Hum in new AB763 DR build Reverb circuit - increases as reverb pot is turned up

[prongster]

I might also help to use short but hopefully better shielded reverb cables. Especially a long, poorly shielded return cable will collect hum.

OK, I have a set of the original style cables, with the shield on the outside, and those don't perform any differently.

Any suggestions for some well shielded cable that I can make my own out of maybe?

[vintagekiki]

It's not from the cables.

1)
Check ?


2)
Check soldering on tube sockets 6V6.
It doesn't see the best, the red arrow looks as if the pin is not soldered.

[Greg_L]

It is a combo, but it's still buzzing even when I remove the tank and place it 6ft (length of cord) away from both the chassis and combo cab (with speaker).

It's never been stock, but I can replace the mod parts with the stock parts and layout if that will make things easier. It might take awhile tho. I'm going to trying shielding the tank on all sides first.

Just spitballin here.....Look at your V3. Do you see that little loop of blue wire connecting the plates? Move that thing around with a chopstick and listen for the noise. I've had issues with that little bastard wire on other AB763s I've messed with.

Also, I'm looking at your pics, and still to me that grounding scheme looks haphazard. You've got grounds going to the pots, and to the chassis, and through the jacks, and wires grounding to the buss for convenience. I think re-thinking that grounding setup could yield benefits. You've got grounds with nearly unlimited pathways. That's not usually a good thing.

[prongster]

It's not from the cables.

1)
Check ?

The PT has a center tap. One of the first things I tried is disconnecting it and adding the artificial center tap w 100R resistors, and that didn’t fix it.

[vintagekiki]

Does that mean you have 2x3.15 VAC according to chassis on both sides pilot light?

[vintagekiki]

OK. That has been eliminated.
look at post #92
... 2) Check soldering on tube sockets 6V6.
It doesn't see the best, the red arrow looks as if the pin is not soldered ...

[prongster]

Just spitballin here.....Look at your V3. Do you see that little loop of blue wire connecting the plates? Move that thing around with a chopstick and listen for the noise. I've had issues with that little bastard wire on other AB763s I've messed with.

Also, I'm looking at your pics, and still to me that grounding scheme looks haphazard. You've got grounds going to the pots, and to the chassis, and through the jacks, and wires grounding to the buss for convenience. I think re-thinking that grounding setup could yield benefits. You've got grounds with nearly unlimited pathways. That's not usually a good thing.

Redoing the preamp ground bus with a single bus wire that’s not attached to the pots, and terminates in a single point at the jack has been on my to do list. It keeps getting bumped back as I try y’all’s suggestions, but I will do that when I convert to original AB763 spec. If for no other reason than it seems to be a good practice.

But I will check the blue wire first - thanks!

[prongster]

Good news - I removed the mods - amp is stock AB763 now, and the buzz in the reverb is gone!

It still hums a bit tho. And this hum is definitely dependent on the orientation of the tank. I added some recordings that I made with the tank oriented for the least noise. I'm curious if this sounds about like you'd expect or if there's still some room for improvement here? I hope to use this amp for recording.

https://app.box.com/s/qopps6wuvcdvi9wd292rphur2jxph5nj

Some of my heater wiring is a little low and not straight up and down in & out of the sockets. I'm thinking about redoing it to see if that improves the hum a bit. Any other thoughts?

Thanks for all the help getting rid of that damn buzz!

[vintagekiki]

Good news - I removed the mods - amp is stock AB763 now ... It still hums a bit tho.

No Good news. Nothing has changed.
Listen.
post #96

https://app.box.com/s/qopps6wuvcdvi9wd292rphur2jxph5nj
DR no mods, at idle, all gain knobs down
DR at idle, all gain knobs down

What is the quiescent current of output tubes 6V6?
Remove all (preamp) tubes except PI (12AT7), 2 x 6V6 and 5U4 rectifier.
Is there buzz?

Check soldering on tube sockets 6V6.
It doesn't see the best, the red arrow looks as if the pin is not soldered.

[prongster]

I left the audio sample with the buzz up there for reference if that's what you're talking about.

[Chuck H]

... this hum is definitely dependent on the orientation of the tank. I added some recordings that I made with the tank oriented for the least noise. I'm curious if this sounds about like you'd expect or if there's still some room for improvement here?

See post #16
I've actually mounted reverbs in head cabinets with success. There's very often a "blind spot" where the hum is not picked up. You just have to look for it. Sometimes a half inch either direction induces hum, but silent right in the sweet spot. Don't discount alternate orientations.

[vintagekiki]

I left the audio sample with the buzz up there for reference if that's what you're talking about.

Listen and compare sample 1. with sample 2. and explain us where is difference.

https://app.box.com/s/qopps6wuvcdvi9wd292rphur2jxph5nj
1. DR no mods, at idle, all gain knobs down
2. DR at idle, all gain knobs down

Read post #97

[Greg_L]

See post #16
I've actually mounted reverbs in head cabinets with success. There's very often a "blind spot" where the hum is not picked up. You just have to look for it. Sometimes a half inch either direction induces hum, but silent right in the sweet spot. Don't discount alternate orientations.

My own Deluxe Reverb build is a head with a full size 4AB3C1B in the bottom, face down, like it's supposed to be. No noise at all.

Reverb tanks are technically supposed to mount a certain way though, aren't they? The last character in the long tank code signifies it's mounting orientation.

[Chuck H]

Reverb tanks are technically supposed to mount a certain way though, aren't they? The last character in the long tank code signifies it's mounting orientation.

Yep. But I had to break the the rules on one occasion to find a hum free orientation. Still not sure why, and it hardly matters because hum is less acceptable than the "incorrect" orientation. I'd still like to try a reverb tank in that amp with the right orientation specs. I didn't have one on hand at the time and I'd already purchased two tanks for that project.

[Greg_L]

Yep. But I had to break the the rules on one occasion to find a hum free orientation. Still not sure why, and it hardly matters because hum is less acceptable than the "incorrect" orientation. I'd still like to try a reverb tank in that amp with the right orientation specs. I didn't have one on hand at the time and I'd already purchased two tanks for that project.

Oh yeah I totally agree. You do what works. An "incorrect" orientation is way better than a bunch of hum and noise.

I also built a standalone 6G15 Reverb. I'm into the drippy surfy reverb thing. The only tank I had at the time was the typical open-side-down-flat orientation that you'd find in a combo. It worked but it wasn't great in that little reverb unit. I then bought a connectors-up mount tank for the front wall of the little cab and cowabunga...surf city.

[vintagekiki]

For correct orientation reverb tank input coil (low-ohm) should be placed below main transformer.
If reverb tank output coil (high-ohm) is located below main transformer, hum from PT (TR1) will be induced in it.
The reverb tank is enclosed in an elastic bag, to decreased whistling (acoustic self-oscillation).

[prongster]

Listen and compare sample 1. with sample 2. and explain us where is difference.


Read post #97

Ah, I would call the remaining noise hum, but ok.

With V1-V5 pulled, the same noise is there. Maybe it's a little quieter, but it seems the same in character. And it's still there if I disconnect the reverb cables.

https://app.box.com/s/qopps6wuvcdvi9wd292rphur2jxph5nj


If you are asking for the bias current of the tubes, they are at ~24mA.

Rectifier tube is a 5AR4 btw.


edit: I pulled the PI tube and no change.