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

[mikepukmel]

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

[Justin Thomas]

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

[stokes]

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.

[mikepukmel]

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.

[mikepukmel]

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.

[mikepukmel]

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.

[mikepukmel]

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).

[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 Phillips]

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 ,

[mikepukmel]

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.

[mikepukmel]

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.

[mikepukmel]

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.

[mikepukmel]

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.