Another "hum" debugging thread

[mikepukmel]

Hi All,
Hope you're all doing well out in tube amp land. Finally got some time to start looking and finishing my handful of amp builds. (after a rather long run of hideous hours at work ) The first one, Deluxe Reverb AB763 (slightly franken-modified) has a hum. Turn on, turn standby off, nothing plugged in, all pots on 0 ( or is that 1) min anyway. its more than an annoyance. Thought to make a todo list rather than jumping in.

I've read sometimes:
- pull each preamp tube starting with the first one, (shut down, pull tube, restart).
- Then pull the reverb tubes

Not sure what to look fore, other than, less pronounced or no hum? Isolate to some part of the circuit?

This is a low freq. hum 60 or 120, don't know how to tell.

I have an ancient scope, can probe, don't know what to look for or where to look.

As mentioned above, its been frankenstiened a bit, Im going to put it back to the original layout as much as possible with new parts and such. But would like to track down and understand the cause.

Thanks
MP

[Chuck H]

Well, extended time on a shelf is very bad for the power supply capacitors. So how much has the amp sat and how old are the filter caps?

Power tubes that are badly mismatched due to age/wear or poor matching can cause hum in guitar amps because the circuits are generally designed to take advantage of some inherent cancellation in the output stage provided there is similar performance from the the output tubes.

Amps that are modified without the benefit of prior experience/knowledge may have poorly considered grounds. This is more often a cause of oscillation, but it can also result in hum when ground loops are accidentally created.

The practice of pulling tubes to isolate a problem isn't limited to hum. And it doesn't always lead you straight to the problem. But it does indicate where the problem is manifesting. This is always helpful for diagnosis. There are other isolation practices that should be done first...

Do any of the front panel controls affect the problem? ie: Does hum increase with the volume control? Is it affected by the tone controls? This provides some isolation that can limit the number of tubes that need pulling and it's the first information we need. After that we can start pulling tubes.

You don't need to power down to pull tubes. Preamp tubes are never hot enough that I can't handle them bare handed. Just put the amp on standby and pull the tube. Be gentle though. The tube IS hot and so is any circuitry it's mounted to or is mounted to it. So pull the tube gently and lay it gently on a cloth rather than any cold, hard surface that sinks heat quickly and may allow it to roll around and knock on other things.

60Hz sounds like a deep hum. 120Hz sounds more like a note in the guitar range. It's not improbable that a 60Hz hum can be carrying a harmonic element or for a 120Hz hum to carry a 60Hz harmonic. This can be a confusing matter so don't judge yourself on inexperience. It's probable that the location of the hum (which we will find with the isolation techniques) will indicate the probable cause being in the filament or power supply, respectively being 60Hz or 120Hz.

Hope this helps.

[mikepukmel]

Thanks for the note and details Chuck. Well, the amp was built about 3 years ago, and I have played it. I do use it from time to time, when work hours aren't a killer. So, 3+ years old, played sometimes.
I will power up and twist the knobs again, but I think this particular hum may go up a little. Will do a careful test tomorrow and post the results.

I did purchase "matched pair" but no idea what that means these days. If they are not matched so well, would that manifest it self in different bias-power dissipation per tube?

Thanks re power down to pull tubes. I always thought that was the case. I have a piece of thin rubber sheet I can grab the tubes with.

Thanks again, yeah, your suggestions definitely help, as always.

Can I measure the frequency of the hum by probing at the speaker with my old scope? I think I did something like that a long time ago, out of curiousity, but it wasn't a nice sine wave it was a really odd shaped thing.

[pdf64]

Has it always done this, or is it a new issue?
I suggest to pull the valve in V6 LTP first and see how that affects the hum, that should quickly indicate whereabouts the root cause is.

[mikepukmel]

Hi Pdf: its always been there. I didn't have time to pull tubes tonight, but ran a few simple tests with various controls.

amp on, but standby on, no hum
turn standby off:

Both volume controls all the way down.
All other controls except reverb on 10, reverb on min (1?)

low freq hum. Very steady

Turn each volume up to max, then back down again
No noticeable change in hum.

Turn all tone pots from 10 to 1.
no change in hum.

With tone pots on min, turn volume pots back up
No change in hum.

Amp does get "hissy" when volume beyond about 6 or 7, so that it masks the hum at high levels.

Turn reverb up, doesn't affect this hum.

Tomorrow I will start pulling tubes and post a chart.
-----------------------------------------------------------------------------------------
Will do this:

1) pull preamp tube 1
2) pull preamp tube 2
3) pull reverb tubes (both send and recv
4) pull PI.

Question: should I leave other tubes in (those before the PI) when I pull the PI? Does it matter?

Last Question: should I try swapping the output tubes around?

Thanks!
Mike

[Enzo]

When you pull the PI, all the tubes before it don't matter.

An unbalanced power stage can hum, make sure there is good B+ on both pins 4 AND 3 of the power tubes. Make sure both sides are conducting.

[mikepukmel]

Cool, thanks, will do right after work. (can't wait!)

[mikepukmel]

Didn't expect this. Pulled tubes:

pull first two (preamp) tubes: no change, hum is as loud as before.

Next, pull both reverb tubes. maybe slight change, hard to determine, but hum is still very noticeable.

Pull all tubes up to but not including PI. Hum is almost gone. Tiny bit left (same freq).

Put reverb tubes back in, but leave tube before the PI out. Hum is back, and about as loud.

Pull reverb tubes, but put back in the tube just before the PI: Hum is back and about as loud.

I thought i made a mistake, so I ran through the whole thing again.
With either:
- both reverb tubes or
- the tube before the PI

hum is there and very loud. (you could not record with this amp)

I think this is odd.

I had made some sketchy mods over the past 2+ years, mostly "experiments". This weekend, Im going to put everything back as close to the original (AB763 layout and schematic as possible.
The only thing I don't have is a good brass plate. I do hav a brass plate but with it installed its REALLY hard to get the nuts on the pots since the shafts are a little shorter than the correct (unavailable?) ones.

I have a brass rod (about 1/8" dia) hanging off the back of two turrets that connect the grounds. I think it will take another round to get that out and back closer to the original layout.

[SoulFetish]

Didn't expect this. Pulled tubes:

pull first two (preamp) tubes: no change, hum is as loud as before.

Next, pull both reverb tubes. maybe slight change, hard to determine, but hum is still very noticeable.

Pull all tubes up to but not including PI. Hum is almost gone. Tiny bit left (same freq).

Put reverb tubes back in, but leave tube before the PI out. Hum is back, and about as loud.

Pull reverb tubes, but put back in the tube just before the PI: Hum is back and about as loud.

I thought i made a mistake, so I ran through the whole thing again.
With either:
- both reverb tubes or
- the tube before the PI

hum is there and very loud. (you could not record with this amp)

I think this is odd.

I had made some sketchy mods over the past 2+ years, mostly "experiments". This weekend, Im going to put everything back as close to the original (AB763 layout and schematic as possible.
The only thing I don't have is a good brass plate. I do hav a brass plate but with it installed its REALLY hard to get the nuts on the pots since the shafts are a little shorter than the correct (unavailable?) ones.

I have a brass rod (about 1/8" dia) hanging off the back of two turrets that connect the grounds. I think it will take another round to get that out and back closer to the original layout.

So, Here are a couple of questions:
Do you have a schematic of the amp as currently configured?
Did you follow someone else's layout? is this from a Kit?
Are you sure you installed the power supply board facing the "correct' way, with all the filter grounds and nodes connected to the correct filter caps?

[Helmholtz]

Can I measure the frequency of the hum by probing at the speaker with my old scope? I think I did something like that a long time ago, out of curiousity, but it wasn't a nice sine wave it was a really odd shaped thing.

Sure, if you can scope the hum, the fundamental frequency is given by 1/T. T is the time between 2 successive identical positive (or negative) peaks.
If not sure how to do it, just post a good scope picture and specify time base setting.

[mikepukmel]

So, Here are a couple of questions:
Do you have a schematic of the amp as currently configured?
Did you follow someone else's layout? is this from a Kit?
Are you sure you installed the power supply board facing the "correct' way, with all the filter grounds and nodes connected to the correct filter caps?

I don't have a schematic as it is currently, will try to make one, I have a schematic software on one of the other computers.
Its not from a kit but I did get a turret board and chassis from what i thought were decent sources. The turret board guy is now out of business. The chassis from ebay, guy zachmdhunter says they are "just like" mojotone. So, dimensions should be ok?

Ah, I don't know about the power supply board, you mean the big cap board on top, under teh dog house? If so, then, yes, I went back and forth a whole bunch of times, and Im sure the wires are feeding down into the chassis in the right spot.

I'll make a schematic, and get some photos of the dog house and how wiring goes through.

[mikepukmel]

Sure, if you can scope the hum, the fundamental frequency is given by 1/T. T is the time between 2 successive identical positive (or negative) peaks.
If not sure how to do it, just post a good scope picture and specify time base setting.

Thanks Helmholtz, will do. Been dying to crank up my old Tek scope to see if it still works. I have a good camera, can put the shades down, and get a photo of the trace. I know a little about how to judge the trace. So, I should scope at the speaker?

[Helmholtz]

So, I should scope at the speaker?

Sure, the speaker signal is what you hear.

[g1]

Its not from a kit but I did get a turret board and chassis from what i thought were decent sources. The turret board guy is now out of business. The chassis from ebay, guy zachmdhunter says they are "just like" mojotone. So, dimensions should be ok?

So were you going by a layout, or did you have to make one up? It sounds like you are on turret rather than eyelet board?
In particular, I'm wondering what tube is 'tube before the PI'. Normally it is the tremolo tube, is that the case here?
If you are not using the standard AB763 tube numbering, you will need to call out the tube functions rather than calling by number.

[mikepukmel]

So were you going by a layout, or did you have to make one up? It sounds like you are on turret rather than eyelet board?
In particular, I'm wondering what tube is 'tube before the PI'. Normally it is the tremolo tube, is that the case here?
If you are not using the standard AB763 tube numbering, you will need to call out the tube functions rather than calling by number.

Sorry g1 (rank amateur here). I'll draw up the current layout, yeah, its been fiddled with a bit. So I started with a non reverb board because I uh, bought the wrong board. I got the whole thing together, with the first ever tube amp wiring job. You can imagine how it turned out. heh. So, I ordered a reverb board, got that wired up. The second re-wiring after taht got rid of most of the problems (hum, some oscillation). This is the Nth attempt.

I'll draw up as well as I can the current layout. its very close, but no tremolo.