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

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

    While adjusting the bias for the first time, I had the chassis upside down on my desk, with a
    fluorescent mag light over the chassis. It took a while to figure out that the loud hum I
    was hearing through the amp was due to that. After adjusting the bias, discharging the caps,
    I insatlled the chassis in the cabinet, restarted, and still hear some hum. its not terrible
    but louder than expected and for this amp, should not be there. Amp seems to be working OK
    otherwise.

    I started searching this blog site for "hum" and "diagnosing hum"s, cine there should be many
    such threads, but didn't see what I was looking for, a basic "getting started" for debugging
    hum.

    Anyone can point me to a few of those threads, that would be great.

    Thanks,
    Mike

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    Geofex.com

    Courtesy of R.G. Keen. Top left corner, tube amp debug. Lots of help there.
    As a start, pull a preamp tube one at a time. Also, do this with no cord plugged in...

    Justin
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    To expand on what Justin said,start with the input or V1,pull it,if the hum/buzz goes away that is where the noise is coming from.If it still hums,replace and pull V2 and so on,til you find the source.On another point,do you have the inside of the cab over the chassis opening shielded?The reason I ask is that you said it was there with the chassis on the bench and was "still there" to a lesser degree.Without the shield you would still get some interference.Youhave to make sure the shielding has good contact with the chassis so it is grounded.
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    Quote Originally Posted by Justin Thomas View Post
    Geofex.com

    Courtesy of R.G. Keen. Top left corner, tube amp debug. Lots of help there.
    As a start, pull a preamp tube one at a time. Also, do this with no cord plugged in...

    Justin
    Thanks for the link, lots of great stuff there.

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    Quote Originally Posted by stokes View Post
    To expand on what Justin said,start with the input or V1,pull it,if the hum/buzz goes away that is where the noise is coming from.If it still hums,replace and pull V2 and so on,til you find the source.On another point,do you have the inside of the cab over the chassis opening shielded?The reason I ask is that you said it was there with the chassis on the bench and was "still there" to a lesser degree.Without the shield you would still get some interference.Youhave to make sure the shielding has good contact with the chassis so it is grounded.
    Thanks, doing that now. Well, there's a low hum in the speaker when I flip the standby "on" (conducting) and it is still there when the volume controls are all the way down. I think the input jack ground is working ok.

    Then there is another hum, more like a buzz, when I turn the volume up. I think I hav a bad ground in the preamp area, looking now.

    I do have shielded wiring from the input jacks to the preamp tubes and from the preamp tubes back out to the vol control. Checking the wiring now.

    Hoping its not a tube or cap, since they are brand new.

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    Quote Originally Posted by mikepukmel View Post
    Thanks, doing that now. Well, there's a low hum in the speaker when I flip the standby "on" (conducting) and it is still there when the volume controls are all the way down. I think the input jack ground is working ok.

    Then there is another hum, more like a buzz, when I turn the volume up. I think I hav a bad ground in the preamp area, looking now.

    I do have shielded wiring from the input jacks to the preamp tubes and from the preamp tubes back out to the vol control. Checking the wiring now.

    Hoping its not a tube or cap, since they are brand new.
    Can't quite pinpoint one thing. With v1, v2 out, still have steady low freq hum, independent of volume ctl.
    The buzzing hum is almost gone, though, so I have some wiring trouble in my preamp area.

    With all tubes in, when I tap on the grid stop resistors, they are microphonic. Not so with the tubes.


    Did notice that when the amp is in standby mode (standby off), with power on, the voltage going into the first filter caps is over 473 range within a couple of volts. Comes back down when I turn the standby to "on". I think I need to put higher voltage rating caps on the first two filter caps.


    Double checked heater wiring, can't see anything terrible there. Moving wires around with a chopstick didn't seem to change any of the noise.

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    1) Might be part of the issue: I put 3 lug terminal strips next to the preamp tubes, and put the grid stop resistors from the strip to pin2 on the tube socket. Then the shielded wire runs from the terminal strip to the input jacks. The shielded wire is grounded at the jack end.


    2) Also might be part of the issue: The only offsets I could get to mount the eyelet board were pretty tall, something like 3/4". When I look at photos of lots of other builds, the eyelet board is mounted very close to the chassis, so that the wiring esp to the tube sockets runs down to the chassis very quickly, then across the chassis to the tube pins.

    3) maybe I toasted the 2 16uf filter caps on the transformer side of the standby switch. I had done some measurements with all tubes out except for the rectifier and got voltages in the 480 range, maybe a little higher. Also, even with all tubes in, when the standby switch is off (no signal) the voltage to those two caps is at or a little over rated 475. I did notice on the last few runs that the plate voltage doesn't get all the way to 430v its more like 405v now. Could this be a sign that the filter caps are toasted (partially) and leaking AC into the output tubes?

    (Geez, wish I had a cheap scope, too many bills this month, will try to get one in the spring).

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    To check for AC ripple on the B+ rail just use your meter set to ac on each filter cap,no need for a scope.At the first filter cap you may see up to 5vac.As you move down the rail you should see less than 1vac at each cap
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    None of the three things you mentioned in your post number 7 are of any concern to me. I don't believe they would have anything to do with the hum or Buzz that you are troubleshooting.

    You could pull the phase inverter tube and determine if you still get the low hum. If you do then you have isolated the problem to the power amp section. That type of hum can be due to Output section imbalance, bias Supply Ripple or ground loops. Let us know what you find and we will go from there. From your reports there is more than one thing going on. It's also hard to tell from the description if the level of hum and or Buzz is worse then it would have been in an old amp design. There are lots of methods to make amps quieter than the vintage designs originally were and so far we have only touched on a few of them.
    Last edited by Tom Phillips; 11-29-2017 at 09:01 PM.
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    With the shielded wire you used, have you grounded it at only 1 end?

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    Post some pics ,
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    Stokes
    To check for AC ripple on the B+ rail just use your meter set to ac on each filter cap,no need for a scope.At the first filter cap you may see up to 5vac.As you move down the rail you should see less than 1vac at each cap

    Tom
    None of the three things you mentioned in your post number 7 are of any concern to me. I don't believe they would have anything to do with the hum or Buzz that you are troubleshooting.

    You could pull the phase inverter tube and determine if you still get the low hum. If you do then you have isolated the problem to the power amp section. That type of hum can be due to Output section imbalance, bias Supply Ripple or ground loops. Let us know what you find and we will go from there. From your reports there is more than one thing going on. It's also hard to tell from the description if the level of hum and or Buzz is worse then it would have been in an old amp design. There are lots of methods to make amps quieter than the vintage designs originally were and so far we have only touched on a few of them.

    Mozz
    With the shielded wire you used, have you grounded it at only 1 end?

    Copperheadroads
    Post some pics

    =====================================================
    I'll drag out my camera and get some good photos. Eeek, was hoping you woulndt ask for photos! Will check for AC ripple <= 5v. Then will take out the PI and give a listen.

    Yep, the shielded wire is only grounded on the brass plate side.



    Yes, I think there are a few things going on here with my first wiring job, judging by the symptoms.

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    Here goes. Please don't laugh. OK go ahead.

    https://www.flickr.com/photos/145832...7689043482311/

    Update:
    With the phase inverter tube removed, the low hum (can't tell if its 60 or 120hz) that is independent of the vol control is almost zero. Just a tiny bit left.

    Q: Can I disconnect or short the two wires leading into the 2 x 220k resistors leading into the PI? Not sure what that would prove. Thinking maybe some noise is getting in through the 2 wires leading from channel 1 and channel 2.
    Last edited by mikepukmel; 11-30-2017 at 03:36 AM.

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    Quote Originally Posted by stokes View Post
    To expand on what Justin said,start with the input or V1,pull it,if the hum/buzz goes away that is where the noise is coming from.If it still hums,replace and pull V2 and so on,til you find the source.On another point,do you have the inside of the cab over the chassis opening shielded?The reason I ask is that you said it was there with the chassis on the bench and was "still there" to a lesser degree.Without the shield you would still get some interference.Youhave to make sure the shielding has good contact with the chassis so it is grounded.

    Hi Stokes, eek Re "On another point,do you have the inside of the cab over the chassis opening shielded?" nope, the chassis isn't shielded over top. I shut off all the lights and other stuff in my office, though.

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    Powered down, discharged the caps, swapped the power tubes. Seems the low freq hum is a *little* lower now.


    Also, while discharging the caps, I noticed something: the first two caps on the rectifier side of the standby switch take a long time to discharge and hold a lot of "juice", like 400 something volts. By the time I get to the other 3 caps, they are almost at zero. Do the 3 upstream filter caps discharge through the circuit?


    Powered down again, sprayed a tiny bit of DeOxit in the pots. let sit, started the amp again. Before the deoxit, when I turned up the vol pots, I would hear a little crackle in the speaker. That is gone now. And when I turn up the vol, it doesn't amplify the buzz as much, its more white noise.

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    Supporting Member The Dude's Avatar
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    A little bit of noise is to be expected. Have you checked bias yet? There's the possibility that you're idling hot and a small noise is made large by excessive bias.
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    Quote Originally Posted by The Dude View Post
    A little bit of noise is to be expected. Have you checked bias yet? There's the possibility that you're idling hot and a small noise is made large by excessive bias.
    Thanks Dude, yeah, checked a couple of times. I have a really crappy meter, but I think its OK for this. The tubes are Electro Harmonix 6V6's, rated 14w max. I have them biased pretty cold, around 6.5W to 7.5W (depending on if measuring across the 1 ohm bias resistor, or using the OT resistance and drop method. When I crank the amp, it doesn't break up, that's how cold it is.

    The hum that comes on with the vol on min and when I switch the standby "on" is bothering me most, worried something is wrong with the power supply caps or output tubes.

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    Supporting Member The Dude's Avatar
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    Have you checked for ripple on the B+ as suggested yet? That will tell you if you have power supply issues.
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    Quote Originally Posted by The Dude View Post
    Have you checked for ripple on the B+ as suggested yet? That will tell you if you have power supply issues.
    Tried to, didn't work. I think its because of my toy meter. I can measure sine wave AC, e.g. wall outlet voltage, and the HV secondaries OK, but when I tried the AC setting on the voltage coming out of the rectifier around the filter caps I get a garbage number (meter is reading something like 800 on a 500 scale) Will order a new meter soon.

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    Set the meter to AC, and add a capacitor in series with the probe. A 0.1uf or 0.047uf would work. That cap will block the DC from your meter, and the cap will pass the AC.
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    Quote Originally Posted by Enzo View Post
    Set the meter to AC, and add a capacitor in series with the probe. A 0.1uf or 0.047uf would work. That cap will block the DC from your meter, and the cap will pass the AC.

    Thanks Enzo, can't wait to get back from work and try it tonight!

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    I've typed up guides to grounding and hum several times. It's worth seeing if you can find one. I'm not in a typing mood right now.

    However, I can tell you a few gotchas to look for.
    One is nearly always missed by beginners. This is the critical wiring of the transformer lead to the negative side of the first filter cap. In an amp with a typical center tapped HV secondary, the center tap of the power transformer must be wired ONLY to the negative lug of the first filter cap. Not to the chassis, not to a star ground, not to anything else, only the negative lead of the first filter cap. This is because large currents go into and out of the first filter cap synched to the power line and caused by rectification. You simply must not let the return wire to the transformer CT share any other ground wire in the amp, or you will get small pulses on ground at 120Hz, and this will be heard as hum.

    Another: magnetic field transmission of hum depends on large conductor loop area for effective transmission. To normal humans, this translates as "keep current loops small" and that devolves into "to the extent possible, make all AC-power wiring be twisted pair". It minimizes the area of the hum loop and keeps mag-field transmission low. Make AC heater wiring as much twisted pair as possible; there are people who think this doesn't matter, but it sure can't hurt.

    Another: use tube rectifiers or fast, soft recovery silicon diodes to make rectifier turn-off soft and stop these every-120th-second transients from shock-exciting all the wiring and giving you a 120Hz buzz much like fluorescent lights.
    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Quote Originally Posted by R.G. View Post
    I've typed up guides to grounding and hum several times. It's worth seeing if you can find one. I'm not in a typing mood right now.

    However, I can tell you a few gotchas to look for.
    One is nearly always missed by beginners. This is the critical wiring of the transformer lead to the negative side of the first filter cap. In an amp with a typical center tapped HV secondary, the center tap of the power transformer must be wired ONLY to the negative lug of the first filter cap. Not to the chassis, not to a star ground, not to anything else, only the negative lead of the first filter cap. This is because large currents go into and out of the first filter cap synched to the power line and caused by rectification. You simply must not let the return wire to the transformer CT share any other ground wire in the amp, or you will get small pulses on ground at 120Hz, and this will be heard as hum.

    Another: magnetic field transmission of hum depends on large conductor loop area for effective transmission. To normal humans, this translates as "keep current loops small" and that devolves into "to the extent possible, make all AC-power wiring be twisted pair". It minimizes the area of the hum loop and keeps mag-field transmission low. Make AC heater wiring as much twisted pair as possible; there are people who think this doesn't matter, but it sure can't hurt.

    Another: use tube rectifiers or fast, soft recovery silicon diodes to make rectifier turn-off soft and stop these every-120th-second transients from shock-exciting all the wiring and giving you a 120Hz buzz much like fluorescent lights.

    Thanks RG! Reading now. Also, I'll search for your other posts.


    Re "This is the critical wiring of the transformer lead to the negative side of the first filter cap. In an amp with a typical center tapped HV secondary, the center tap of the power transformer must be wired ONLY to the negative lug of the first filter cap. Not to the chassis, not to a star ground, not to anything else, only the negative lead of the first filter cap."

    Aha, ok, I did the exact opposite here, like in the photos of the old period (noisy as hell) AB763 chassis. My center tap wire goes right to ground very close to the power transformer. And the first ground wire from the first 2 filter caps is grounded to the same lugs. So, I should lift the ground wire leading back from the first filter caps, lift the center tap, and wire those together, no ground. Will do.

    Re twisted wiring, yes I tried to do that as best as possible, all bundles are twisted.

    Not sure i understand the last one.

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    Tried the capacitor/ac thing, didn't have a big enough cap, could not measure anythig. Will order a couple of 0.1 and 0.047 non polar caps, or try to pick some up on the way home from work tomorrow if I can find an electronics place (very rare these days around here).

    Downloaded 60 and 120 hz test tones. Fairly sure that the low hum in my franken-amp is 60hz. (could hear the beats between the amp and my computer speakers. So, this seems to indicate heater wiring inducing into a signal someplace, and probably not hv from the rectifier?

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    RG, read through a bunch of hum/grouding threads, could not find one of yours, but some good ones. I'm still not completely down on the wiring routing. I got the idea why the CT from the power trans HV needs to first get routed to the neg filter caps, but no ground at all? I found a few threads that had a whole bunch of different configurations and looked like the gurus were saying to route first to the filter cap, then run another wire from that connection to ground. I mean, the filter negative has to get grounded at some point, its not left floating, right?

    Thanks,
    MP

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    He wasnt saying they should not be grounded.What he said was to ground the first cap with the PT CT at the same place,usually one of the transformer mounting bolts.I had gone thru this in a different post from you.I said to move the screen cap ground from the point where the preamp cap is grounded to the point with the first or main cap and PT CT.
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    What's going on here is that (1) wires are not short circuits, they're low value resistors and (2) single ended tube circuits have zero ground noise rejection.

    Taking #2 first, any noise on the "ground" that a tube's grid is referred to will ride right in with any signal and be amplified by the tube. So you have to keep the ground with as little noise pollution as you possibly can.

    That's where #1 comes in. Wires are resistors - if you shovel current through them, the only way to do that is with a voltage across them. This is an expression of Ohm's law: V = I*R. R may be small, but if current is big, or the use of the voltage is to be amplified a lot, the resulting voltage can be truly annoying. Well, Ohm didn't say truly annoying, but you know what I mean.

    We have to have some place to designate the One True Source of Zero Volts in the amp. That's usually the negative side of the first filter cap, although it may be in other places depending on special needs. So we want all those triode grid resistors to go to the filter cap ground and nowhere else. Problem solved. Well, except that the other part of the tube are busy funneling current through the plate, into the cathode, and draining that off to ... ulp! ... ground. The wire leading from the cathodes and cathode resistors carries current, and that current, times the wire resistance, is a voltage. For a single stage, it's not too bad to connect the grid wire to the cathode ground, it's a bit of positive feedback, and small at that, but where many tubes and higher currents - like the output stage, and speaker ground - get into it, you can come up with screaming oscillations because of the unintended feedback.

    Same with rectifier ground noise. The rectifiers have only a tiny part of the AC half-cycle to get all the current stored into the first filter cap to hold up the supply voltage till same time, next half cycle. So there are BIG current pulses when the rectifiers turn on into the caps. Big enough to cause the wires back into the transformer at the CT to wobble up and down with it. If you've connected the CT somewhere off on the chassis and the signal ground reference to the first filter cap, and the input jacks to the chassis, the input jacks' "ground" is being jerked up and down by the rectifier pulse currents, and you hear it in the output.

    The way to fix this whole mess is to force currents to flow only in specific wires, so the voltage drops in the wires are at least known. And if you have critical "reference" ground wires, make it so no currents flow in those wires, so the voltage drop in the wires is zero.

    We designate the negative side of the first filter cap as the One True Ground. The CT connects here. The current pulses from rectification cross only the wire back into the transformer, so they can't yank the first filter cap's negative lead up and down. A second wire leads from the first filter cap to the output tube cathode circuits. Big currents here, not so big as the rectifiers, but they have to follow their own wire, so they can't muck up the input stages. Speaker ground? Goes right back to the first filter cap negative, not the chassis, but for different reasons.

    The second, third, fourth, etc. bypass/decoupling cap negatives connect by wires to the One True Ground. The AC they "bypass" goes through those wires and not into mucking up signal references. Finally, the grid and cathode grounds for tubes - they get one or more wires back the One True Ground. The currents are small, the resistances are small, so the perturbations are small.

    Ground may mean:
    >> A source of zero volts as a reference
    >> A shield for shunting interfering radiation or conduction away from sensitive points
    >> A "sewer" that returns used electricity back to the power supply for recycling.

    Mixing up the uses of these and forcing them onto a single wire can cause heart - and head-aches.
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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Quote Originally Posted by mikepukmel View Post
    Tried the capacitor/ac thing, didn't have a big enough cap, could not measure anythig. Will order a couple of 0.1 and 0.047 non polar caps, or try to pick some up on the way home from work tomorrow
    The cheap meter is probably half wave rectified so it won't work with just a cap like a bridge rectifier would. Try a 0.1u cap in series with the +ve meter lead and a 1M resistor across the meter inputs on the meter side of the cap. That might work (or not) depending on the meter's circuit.

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    Senior Member Enzo's Avatar
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    Why won't a cap work in series with a half wave? Isn't that how most taps off HV bias supplies work?
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    Have to ask,Mike,how are you connecting the leads to measure the ac on the filter caps?Even the cheapest of meters should get a reading.Dont quite understand what you are saying here "(meter is reading something like 800 on a 500 scale)"
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    On cheap meters, DC measures OK, but if you set it to AC and try to read the ripple sitting on a 500v rail, it can't ignore it, and so reads WAY high. They lack an isolation cap at the input. AC is read on the DC function through an internal diode. The assumed calculation causes the reading.

    I always suggest measuring a 9v battery on AC function. If it measures zero, then the meter is OK, if it measures like 12v or something, then the meter is not suited to read AC on top of DC.
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    Quote Originally Posted by stokes View Post
    Have to ask,Mike,how are you connecting the leads to measure the ac on the filter caps?Even the cheapest of meters should get a reading.Dont quite understand what you are saying here "(meter is reading something like 800 on a 500 scale)"
    Hi Stokes, all I did was flip the dial to AC, the max is 500 on this meter, then probe the hot side of the first filter cap lead with the red lead, and clamped the black lead to ground, same ground where the filter cap ground wire goes. The meter goes flooey. The numbers jump up and down 884 729 645 .... I thought I blew it out. I checked the AC wall voltage and it read about 122 steady, then set it to DC and got 410 or so on the same filter cap lead.

    If you saw this meter, you'd laugh. Its a Kelvin 100 Made In Taiwan, probably close to 30 years old, cheap back when it was made. I took some audio courses at a school in Manhattan in the early 90's, they gave them out in one of the classes.

    I have a line on a low end Fluke 115 on sale for 125.00 range, will order that soon.

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    Quote Originally Posted by Enzo View Post
    On cheap meters, DC measures OK, but if you set it to AC and try to read the ripple sitting on a 500v rail, it can't ignore it, and so reads WAY high. They lack an isolation cap at the input. AC is read on the DC function through an internal diode. The assumed calculation causes the reading.

    I always suggest measuring a 9v battery on AC function. If it measures zero, then the meter is OK, if it measures like 12v or something, then the meter is not suited to read AC on top of DC.
    Enzo, yeah, man, it went insane! I have to find a way to get something done this weekend, without a good meter (won't be here for a few days, but that puts me smack in the middle of a tough work week). Will see if any holiday sales on meters around here. I will try reading a 9v on AC but I think you pointed out the problems with the "free with 10 cereal box top" meters.

  34. #34
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    Back to Stokes and RG's great posts:

    I'm fairly sure I have two problems. The very low freq hum I think is 60hz or 120hz (hard to tell with overtones) is not normal for that amp, even with my terrible grounding scheme. I screwed up something. Id like to fix this first. I think something is broken here. If I pull out the first two preamp tubes it goes down a little and down a little more if I pull the phase inverter. So, I think something is building up, like a bad signal fed to each tube. (Will test for AC in the DC line when the new meter arrives)

    Then, Id like to go back and figure out a layout that better fits the suggestions RG mentioned above. The Fender scheme of having the filter caps on top of the chassis and a bundle of wires running all over to get to and from the caps seems to cause some trouble here. I got the idea (mostly) about reducing noise propagation, but Im not sure how to physically rewire all of the grounds. E.g. how to rewire the preamp and PI tube cathode grounds that come off the cap and resistor, or the 1 ohm resistor grounded to the chassis from the output tubes.

    There's an eyelet on that side of the eyelet board with the 3 (soon to be 5) cathode grounds. If I put a bus bar there just next to the eyelet board, and solder the cathode grounds there, should I wire one end of the bus bar to the - filter cap, i.e. run a wire through the chassis up to the cap? or can I make the physical connection at the bus bar itself?

    As an aside ive seen photos of many home builds where all the 'ground' stuff, e.g. cathode wiring, input jacks, etc, go to a bus bar, but then the bus bar is wired right to the chassis at some point. This seems to be opposite of what RG suggested.

  35. #35
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    Someone mentioned to avoid posting photos directly, but to host them someplace and put a link Here are some chassis photos on my flickr account. I put a link above but its not an eye catcher.


    Here you can see the terrible mess of a grounding on the brass plate.
    https://www.flickr.com/photos/145832...7689043482311/

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