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

[Paul P]

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.

Just a few minutes ago I was moving my long reverb tank around on some long cables and found that orientation seems more important than proximity. With a particularly noisy orientation (still with the closed side facing the amp) moving closer or farther away didn't make much difference. I found that turning the tank 90 degrees off to the side of the amp (so one end pointing forward, the other back) was the quietest.

Then tried upside down to its original position (turned 180 degrees, so jacks on top instead of underneath) and it was as quiet or just about. That is a very easy change. I was considering moving the tank to the speaker cabinet, but wasn't too happy with that idea.

[vintagekiki]

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.
edit: I pulled the PI tube and no change.

The buzz (not hum) has nothing with a reverb tank.
If I understood you, all preamp tubes (V1-V5) and PI (12AT7) is removed, only output tubes (2 x 6L6) and rectifier (5AR4) is remained, but AB763 still buzz.

Buzz occurs in power amp or rectifier.
In troubleshooting, always start from output to input.
post #78 The buzz is constant and reminds, heater is no grounded, or some of tube have poor insulation between cathode and heater.

Check AB763 DR output tubes (2 x 6L6) and rectifier (5AR4) in some correct amp for buzz.
Check whether AB763 DR buzz with remove all tubes
It looks stupid, but maybe buzz is caused by mutually unfortunate mounting transformers in AB763 DR, so there becomes to unwanted magnetic coupling (feedback) that manifests as buzz.

1)
https://www.mercurymagnetics.com/what-you-need-to-know-about-transformer-mounting-styles/
What you need to know about Transformer Mounting Styles

https://ampgarage.com/forum/viewtopic.php?t=14949
How to Mount Power Transformer on Chassis

https://www.youtube.com/watch?v=6Mp_8mV9oLo
power transformer mounting orientation, Horizontal vs Vertical for DIY tube amplifier

https://www.youtube.com/watch?v=Ublo6uSBwfw
Transformer Orientation

https://www.tdpri.com/threads/transformer-orientation.1003730/
Transformer Orientation

[prongster]

It does not hum with the power tubes removed (rectifier is still installed). What does that suggest?

I just installed a new set of power tubes and it still hums, making this the 4th set of power tubes I've tried, so I'm feeling it's likely not the tubes themselves. Tomorrow I'll see if I can try these tubes in another amp, just to be sure.

I've also tried a different rectifier tube with the same result.

To be clear, the reverb buzz I was having earlier was resolved when I removed the mods - thanks everyone who suggested that! There is still some low level hum that is present at idle, however and not associated with the reverb tank orientation. (Thanks for all the input on that, btw. I will be using that info when I reinstall the tank in the combo cab.) I was initially confused because I was hearing this hum and the buzz amplified by the reverb circuit, and didn't realize they were 2 different problems.

Should I start a new thread?

[vintagekiki]

It does not hum with the power tubes removed (rectifier is still installed).
I just installed a new set of power tubes and it still hums ...
I've also tried a different rectifier tube with the same result.
Should I start a new thread?

Do not open a new thread, until you remove trouble with your AB763 DR, we will forget what we wrote.

What did we conclude?
- AB763 Dr does not hum with power tubes and rectifier. This means no unwanted magnetic coupling (feedback) between transformers.

- AB763 DR has buzz with new set of power and rectifier tubes. This means tubes are correct and that buzz is formed within power amp and rectifier.
While buzz do not eliminate, no put preamp tubes (V1-V5).
Pay attention for post #92 (comment and picture 2) and check soldering on power tube sockets (6V6). It seems that some pins are not best soldered.
If some power tube "float" or has larger quiescent current may be come to buzz.

- Temporarily disconnect resistor for negative feedback (820). Is there buzz?
- Check all points where ground wires are connected over screw to chassis.
- If all of the above is OK, check with scop how much is buzz on + HV (First Cap)

I would ask you to upload box image with caps.

1)
Check that the NFB (black) wire (1, 2) is connected to GND point (green wire) and at same time via wire 2 soldered to wire 3 (PI)

[prongster]

I uploaded updated pics showing my re-wiring of the grounds, the heater wires, and a pic of the cap board, so you have current information to work with.

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

I will check the items on your list next, but first - where to I attach the scope probe to see if the noise is on + HV?

[vintagekiki]

Scope probe attach to oscilloscope.
Check noise voltage on + HV cap board.
Pay attention when when adjust oscilloscope on high voltage, order of 400-500 VDC.

https://www.tek.com/blog/what-is-an-oscilloscope-probe
What is an Oscilloscope Probe?

https://www.testandmeasurementtips.com/basics-of-oscilloscope-probes-part-one-faq/
Basics of oscilloscope probes – Part one

[prongster]

Exactly where do I attach the probe on the cap board, please?

[vintagekiki]

Hot end probe on + pole on caps (green arrow), gnd end probe to chassis.
If you are unsure with oscilloscope, do not take HV measurements.
Check rest what is written in post #109

[prongster]

My scope probes are only rated for 300V, so I think that means I shouldn’t put them on the + HV. Is there a way to check for noise w my meter?

[Helmholtz]

You can measure ripple voltage on supply nodes with a decent DMM (check if it separates ACV from DCV: You should get a 0 ACV reading when connecting to a battery).

Scoping the hum at the speaker output might give some clue regarding the source.

[vintagekiki]

My scope probes are only rated for 300V, so I think that means I shouldn’t put them on the + HV. Is there a way to check for noise w my meter?

Do not take HV measurements not to damage measuring equipment.
With multimeter (range 2VAC) via diy DC/AC probe can read superimposed AC or ripple voltage.

[prongster]

Do not open a new thread, until you remove trouble with your AB763 DR, we will forget what we wrote.

What did we conclude?
- AB763 Dr does not hum with power tubes and rectifier. This means no unwanted magnetic coupling (feedback) between transformers.

- AB763 DR has buzz with new set of power and rectifier tubes. This means tubes are correct and that buzz is formed within power amp and rectifier.
While buzz do not eliminate, no put preamp tubes (V1-V5).
Pay attention for post #92 (comment and picture 2) and check soldering on power tube sockets (6V6). It seems that some pins are not best soldered.
If some power tube "float" or has larger quiescent current may be come to buzz.

- Temporarily disconnect resistor for negative feedback (820). Is there buzz?
- Check all points where ground wires are connected over screw to chassis.
- If all of the above is OK, check with scop how much is buzz on + HV (First Cap)

OK, disconnected the 820R NFB resistor and the amp still hums - a little worse than before I'd say

Went over the grounds and they all seem to read < .3 ohm to the chassis and each other.

The AC voltage on the heaters read the same on either side of the center tap - V8 = 3.49VAC, V7 = 3.47VAC, V6 = 3.45 VAC if that little drop from tube to tube means anything.

And I did not detect any AC voltage on the filter caps, measuring them from the + side to their respective grounds.


Scoped the output and posted pics of the readings (AC & DC):

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

Did I miss anything?

[Helmholtz]

And I did not detect any AC voltage on the filter caps, measuring them from the + side to their respective grounds.

Scoped the output and posted pics of the readings (AC &amp; DC)

Spacing of negative peaks is about 16 ms, meaning that the fundamental is 60Hz.
So we can exclude power supply ripple.

To also exclude the bias supply, see if the hum stops when you pull the PI (V6).

[prongster]

Spacing of negative peaks is about 16 ms, meaning that the fundamental is 60Hz.
So we can exclude power supply ripple.

To also exclude the bias supply, see if the hum stops when you pull the PI (V6).

There is a tiny bit of hum with the PI pulled - I have to put my ear up to the speaker to confirm that I hear it, but it is quite a bit louder the PI tube (V6) in, ie it's audible in the room as soon as the amp warms up.

[vintagekiki]

I do not understand, when PI tube pulled (remove) in post #105there is hum and in post #119 there is no hum.

#105
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.
edit: I pulled the PI tube and no change.

#119
There is a tiny bit of hum with the PI pulled - I have to put my ear up to the speaker to confirm that I hear it, but it is quite a bit louder the PI tube (V6) in, ie it's audible in the room as soon as the amp warms up.

I listened several times .m4a clips.

DR no mods, at idle no V1-V5 .m4a
DR no mods w tank rev on 10.m4a
DR no mods, rev on 6.m4a
DR no mods, at idle all gain knobs down.m4a
DR w tank, rev on 6.m4a
DR at idle, all gain knobs down.m4a
DR no tank, rev on 6.m4a

With exception of DR no mods w tank rev on 10.m4a and DR w tank, rev on 6.m4a other .m4a clips are identical in structure.

From these 120 posts is conclude that everything is properly connected, but it will not work.
It is clear that hum does not form in preamp. Let's try to narrow source hum.

Tube V6. Temporarily disconnect cap .001 from point 2 x 220k resistor, and connect cap with wire to gnd (point where 47 ohm and LDR resistor is grounded)
Is there a hum?
Looking at your .jpg in several places (picture can be deceive), it can be noticed that some solders are not very jubilee.
Regarding hum, did you try to measure how strong it is? Connect AC DMM parallel to speaker (of course without guitar) and measure how much is AC voltage on speaker.
The general question, how hum is audible on stage (not in room, not ear in speaker) during live music.