Preamp Ground Bus Hum

gabasa

Tele-Holic
Joined
Jan 29, 2018
Posts
676
Age
50
Location
Toronto, ON
Hey everyone,

I just fired up my new tweed Bassman build and I'm really happy with how it turned out. At really high volumes though, perhaps 7 or 8 and up, it seems to hum a bit more than I want. It's not at a problematic level; I think that most players would find it acceptable and I'm being picky.

The hum is only affected by the volume knob settings and not by the tone controls. My gut tells me that perhaps something on my preamp ground bus shouldn't be there. The bus terminates at the normal channel, input 1 ground lug. Connected to it are the following:

V1 cathode resistor/bypass cap
V2 cathode resistor
Volume pots
8uf filter cap
The fourth 20uf filter cap
The middle control
The presence control

Does anything look awry here? Any glaring errors? I'm thinking that perhaps the presence control shouldn't ground to the preamp bus, or perhaps the 20uf filter cap is doing something. Any troubleshooting ideas would be greatly appreciated, thanks so much.
 
Last edited:

gabasa

Tele-Holic
Joined
Jan 29, 2018
Posts
676
Age
50
Location
Toronto, ON
Yeah, for the filter caps, I followed the grounding according to the layout drawing on RobRob's website:

Still not sure, but I'm definitely going to test out a number of different things.
 

gabasa

Tele-Holic
Joined
Jan 29, 2018
Posts
676
Age
50
Location
Toronto, ON
Not sure if it's 60Hz or 120Hz.

However, my 4x10 cab isn't ready, and I've tried it out running an impedance mismatch; a 2-Ohm amp output into a 4-Ohm speaker load. I've read that this alone can cause some humming, so I'm thinking of investigating more once it's in its cabinet and properly loaded. In the meantime, I'm just wondering if I did anything glaringly wrong in my grounding that anyone might notice.

Swapping tubes didn't help, so I thought that maybe grounding is the issue.
 

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,222
Location
Salt Lake City
Hum has so many causes, I'm not sure why we'd only look at the preamp ground bus. We need to look at all likely causes, but also try to narrow down the field, since there are so many. One BIG clue is it varies with the volume knob -- that means it enters the circuit before the volume knob.

You *really* want to figure out if it's 60Hz or 120Hz or something else. 60Hz will be well below the lowest E on the sixth string (at ~82Hz); 120Hz will be lie between Bb (~117) and B (~123). Does it change when you insert a short patch cable or bare plug? All inputs? Is the chassis buttoned up in the *shielded* cab in an electronically quiet room on a good outlet known to be quiet with other amps?

It was smart to swap tubes -- tubes can hum. It may or may not tell us more than swapping, but some folks check the hum as they pull the tubes one at a time, starting with V1...
 

dan40

Friend of Leo's
Joined
Aug 19, 2015
Posts
2,731
Location
Richmond Va
Have you tried swapping in a different 12ay7 tube into the v1 slot yet? Some tubes can exhibit excessive heater to cathode leakage that will result in an extra bit of hum.

You could try landing those preamp filter cap grounds with the power section grounds and see if it helps. That was the method that I used for many builds but I have now converted to the "single ground buss" scheme that many of the fellas here use. It lands every ground from the PT all the way through to the preamp on one single ground buss that connects to the chassis near the input jacks. I have found this method to work very well in all of my recent builds.
 

gabasa

Tele-Holic
Joined
Jan 29, 2018
Posts
676
Age
50
Location
Toronto, ON
Thanks for the responses, I appreciate it.

The first thing I did was roll through some tubes and it didn't make a difference. As soon as I take another look at it, when it's in it's proper cabinet, I'll go through your suggestions and report back.
 

NTC

Tele-Afflicted
Joined
Mar 17, 2003
Posts
1,211
Check the grounding tabs on the input jacks. It PROBABLY isn't one of those since that tends to be louder. I had that issue on my 5E7. On that it turned out to be cheap jacks and then a missed solder joint on a wire.
 

Paul G.

Friend of Leo's
Joined
Mar 17, 2003
Posts
4,185
Location
Rhode Island
Are you saying it hums without anything plugged in when you turn up the volume?

Your ground scheme sounds correct. Are you sure you're not 60 cycle heater hum? Do you have an artificial heater center tap (or a real one on the transformer)? When you measure your heater to ground, are both sides very close? Are your heaters flying or down against the chassis? If they're down, any signal wires should cross at right angles, never have anything parallel.

Is your power transformer screwed down tight (they vibrate)? Double check your ground connections, especially the cathodes and filter caps.
 

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,434
Age
62
Location
Finland
Yeah, for the filter caps, I followed the grounding according to the layout drawing on RobRob's website:

Still not sure, but I'm definitely going to test out a number of different things.
That layout seems to use Chassis for pre amp return current! It looks very easy to fix!!!

Isolate ground lug near PT and solder a piece of wire between 3rd and 4th capacitor negatives in dog house!

If it lessen hum perhaps Robrob can edit his layout too if you ask?
 
Last edited:

gabasa

Tele-Holic
Joined
Jan 29, 2018
Posts
676
Age
50
Location
Toronto, ON
Are you saying it hums without anything plugged in when you turn up the volume?

Your ground scheme sounds correct. Are you sure you're not 60 cycle heater hum? Do you have an artificial heater center tap (or a real one on the transformer)? When you measure your heater to ground, are both sides very close? Are your heaters flying or down against the chassis? If they're down, any signal wires should cross at right angles, never have anything parallel.

Is your power transformer screwed down tight (they vibrate)? Double check your ground connections, especially the cathodes and filter caps.
It hums the same, plugged in or not. The volume control changes it but the tone controls don't.
Swapping tubes doesn't help. All the connections look solid and the PT is quite firmly bolted to the chassis.
Moving wire positions, including the heaters. doesn't seem to change a thing.

Here is a pic of the chassis I posted in my build blog, the heater wires literally go straight up from the tube sockets:

 

gabasa

Tele-Holic
Joined
Jan 29, 2018
Posts
676
Age
50
Location
Toronto, ON
That layout seems to use Chassis for pre amp return current! It looks very easy to fix!!!

Isolate ground lug near PT and solder a piece of wire between 3rd and 4th capacitor negatives in dog house!

If it lessen hum perhaps Robrob can edit his layout too if you ask?
Have a look at the ground lugs in the photo I posted here:


In the preamp, the ground bus terminates at the input jack. You can see the little green wire on the right side of the photo.
In the power amp, everything terminates on the lugs on the left side of the chassis.

I have a question regarding your comment. I will try to disconnect the 4th capacitor from it's grounding on the preamp bus and solder a piece of wire between it and the the 3rd capacitor negative. What do you mean by, "Isolate ground lugs near PT?"
 

gabasa

Tele-Holic
Joined
Jan 29, 2018
Posts
676
Age
50
Location
Toronto, ON
Here's another question. Should the power cord ground wire have it's own ground lug separate from the power amp grounds?
 

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,222
Location
Salt Lake City
Here's another question. Should the power cord ground wire have it's own ground lug separate from the power amp grounds?

Yes. This the first and most important point in ground theory. See Rob's layout or read Merlin Blencowe on grounds. This is strict code in the US and EU for safety reasons, tho I imagine there is such a thing as noisy ground.

This thread reads like we're all heading off guessing at individual suspects. Not unexpected with crowd-sourced problem solving -- sure, some of us have a pet theory, but also, given the dozens of possible causes, folks will have totally valid *experience* with different fixes. But... when I've had surgery in the past, I was happy the docs did a thorough stepwise workup to get as clear as possible on likely causes before they scheduled OR time.

I support your idea of trying the amp in a cab, but if you don't have a nice shielded cab yet, at least play it in your best known-quiet music space on a solid support and cover the chassis with a cookie tin or something. We really need to figure out if it's 120 or 60 -- if the 6th string test is unclear, you can listen to each frequency on the internet, or I think there are free frequency analyzer apps for smart phones. And in addition to tube swaps, I'd try tube pulls, first V1, then also V2.
 

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,434
Age
62
Location
Finland
Have a look at the ground lugs in the photo I posted here:


In the preamp, the ground bus terminates at the input jack. You can see the little green wire on the right side of the photo.
In the power amp, everything terminates on the lugs on the left side of the chassis.

I have a question regarding your comment. I will try to disconnect the 4th capacitor from it's grounding on the preamp bus and solder a piece of wire between it and the the 3rd capacitor negative. What do you mean by, "Isolate ground lugs near PT?"
Pre amp Common is not connected to PT CT when there is no wire between 3rd and 4th filters. Then pre amp current flow to chassis thru amp input and when there is ”power amp ground lug” the pre amp current return to PT CT thru it.
 

Lowerleftcoast

Poster Extraordinaire
Joined
Dec 30, 2019
Posts
5,534
Location
california
And now for something completely different.

How does the soldering on this look to you?

InkedTweed Done 2_LI.jpg
 

gabasa

Tele-Holic
Joined
Jan 29, 2018
Posts
676
Age
50
Location
Toronto, ON
They will often hum a lot when not in a cab that has the shield in the top.
That's a good point, I'm looking forward to getting it in the cabinet. Do vintage Fender amps typically have a shielding foil on the open side of the chassis? The tweed cabinets I've had built for me in the past have had foil glued to the back banel, but I wonder what's normally done to the underside of the the top of a blackface cabinet. Do people glue foil in this position? This would be a bit scary because if the foil comes loose, it could short circuit something.
 




Top