Noisy 6G15

[warioblast]

Hi guys,
I built a 6G15 unit from scratch 3 years ago. I had noise issues and I gave it to a tech friend of a friend 2 years ago. I got it back last month...
He told me it was working fine for him. In the meantime, he also worked on my amp which I got back at the same time, and it wasnt properly working...
He told me it could come from a difference of impedance. He tried it on a Musicman amp. My amp is a DIY "Marshall".

To me, it doesn't really sound like a hum or a buzz...
On my clean channel, this background noise is lower than on my dirty channel.
My 6G15 has an isolated ground buss.
Nethertheless I did this test https://www.thegearpage.net/board/index.php?threads/6g15-fender-reverb-unit-build-questions.1690482/#post-21700467 "if you want to hear the ground loop hum of a unit built without the isolated ground buss at its worst, plug the reverb unit into an amp with an instrument cable, plug in the power cords of both the amp and reverb unit, turn on the amp, and crank the volume all the way up. The reverb unit doesn't even need to be turned on. Then, while listening to the hum, unplug the power cord of the reverb unit only. The hum that goes away is the ground loop hum."
I didnt notice any difference with the unit powered or not. But I noticed the difference going straight into the amp VS going through the 6G15.
I had written a few tweaks I could do before this test, but I'm realizing now that most of them are logically useless, because  the noise is there even when the unit is not powered.

I have no experience with a real 6G15, but this doesnt seem normal to me. Also I only have 1 amp. (Which is pretty quiet on its own)
I use the same power outlet for the amp and the reverb.
I will post some pictures and also will try to record this noise the best I can.

 
 

[vampwizzard]

You'll need to identify what kind of noise youre hearing.. if its 120hz, 60hz etc. If its RF noise, make sure you've used shielded cables from your inputs to the 12AT7. Make sure your heater has the virtual ground or has a center tap that is grounded.

After that, id have the case open and start chopsticking to see if any components are causing this noise.

Looking forward to any audio you can get of this problem.

[sluckey]

Disconnect the power cord green wire from chassis. Any better?

[warioblast]

While I was doing some poor recording attempts. I made a discovery, pretty much all the added noise I'm hearing comes from the mix pot.
With the tone pot it gets a little noisier as you turn it clockwise, but I'm guessing it's normal.
Here's a clip where I'm only turning the mix pot.

[warioblast]

Some pics.

I don't remember if my footswitch jack is insulated 

[warioblast]

Uninspired short demo.

[tubeswell]

My 2CW


The way the signal ground elevation is wired in is arse-about-face. The ground buss for the low current returns is presently riding 'on top of' any 'micro wobble' in the high current returns

[warioblast]

tubeswell,
I was hoping you would chime in. I saw your grounding schematic on another forum. I will give a try.

Meanwhile I found 2 other leads.

https://el34world.com/Forum/index.php?topic=13428.0

[warioblast]

Disconnect the power cord green wire from chassis. Any better?

It didn't make a difference.

My 2CW


The way the signal ground elevation is wired in is arse-about-face. The ground buss for the low current returns is presently riding 'on top of' any 'micro wobble' in the high current returns

I separated high and low current returns as per the schematic posted above. It didn't do anything.
I didn't elevate the heater center tap though. I'll give it a shot.

You'll need to identify what kind of noise youre hearing.. if its 120hz, 60hz etc. If its RF noise, make sure you've used shielded cables from your inputs to the 12AT7. Make sure your heater has the virtual ground or has a center tap that is grounded.

After that, id have the case open and start chopsticking to see if any components are causing this noise.

Looking forward to any audio you can get of this problem.

I gave it a bit of chopsticking.
I found some sensitive spots (I don't know if it is normal or not). See my attached layout.
Particularly the wire that goes from pin 2 of the mix pot to the output jack. The ground buss on the controls is a bit sensitive, I'm going to redo this area; controls & input / output jacks.

I also tried another RCA cable.
I haven't tried other tubes yet. I only have some spare 12AX7 at home.

Question:
On a perfectly working 6G15, how silent is the mix control ?

[DummyLoad]

you may be picking up noise from the ground loop that the tank creates. does the tank have isolated in/out jacks and is the tank bolted to the chassis? 

that second video is 120Hz PS buzz. when you're rotating the MIX control is the buzz louder when the control is 100% effect or 100% dry signal? if buzz is loudest with the control set with 100% effect, then the buzz is coming from the reverb signal path, if the buzz is loudest with the mix control is 100% dry signal path, then the buzz is coming from the cathode follower.

suspect that buzz is coming from the effect (amplified) path.   

some light reading and instructions on how to isolate & resolve ground loops in link below - for unbalanced systems (this effect device and 99.9999% types of guitar amps) you can skip to page 12 and start there. there is bias to Jenen isolation products but the information and T-shooting approach are solid.

if you use and build the test probe in the reverb tank path and you are testing link from tank driver output to tank input _ DO NOT FORGET TO APPLY A LOAD to the reverb OT secondary or you can fry the reverb driver OT with flyback currents.


https://web.mit.edu/jhawk/tmp/p/EST016_Ground_Loops_handout.pdf


--pete

[tubeswell]

If you correctly follow the ground scheme on the schematic you posted, it should be a lot quieter if it is ground related noise. If it is still noisey, then either you haven’t followed it properly, or there is some other fault.

[warioblast]

you may be picking up noise from the ground loop that the tank creates. does the tank have isolated in/out jacks and is the tank bolted to the chassis? 

that second video is 120Hz PS buzz. when you're rotating the MIX control is the buzz louder when the control is 100% effect or 100% dry signal? if buzz is loudest with the control set with 100% effect, then the buzz is coming from the reverb signal path, if the buzz is loudest with the mix control is 100% dry signal path, then the buzz is coming from the cathode follower.

suspect that buzz is coming from the effect (amplified) path.   

some light reading and instructions on how to isolate & resolve ground loops in link below - for unbalanced systems (this effect device and 99.9999% types of guitar amps) you can skip to page 12 and start there. there is bias to Jenen isolation products but the information and T-shooting approach are solid.

if you use and build the test probe in the reverb tank path and you are testing link from tank driver output to tank input _ DO NOT FORGET TO APPLY A LOAD to the reverb OT secondary or you can fry the reverb driver OT with flyback currents.


https://web.mit.edu/jhawk/tmp/p/EST016_Ground_Loops_handout.pdf


--pete

My reverb tank is a brand new TAD 4AB3C1B https://www.tubeampdoctor.com/en/4ab3c1b-tad-reverb-can-2x2-springs
Connectors:    C = Input Insulated / Output Grounded 
It is (was) bolted to the cabinet not to the chassis.

Buzz is loudest with the control set with 100% effect.

[warioblast]

BTW I have used this reverb tank with a Surfybear FET Reverb kit for a few months and I don't remember having this noise issue. I'll check it back tomorrow.

[warioblast]

From robrobinette.com

It means I should focus on the green path right !?

[tubeswell]

Focus on the ground returns including isolating the input and output jacks and pan send return jack sleeves from the chassis (and running separate wires from the sleeve returns to the ground buss.


Also the hum loop blocker doesn't have any clamping diodes on it, so the signal buss voltage won't be very tightly clamped

[warioblast]

Clamping diodes  You can't see them but I have 2 diodes underneath the 22ohm R & 0.1uF cap, is this how you call them.

[tubeswell]

Clamping diodes  You can't see them but I have 2 diodes underneath the 22ohm R & 0.1uF cap, is this how you call them.

'
I see now - I missed that. They look small. Whats the rating on them? (You want 6A diodes)

[shooter]

Expensive and exotic cables, even if double or triple shielded, made of 100% pure unobtainium, and hand-made by virgins, have NO significant effect on hum and buzz
problems!

 

That's my kind of Teaching 
the graphics are great also
Thanks Pete!

[warioblast]

Clamping diodes  You can't see them but I have 2 diodes underneath the 22ohm R & 0.1uF cap, is this how you call them.

'
I see now - I missed that. They look small. Whats the rating on them? (You want 6A diodes)

They are most likely 1N4007. If they are good enough for Jeff Gehring.   


This afternoon, I took out the pots and jacks and redo my ground bus. I tried 2 different groundings and it is still the same.
I also tried a 12ax7 in place of the 7025.
Tomorrow I'll make sure my tank is fine. Before any further tweakings.

[warioblast]

Quick update:
I finished working on my amp earlier this week, so today I decided to have another go with the 6G15.

I worked around V1 (7025) and freed my wires from the holes they were "locked in" on the board. And I used shielded wire from V1 pin 2 to the reverb output jack. I also unsoldered the footswitch jack.
This resulted in a dead quiet reverb !
I play for a few minutes with a grin on my face.
I decided to put back the footswitch jack in...

The noise came back. I unsoldered the footswitch back again; still the same noise. 

Oh well, at least , now I know the ground scheme is good. 

[PRR]

The signal from the RT to the tank is strong, should not need shielding.

The signal from the tank to the reverb recovery stage is weak. And it is in the middle of the amplifier so grounding is not clear. If taking the jack off the chassis "fixes it", the no-nonsense cure is insulating washers so it sits solid without electric contact. (Obviously you have contact some other way.) The alternative is to free-up all grounds on the reverb recovery stage and run them ONLY to the reverb return jack.

[warioblast]

Today, I arranged V1 wires. I turned the reverb on and it was noise free.   
I took pictures for future reference.
I soldered the footswitch jack in. So far so good. Keeping my fingers crossed.
Thank you guys for all your assistance.