What a dreadfull hum - Hoffman AB763

[riftamps]

Hi guys,

You know that feeling where you've been looking at something for so long that you can't work out who you are anymore? Well I'm at that place....

I've got a SF Pro Reverb converted to an AB763 with a Hoffman board in for a cap job, there's also a hum that does two things:

1) Increases in volume when I DECREASE the bias voltage (turning pot clockwise)

2) Decreases in volume when I turn the Vibrato channel volume UP but returns when I turn the Bass control up


I have done the following:

1) Full cap job - every electrolytic (this was due anyway)

2) Measured all resistors in both the bias and tremolo (6G16 spec) circuits, all are within tolerance - none are noisy, burned, charred, microphonic. The bias diode also tests ok.

3) Replaced PI with various new and NOS valves, including both 12AT7 and 12AX7

4) Rewired heaters and replaced both 100r CT resistors

The output valves are relatively new, here are the bias readings:

Inner - PC 37.5mA       PV  473v
Outer - PC 36.7mA      PV  473v

As you can see, they are matched and still sound great

The only thing that I can see is different is the Hoffman schematic calls for a 22k resistor connected to the bias pot but this has a 47k installed. It's been added in after the initial conversion by another tech.

Before I pull the last of my hair out....someone please point out the obvious, I'm blinded by it all now

"He couldn't see the wood for the trees"

Thanks

[HotBluePlates]

Pull tubes to figure out where the hum is.

Start with the phase inverter tube. If you remove it, is the hum still there? If no, the hum is earlier in the preamp (or in the phase inverter itself).

If the hum is still there, return the phase inverter tube to its socket, and pull the preamp tube next-closer to the input jack. Repeat the process until you find the tube whose removal no longer stops the hum.

Let us know what you find.

[sluckey]

Did the amp ever work properly after the Hoffman board was installed?

[TeslaRect5150]

I know the feeling rift. I have stared at the inside of an amp for hours at my wits end. Just makes it that much better when the problem is fixed though. You'll never forget what caused the problem either. :)
Is it doing this with a guitar plugged in or not? Built an amp with an old input jack that wasn't making good switch contact...made a horrible hum. Even if it's not the jacks, still good practice to make sure those old jacks are good to go.
I would check all grounds everywhere. Those chassis can get old and oxidized making some ground connections bad.
Your said you changed the filter caps. How and where you ground them can make a difference in hum. Grounding your High Voltage CT at the first cap helps. Sometimes if they are all grounded together it can be a problem.
I once had a bad solder joint on a first stage heater wire that caused hum. Voltage measured fine, but after reflowing the joint, hum went away.
If the amp didn't have the hum before you replaced the electrolytic caps and rewired the heaters. I would quadruple check everything I did.
 
 
Just my .02
Peace,
Aaron

 

[HotBluePlates]

Having a long list of things to check might be helpful, but the fastest way to solve any amp problem is to eliminate things you don't need to check.

Pulling tubes can help you pinpoint what section of the amp has the noise. Then you know you don't have to look for issues in any other section, and maybe reduce the list by 50-90%. It can also help you look at things you would never think to check.

At one time, I had never had a case where pulling the phase inverter tube ever resulting in a sound coming from the speaker. Troubleshooting hum in a customer's amp, I yanked the phase inverter, and there was still hum coming from the speaker. "WTF?" The output tube plate would be insensitive to hum, unless all filter caps were 0uF. The phase inverter is gone, so nothing is feeding the output tube grids... except the bias supply! There is only the bias cap in that supply that could fail and result in a.c. at the output tube grids (except a failed bias diode, but the bias cap would have exploded due to reverse-voltage), so a swapped bias cap and the amp was hum-free. It took longer to remove the chassis from the cabinet (and type this reply) than it did to find & fix the problem.

That stuff matters if you ever fix amps at a music store, where the hourly rate the store charges the customer is steep and there's a 1-hour minimum. Getting a 2nd amp to fix from a customer is important to your business.

[Colas LeGrippa]

check for a bad solder at grids of each and every preamp tube, if not sure, redo the solders. A bad solder at a grid will cause low freq, hum.

Good luck.

Colas

[EL34]

Hard to say what is wrong
Post some very clear, in focus pics of your build

[riftamps]

Ok, back onto this today, just a few updates:

1) The hum was there before the cap job
2) Pulling V6 PI removes the hum completely
3) Pulling V4 lessens the hum slightly, but does not eliminate
4) No other valves effect the hum
5) I've cleaned all pots, sockets, switches etc
6) The chopstick test yields no clues, I've bashed ever resistor, cap, soldier joint and wire to no avail

I've never spent so much time hunting down hum in an amp....this one is escaping me

[floyd]

Had a maddening hum in an AB763 for two weeks... tried EVERYTHING. Then when I thought I had exhausted all possibilities. I changed the GZ34 rectifier tube for another one.. the hum was gone. I've never had a tube rectifier cause that before.

[eleventeen]

"I changed the GZ34 rectifier tube for another one.. the hum was gone. I've never had a tube rectifier cause that before."

I must confess I've never heard of that. It hardly seems possible. What do you imagine might have caused that....the heater is connected to the cathode, so there is no H-K short that should cause such a thing. A partial short from one of the plates to the cathode?

[sluckey]

I've had a bad GZ34 to cause hum. It was extremely loud though, and the amp was totally unusable. I seem to remember fuse blowing too, but it was 40 years ago. Can't say for sure. The tube was shorted plate to cathode. Don't remember if it was just one plate or both that were shorted, or if I even knew.

[floyd]

I've had a bad GZ34 to cause hum. It was extremely loud though, and the amp was totally unusable. I seem to remember fuse blowing too, but it was 40 years ago. Can't say for sure. The tube was shorted plate to cathode. Don't remember if it was just one plate or both that were shorted, or if I even knew.

  I thought it was my imagination at first , but the hum was getting louder over time , and eventually it probably would blow a fuse. I'n my extensive search for the hum , I even unbolted the OT and changed it's orientation , not thinking that the rectifier could be the cause.

[tubeswell]

Put the amp on standby. Does the hum go away instantly or does it fade out within a second or two?

[riftamps]

Ok, here's the hum on a scope, thoughts?

http://i881.photobucket.com/albums/ac12/fantana54/photo.jpg

[HotBluePlates]

1) The hum was there before the cap job
2) Pulling V6 PI removes the hum completely
3) Pulling V4 lessens the hum slightly, but does not eliminate
4) No other valves effect the hum

Post some pictures of the amp board/wiring, especially V4 & V6. The hum is probably related to either the phase inverter tube or the wiring from V4 over to the phase inverter.