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Thread: getting started Debugging a buzz/hum (AB763 deluxe, new build)

  1. #36
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    Cleaned up the solder joints, ready to plug in and give a listen. I bet that hum is gone now:

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    What is with that pot in that last pic?Did you see the pm I sent you?

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    Wow, how did you get a hold of that first guitar I ever modified, a Tokai TST-62?

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
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    When a low cost meter gives you a strange reading on AC Volts or Ohms, reverse the Red and Black leads to see of you get the same reading. In the AC Volts case, the meter can't deal with the DC offset. In the Ohms case, there is probably some DC Voltage coming out of the circuit you are trying to test.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    Quote Originally Posted by stokes View Post
    What is with that pot in that last pic?Did you see the pm I sent you?
    Just a bad joke, Stokes! Re your pm, yep, just replied.

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    Quote Originally Posted by loudthud View Post
    When a low cost meter gives you a strange reading on AC Volts or Ohms, reverse the Red and Black leads to see of you get the same reading. In the AC Volts case, the meter can't deal with the DC offset. In the Ohms case, there is probably some DC Voltage coming out of the circuit you are trying to test.
    Thanks will do!

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    Quote Originally Posted by mikepukmel View Post
    Just a bad joke...
    I thought it quite good, actually... I put that pic up on Farcebook & got a few bewildered responses, thinking I actually did that...

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  8. #43
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    All the good and experienced advice on grounding is a great thing to follow up on and idealize. But DO try other tubes as well. You mention that you "hope it's not the tubes because they are new". And I'm sorry that it's just not like that anymore. When I order tubes for a new project I buy extras on the likelihood that I'm going to get a couple of noisy preamp tubes and possibly even a set of power tubes with a bad one in there. Since you're questioning your own work that's the first thing you're going to suspect, but I can tell you from experience that a new, noisy preamp tube or even a questionable new power tube is always a possibility (for me it's actually been a probability). The first time it happened to me was about seven years ago and I spent no less than two days trying to troubleshoot my circuit when the hum was cased by a bad, new tube. Buzz from new tubes is also common. And a bad power tube can cause hum in the power amp. If such a power tube isn't bad in some obvious way (exploded, melted, doesn't light, etc.) you will need to take individual bias measurements of the power tubes to discover it.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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  9. #44
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    Quote Originally Posted by mikepukmel View Post
    Just a bad joke, Stokes! Re your pm, yep, just replied.
    Ok,you got me,I was gonna give you a head slap.

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  10. #45
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    After cleaning out the gutters (most of them anyway), and searching for a roof leak, that I didn't find, I spent the afternoon removing the brass plate from my Franken-Deluxe. I had a whole bunch of ground points all over the place, mostly because i didn't want to solder to the back of the pots. I removed all of the grounds, put a piece of the biggest bus wire I have, and put most grounds to that, then grounded the end of the bus wire, and a few other grounds to a star like thing I put way on the other side of the amp from the power supply. Its part way there. The changes reduced the hum so that I can barely hear it now. Barely! It was so quiet that I practiced until after 9pm when my wife got home from her church meeting.

    I now know how to make *excellent* ground loops!

    Chuck, thanks for the info! Got lucky, in one of the dozen things I tried, I had swapped the 2 output tubes and it helped some. I was surprised that the bias measurements I took, the two tubes were off by so much. One was 6.45 watts the other 7.3, at idle. Thought matched tubes would be closer. The output trans resistance measurements may be a bigger factor than the tubes, though, I don't know what is good and what is bad. I measured 210 ohms on one side 199 ohms 0n the other. later on, after heating up 212ohms and 202 ohms.


    Thanks to everyone again. Thanks to Randall for putting up photos of your awesome builds for me to check out how the grounding scheme should look like.

    Since the standby voltage is right around 475v Im going to upgrade the first two filter caps to 500v.

    Then will go back and do a more careful job finishing fixing up the grounding as per your suggestions.

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  11. #46
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    Quote Originally Posted by stokes View Post
    Ok,you got me,I was gonna give you a head slap.
    I deserve it!

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  12. #47
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    Before. The big messy stuff on the brass plate was ~90% of the evil. (stoopid stuff). Thanks everyone for the great suggestions and explanations of what is going on there.

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    After photos to come. I removed the brass plate, put a piece of the heaviest buss wire i had, moved all the grounds I could fit onto that, and a few other grounds to a single point (as close as I could get) way on the other side of the chassis, under the first input jacks. This made a huge difference.

    I tried routing the HV secondary center tap to a wire that ran up through the chassis to the first filter cap negative, then another wire on the other side of a little piece of bus wire that hooks the first filter caps together (the two caps before the standby switch) that didn't have any noticeable effect, probably because I had so many other wiring problems there.

    I would like to move the 2 x 100 ohm resistors to ground at the point where the rest of the power supply stuff is, mostly since its a hell of a lot easier to work (more room) and if I twist the pilot light socket, it wrenches the two resistor connections a bit.

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  13. #48
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    Progress.

    After:
    Much lower hum with vol at any level, almost not noticeable at lowest vol. Still kinda messy over by the input jacks. have to get a heavier piece of something for a buss bar, this one not sure if its 16 or 18 ga.

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    Some questions on wiring: one of the blog posts had a nice thread on where to put the grid stops for both sides of V1 and V2 (preamp tubes). The conclusion was to mount them right on the socket pin for best results (like the output tubes). My skills being what they are, the result looks a little too messy. I put a terminal strip and mounted one end of the resistor there. There wasn't enough room to mount another terminal strip for the other side of V1 and V2 (that go to the vol control center lug), so I cut the lead really short soldered the conductor of the shielded wire there, and put a piece of heat shrink on top. Is it OK to do this way? How else can I do this? All the photos and custom/boutique type Deluxe builds didn't have any grid stops on the second side of V1 and V2, and put the grid stops on the input jacks like Fender did back in ancient times.


    Other main question: all the the builds I've seen, have the eyelet or turret board mounted REALLY close to the chassis, so that it was really easy to run the wiring right to the chassis, then along the chassis to the tube socket. Dose it make a difference having that 1/2" or so wire in mid air as it snakes down to the chassis?

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    Last edited by mikepukmel; 12-03-2017 at 07:46 PM.

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    I wouldnt be concerned with the clearance under the turret board.As for the grid stoppers its best to cut the lead short enough so the body of the resistor is as close to the tube socket pin as possible.Then cut the other lead as short as you can and attach the incoming lead to that.I wouldnt bother with the terminal strips.

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    Quote Originally Posted by stokes View Post
    I wouldnt be concerned with the clearance under the turret board.As for the grid stoppers its best to cut the lead short enough so the body of the resistor is as close to the tube socket pin as possible.Then cut the other lead as short as you can and attach the incoming lead to that.I wouldnt bother with the terminal strips.
    Thanks Stokes! Great news. I would really like to ditch those terminal strips they really take a lot of space and makes it miserable to route the wiring nicely. Also, thanks re. the ckt board height. I was worried Id have to mount it lower and rewire everything. (although, its fun stuff to do, also nice to have a working amp to play through!)

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    The 500v caps arrived this week, had time to put them in today after getting the old clunker snow blower running (barely). With just that change, the bass sounds different. At 10, its not as "boomey", not sure the right term. Amp sounds good, just different. The 500v caps are 22uf, could not find smaller. I only replaced the two nearest the power supply, before the standby switch. Would never have thought just that change would affect the tone as much. Not complaining, just different. At least Im confident Im not going to pop the 16uf 475's when the amp is on standby since it was going over that.

    As Stokes suggested (thanks!), ditched the terminal strips next to the preamp tubes, and fought tooth and nail with the mogami shielded wire. Tried two ways: 1) take the resistors off the preamp pins, solder the really thin conductor wire on the mogami to one side of the resistor, then shrink some tubing over the connection to shield it but also make it stiffer, then solder the resistor to the tube pins. 2) clip the leads first, solder one side of the resistor to the tube pin, then solder the mogami conductor to the resistor, then shrink tubing over that. What I ran into is that the solder wicks up into the really thin conductor on the shielded wire. Then if I bend it back and forth a bit, it snaps off.

    Tried to make the ground bar nicer looking, put in a thicker copper wire, tough to heat it enough to solder all of the preamp grounds to. Plus my wife was yelling to shut the soldering iron off (she could smell the burning flux). End result, the grounding bar re-do looks a lot worse than the first trial. So, have another afternoon of fiddling with that.

    with the terminal strips gone, and the wiring on the tube side done, it looks a lot neater! Hopefully will have good longevity.

    Need some time to crank it up to breakup and see how it sounds loud, and how it behaves re blocking disto.

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  17. #52
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    The reason the 22uf caps make the bass sound "different" is when you use larger caps it tightens up the bass.I have used as high as 40uf as the input cap in many similar amps for just this reason.In all the amps I've built I never used the 16uf for the input,plate and screen caps,I usually use a 40uf for the input or main and screen but keep the lower values on the preamp sections.When these amps were designed the 16uf was adequate because they didnt think anybody was going to push them as we do today. Most Fender and Boogie amps,when playing loud it is important to keep the bass down around 2-3 to keep it from "farting out".Even when using the larger filters.Boogies use around 100uf as the input and you still have to keep the bass control down around 2.

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  18. #53
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    Quote Originally Posted by stokes View Post
    The reason the 22uf caps make the bass sound "different" is when you use larger caps it tightens up the bass.I have used as high as 40uf as the input cap in many similar amps for just this reason.In all the amps I've built I never used the 16uf for the input,plate and screen caps,I usually use a 40for the input or main and screen but keep the lower values on the preamp sections.Most Fender and Boogie amps,when playing loud it is important to keep the bass down around 2-3 to keep it from "farting out".Even when using the larger filters.Boogies use around 100uf as the input and you still have to keep the bass control down around 2.
    Thanks Stokes. Ive heard the term "tighten up the bass" but never knew what it meant. Didn't know Boogies had the blocking issue as well. Never played through one.

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    This is all good, but please tell us more about the snowblower.

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    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

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    Quote Originally Posted by mikepukmel View Post
    ...my wife was yelling to shut the soldering iron off (she could smell the burning flux)...
    If she could smell it, the fume must have been a lot more concentrated for you.
    It's best to avoid the risk of becoming sensitised to it, as an asthma response can result and put paid to your soldering days.
    I acknowledge that getting some suitable ventilation in place in a domestic scenario can be tricky,especially in winter.
    Occupational Asthma Causative Agent: Colophony (Solder)

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  21. #56
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    Quote Originally Posted by mikepukmel View Post
    Thanks Stokes. Ive heard the term "tighten up the bass" but never knew what it meant. Didn't know Boogies had the blocking issue as well. Never played through one.
    I wouldnt call it a "blocking issue", just the nature of the circuit.Have never heard of anybody diagnosing it as a blocking issue or attacking it as if it were and adding grid stoppers to address it.Was always just a matter of turning the bass control down if you put the volume high.

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    Quote Originally Posted by g1 View Post
    This is all good, but please tell us more about the snowblower.
    Done:
    http://music-electronics-forum.com/t45588/#post473322


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    Quote Originally Posted by pdf64 View Post
    If she could smell it, the fume must have been a lot more concentrated for you.
    It's best to avoid the risk of becoming sensitised to it, as an asthma response can result and put paid to your soldering days.
    I acknowledge that getting some suitable ventilation in place in a domestic scenario can be tricky,especially in winter.
    Occupational Asthma Causative Agent: Colophony (Solder)

    Yeah, PDF, thank you very much for the warning. I am stupid with this stuff, esp in the winter. In the fall I put a fan in the window blowing out, helped a lot, but still get the rosin smoke in the room. I had plans to build a mini fume hood, but could not locate all of the parts so I got lazy. There's a cool techie Fran Blanche on youtube who has some videos on making your own circuit boards. She has a home made fume hood in one of them, went over how it was built. I wanted to make, located everything except for a fan motor. I got lazy and just soldered with the fumes wafting up into my face. Stoopid. Since I want to do a lot more of these kinds of projects, will definitely build a small hood.

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    Quote Originally Posted by stokes View Post
    I wouldnt call it a "blocking issue", just the nature of the circuit.Have never heard of anybody diagnosing it as a blocking issue or attacking it as if it were and adding grid stoppers to address it.Was always just a matter of turning the bass control down if you put the volume high.
    Thanks Stokes. Im not up to speed on all of the concepts. Learning. I guess, if its to the point where someone wants to hear more bass (within reason!) and can't because the amp starts to block, then its an issue, but most of these amps were designed to work within a reasonable. range. have to learn that range for this amp.

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    Even if you can't get a hood or exhaust fan, you can get a small benchtop unit that will pull the fumes away and run them through a filter.
    search 'solder fume extractor'

    For example: https://www.aliexpress.com/item/high...yABEgIsY_D_BwE

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    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

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    Quote Originally Posted by g1 View Post
    Even if you can't get a hood or exhaust fan, you can get a small benchtop unit that will pull the fumes away and run them through a filter.
    search 'solder fume extractor'

    For example: https://www.aliexpress.com/item/high...yABEgIsY_D_BwE
    Pretty good THANKS, small unit and only a little over 30 bucks with shipping.

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