First 5F1 Build - Houston We (May) Have A Problem...

Discussion in 'Shock Brother's DIY Amps' started by corrado15, May 11, 2021.

  1. corrado15

    corrado15 TDPRI Member

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    Evening All,

    Built a 5E3 a few weeks ago. Surprisingly uneventful but very enjoyable. This subsequently led to a 5F1 build last weekend. Again relatively uneventful other than a quick last minute trip to Guitar Center when I opened the (undamaged) 6V6 box to find it had imploded.

    Anyhoo....kit is a Mojotone, though I used the StewMac layout which is slightly different as far as the ground bus and incoming 110V wiring are concerned, but should not be relevant to my potential problem.

    Amp fired up fine although all voltages are reading about 10% above that noted on the MT wiring with the exception of the readings at the 1.5K resistors which are about 10% low.

    Possible problem is that there is a low hum which starts after 10 seconds or so. Hum is there whether there's a guitar plugged in or not. It does not increase or decrease in volume when the amp volume is increased. Some hiss is evident when the amp volume is at full tilt, but it's nothing I wouldn't expect. According to an app on my phone, the peak frequency of the hum is around 198Hz (I was expecting 60 or 120Hz) and a reading of approx -40dB with the phone less than an inch from the baffle. Chopsticking did not appear to have any effect.

    I've read that SE amps are prone to hum but I don't enough experience to now if my amp is performing as expected or if there's an issue.

    Aside from this one niggle, it sounds absolutely fantastic. No crackles, buzz or other unexpected noises. Just the hum....

    Any and all advice and feedback gratefully received. Gut shots are attached...

    Thanks,

    Davy IMG_0191.jpg IMG_0190.JPG IMG_0189.jpg
     
    Last edited: May 11, 2021
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Can you show us exactly what's up with the artificial center tap resistors? That just looks odd to me, but it could be the angle of the wires or the photos.
     
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  3. corrado15

    corrado15 TDPRI Member

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    Thanks for the reply corliss1. Amp is buttoned up at the moment, but the ground side wires from the resistors are soldered together and then further soldered to the green wire going to a ground lug on one of the PT legs. The shape of the two resistors looks a bit weird as I had to bend the legs back on themslves slightly so as they wouldn't be bumping up agains the rear panel.

    Watching a build video at StewMac at the moment and they mention that there should be a low hum, so beginning to think all is good....

    Thanks!
     
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    So that's not right - you either have the center tap wire that gets soldered directly to chassis, or you have 2 100ohm resistors from each heater wire to ground.

    We want it to look something like the image below:

    Screen Shot 2021-05-11 at 9.58.46 PM.png
     
  5. corrado15

    corrado15 TDPRI Member

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    Hmm...I think we're talking about the same thing. Where your two resistors are soldered directly to the lug, I instead have them soldered to a 3" length of wire - that wire is then soldered to the ground lug on the chassis (per the StewMac instructions).

    Davy
     
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  6. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Ah - I thought you meant that wire was one side of the heaters :D

    If that wire goes to ground, you're all good!

    Your build looks pretty clean - you could always try different power/preamp tubes to see if the hum changes in any way, but it could be the standard single-ended hum. When the Champ 600 came out, Fender actually put out a service bulletin telling techs NOT to try and fix the hum with it.
     
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  7. corrado15

    corrado15 TDPRI Member

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    Thanks man - appreciate the help and feedback. Gonna go enjoy my amp now. Can't believe how awesome it sounds....!

    Davy
     
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  8. jman72

    jman72 Tele-Afflicted

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    My 5f1 is dead quiet- no hum. So quiet that I accidentally left it on for 2 hours yesterday when I thought it was off. So, they don't have to hum.

    I don't have those flying heater wires, though. Mine are flat on the chassis. I don't know if that could induce some hum.
     
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  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Ime the power cord wiring should be changed to a safer arrangement.

    The ((hot) black wire should terminate at the back of the fuse. The side of the fuse should connect to the switch. The (neutral) white should connect with the white wire of the transformer.

    To attempt to reduce hum, the 100 Ohm CT resistors could be installed on the 6V6 tube socket. One resistor from pin2 to pin8. The other resistor from pin7 to pin8. This would elevate the artificial CT. (It would look neater than flying those resistors as well.)
     
    Last edited: May 11, 2021
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  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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  11. corrado15

    corrado15 TDPRI Member

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  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Twist the power cord black and white wires as far as practical before they reach their terminals.

    Solder the power transformer red/yellow CT wire to the negative side of the 1st filter cap. This will keep the filter reservoir current path as short as possible. This correction has the potential to reduce buzz.

    I am just a guy on the internet and I just don't like seeing the mains ground wire where you have it placed. Move the power cord ground wire to the other side of the power transformer. Move it away from the power transformer wires. IDK if this will help with hum. Couldn't hurt.

    The 6V6 white cement 5W bias resistor gets hot. Create some space so the bias cap does not get cooked.
     
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  13. bebopbrain

    bebopbrain TDPRI Member

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    Is the hum really 198Hz? If you play your open G-string is it the same pitch as the hum? If so, you may have an unidentified noise source in your house. Do you live in a Cuban embassy?

    Another possibility is that the amp is oscillating. If you disconnect the negative feedback from the speaker, does the hum change?
     
  14. pmjennin

    pmjennin TDPRI Member

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    Running the heaters to the 6V6 through the holes in the eyelet board looks off to me. On mine, I run them straight down to the filaments from the pilot light. But I don't know if that could cause any hum.
     
  15. lathoto

    lathoto Tele-Meister

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    Eliminate the tubes as a potential source first. Does the hum show up only after the amp is up to temperature?
     
  16. Mongo Park

    Mongo Park Tele-Afflicted

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    Another source of hum is the on off switch connected to the volume pot. Twist the power wires going to the vol pot and if possible keep them away from other wires ( hard to do). If this is the source a separate on off switch might eliminate the hum but that would involve drilling another hole.
    This was the source of hum on my first build and is part of the design, one less hole one less part kept Leo from bankruptcy a reality of selling amps.
     
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  17. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    That *is* a darn tidy build. Deserves to be quiet. You're getting a lot of good advice. Naturally, it's diverse; hum has about 1,000 causes, with *one* being single-ended operation. :) I suggest a bit more diagnosis before we take the patient to the OR. Is this a mild little SE hum (which can be very quiet) or not? How about a phone recording of the noise? (Hmm, you may have to link to a video on youtube or sound file on soundcloud).

    If the frequency is really ~200, that's weird. Is it higher or lower than a low E? Higher than a 5th-string A (110Hz)? 4th string D (146)? If so, I endorse the idea to try swapping OT primaries (or secondaries, not both) to check for sneaky oscillation.

    IME, heater wires through the passthroughs probably don't add noise -- and by themselves flying heater wires definitely shouldn't. At least those wouldn't be high on my list to change.

    Agree with LLC that the HT center tap (the red/yel) should ideally connect to the negative pole of the first filter cap. Usually a pretty easy change (you may need to splice in a bit of the red/yel you cut off) and IME a good deal quieter. I also agree you could 'elevate' your artificial CT, but at this point that'd be a tricky operation, so I'd wait on that too.

    Grounding? I agree it can't hurt to move the household ground wire to the left side of the PT, but OTOH my first amp build anchored it lower right (tho flat to the floor) and it is dead quiet. But for your overall ground scheme: Is it like this StewMac layout? If so, yes, Rob's version might be better. But at this point changing yours would be a royal pain, so I'd save that step.

    upload_2021-5-12_8-59-44.jpeg

    Your power wiring may be OK, actually, since AFAIK the white and black primaries from the PT are equivalent -- two ends of one long wire, and just as often not color coded. I *think* you've got the key bit: Hot to fuse to switch to (either) primary, neutral to other primary. I *think* StewMac's white primary to the hot leg is just visually, not electrically, confusing. Yes/no, gentlemen?

    OTOH, having the household AC pass over to the pot is a *notorious* source of 5F1 noise, and so as noted twisting the black and white together (including the hot to and from the switch) is a good plan, and then see if you can tuck the pair well up or down into the left top or bottom corner of the chassis.

    A few more diagnostic steps:

    In an electronically noisy environment (say opened up, in your shop) turn off fluorescent lights and power tools, and at least throw a cookie tin over the chassis for shielding. Can you run your other amp open in the same environment on the same outlet?

    Test input jack grounding switch/prong. Visually check that the switch prong securely contacts the tip prong when the plug is removed. With the plug removed, you should get a continuity beep (or resistance < 1 ohm) between the tip, switch, and ground lugs on the jack, as well as between the tip lug and the chassis.

    To eliminate or ID heater hum, lift the tube-heater chain off the lamp and power it with a 6V (dry cell) battery, which being DC can't induce hum.
     
    Last edited: May 12, 2021
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  18. D'tar

    D'tar Friend of Leo's

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    Hmmm... Is it more or less quiet to switch neutral rather than line?

    This is a good place to save a voltage chart for future reference.

    One may "Really" like a bypass cap on v1a.

    Supply and Grounding may be worth making some subtle changes. However, if you are satisfied with the noise and safety level of the amp, then carry on!
     
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  19. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good point -- I think he's actually switching the hot here, right? But is the 5F1 a case to do it the 'wrong' way, with neutral over to the switch? I flunked AC, but I'd-a thought maybe not?

    And yeah, for sure, if it's not too noisy, sounds good when played, and has decent voltages, cancel surgery and save the patient any risk, cost, or delay in rocking out. :D
     
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  20. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Don't get too hung up on the 198Hz peak reading. Studies show the many different apps available are not all equal. Some apps are better than others. The phone also has to be taken into account. Most microphones in these devices are not precision mics. The phone may also have circuitry to shape the lows to provide a more pleasing listening/recording experience.
     
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