both sides and leave the old one where it is?

    Yea, the noise occurs with an instrument connected. As you'd expect it's a little less noticeable with guitar over it, but it's still loud enough that you can hear it along with the playing. Same behavior on both channels. Interesting about the jacks, so would the test for that be to press the two little parallel prongs for the ground and sleeve together to make sure they're contacting with no plug inserted? I can press them together with a chopstick.

    My assumption at this point is that something (grounding, component failure, lead dress, etc) is causing V1 to go microphonic and what I'm hearing as this static hiss is a result of that. Does that seem reasonable? I guess it helps in my head to know if I'm tracking down one issue or two.

    1.jpg 2.jpg 3.jpg 4.jpg
     
  12. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    I tried running a parallel filter cap on each one - no change.

    The biggest internal offender for the microphonic V1 is the wire going from the grid stoppers to the V1A grid (pin 2). A light chopstick to that wire makes a big pop. The V1B grid wire pops too, but much less loud. Oddly, a chopstick to any of the 4 grid stopper resistors makes no pop at all. Same with the wires between the input jacks and the grid stoppers, no pops. I took the board out, the wires going to the V1 grids aren't touching on the underside.
     
  13. dan40

    dan40 Tele-Afflicted

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    Yes...you can gently press on each switch contact with a chopstick to be sure they are closing properly. Do this with no cable plugged in and see if one of the jacks has any affect on the noise while pressing on the contact.

    I was looking at the pics and from a quick glance I can see that several of the pins on the power tube sockets could use a bit more solder. I would carefully go through and reheat and reflow all connections at the sockets and grounding points and see if it helps.
     
  14. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    I addition to what dan40 said.

    Elevate the heater CT. Connect it to the turret of the junction of 6V6 pin8 and the bias cap/resistor. It helps to reduce heater hum.

    Connect the PT CT to the same bolt as the filter cap negative ground.
     
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  15. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    Wrt the hum I would advice a humdinger (can be used with an elevated heater also). You can optimize the hum level (in an amp with no other grounding problems) very precisely. It has never been exactly at 50% in my 2 builds.

    Theoretically the biggest noise source is the resistance in series with the input grid. I see you've put some metal film resistors on the input jack. Based on color alone and photos of only a very small area of the circuit I would say you've got metal oxides in that position. I would suggest metal film.
     
  16. Nickfl

    Nickfl Tele-Afflicted

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    Grid stoppers are most effective when atteched as close to the socket as possible. If you move those grid stoppers off the board and onto the socket that will probably eliminate the microphonic issue.
     
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  17. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    Agreed.

    I have some trouble with this statement.
    Grid stoppers form a low pass filter with the Miller capacitance of the tube. It is supposed to filter out signal above 20kHz (although it might be used to filter the audio top end on purpose). Putting it right at the tube socket makes sure no new (inaudible) signals and noise are being picked up after it was filtered out. I don't see how audible microphonics will be solved by moving the grid stoppers as close as possible to the tube sockets, even though it is a good idea for the reason explained above.

    Having said that, I'm open to learning something new.
     
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  18. Nickfl

    Nickfl Tele-Afflicted

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    I can't really explain why it happens, but I've experienced something similar to what the OP seems to be describing where the lead between the grid stopper and the socket pin was microphonic, but the lead between the input jack and the grid stopper was not. Moving the grid stopper(s) directly onto the socket effectively eliminated the microphonic lead and solved the issue. I dont know why the wire after the grid stopper is microphonic while the wire between the jack and stopper isn't, but its a thing I've seen at least twice before.
     
  19. tubegeek

    tubegeek Friend of Leo's

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    It's because parasitic (RF) oscillations make all sorts of stable, no-big-deal flaws MUCH worse. When the oscillations ride on top of a LITTLE bit of 60 Hz hum, all of a sudden you can't get rid of the hum, it's everywhere. If a minor microphonic problem exists (and there's always some) all of a sudden it's a huge problem.

    A length of the wire has some inductance and the shorter it is the less it has. In combination with the Miller C it can either resonate at RF or filter out RF, and filtering is much more betterer. This is why you will see ferrite beads on grid leads in RF equipment, so the signals can be controlled selectively and not just allowed to run roughshod.

    I learned about this stuff making phono preamps with a Russian tube 6C45P that has extremely high gm, and is very sensitive to layout and RF precaustions. It's real, and it's not easy to fix if it happens. I had to bypass the heater leads to ground with ceramic caps on some of those builds to get them stable.

    BTW - there is some property of carbon comp resistors - low inductance I guess? - that makes them the preferred type for grid stoppers, they are not usually ideal for the grid circuit due to excess noise. But grid stoppers can be a pretty small value so that's not really a problem in this application.
     
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  20. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    Because of the tone dust and they used to use 'm :)

    Theoretically the noise in these grid stoppers is the main source for noise. I even used wirewounds in my first amp. No problem, no noise.
     
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