Fender Musicmaster Bass amp hummin

[fishinjim]

Just received a 6v6 version Musicmaster Bass Amp.
The amp has a very loud 60hz hum. Loud enough that I call it more of a buzz than a hum.
The amp has had a recent recap along with a tone stack mod.

Pulled the chassis to investigate the hum hoping to see something obvious. All looked clean and tight, but was able to pinpoint at least a good portion of the hum cause to the three wires going from the transformer to the board.

When those three wires are floating in the space between the transformer and board/caps, the hum is at its loudest. Should be noted as well that the wires are also close to the volume pot just before they attach to the board.

If I push the wires away from the volume pot and up against the two big caps on the board, the volume of the hum drops considerably.
I was able to wedge the group of wires behind a small bolt in the chassis to hold it in place this way.

Photos to hopefully help illustrate...

Loud hum...


Less hum...


So at least I've found the area that seems to be creating the bulk of the problem, however, I don't know if the hum is going away because the wire is being pushed against the caps, or because its being pushed away from the volume pot.

My first guess was a cold solder joint where the group of transformer wires meet the board, but after moving those wires around some, it seems that its their location relative the the things around them that are most culprit.

Anyone here able to tell me what might be going on here?
And of course, what the simple fix is. lol.

Not looking for dead silence with this thing, but would like to be able to put a mic in front of it.

Thanks.

 

[xtian]

The red wires from the PT are carrying AC, and radiating that 60Hz buzz into your sensitive input wires at the volume control. Keep them as far away as possible from the signal wires in the first stages of the amp. Push that signal wire at the pots against the chassis. If you still have noise, consider replacing that volume wire with shielded cable.

 

[fishinjim]

Thank ya, xtian.

Volume pot wires and transformer wires are now about as far apart as they can be. Still hummin, so I may give the shielded wire thing a whirl.

Wondering if I shouldn't reroute the transformer wires to come in from the rear side of the tag board.

Question...
Anything that would abruptly cause this hum to start up other than wires getting jiggled during shipping?
Guy who sold the amp to me said that it was quiet before boxed and shipped.

 

[xtian]

Certainly something could have broken during shipping. Use a wooden chopstick to prod all the connections, especially the ground connections to the chassis, to see if you can affect the hum.

I would suspect e-caps, but if we are to believe the seller, it was good before, so...

With no cable plugged into the input, does the hum change at all with the volume control? Bass control?

Pull the tubes, one by one, starting with the first preamp tube. When does the hum vanish?

Try a known good set of 6V6. Badly mismatched tube (or a faulty one) will also hum.

Etc. Basic troubleshooting approach.

 

[fishinjim]

I pulled the tubes when I first noticed the buzz.
Went quiet when I pulled the preamp tube. Tried a couple other tubes in the preamp slot and still had the problem, so then I pulled the chassis and started chopstickin.

Don't have another set of 6v6's right now to test, but I wasn't sure if they were needed after having that result with the preamp tube.

The moving of the wires from the transformer were the only things that changed the volume of the hum.
The wires from the tone pot were pretty close to the chassis. After you suggested that their relation to transformer wires were likely the problem, I pulled the wire out a bit more towards the transformer wires and the hum level increased.
Everything else in the amp seems solid and unaffected when prodded.

I think the source of the hum is pretty well narrowed down.
Was just wondering if there could have been some weird behind the scenes something or other that could cause that source to be to behave the way it is.

Thanks again!

 

[TimmyP]

Increasing the cathode bypass shouldn't decrease hum. Owing to extending the low frequency bandwidth, it should increase any hum that is produced in that part of the circuit. And it should increase the amp's tendency to sound raunchy or farty when cranked.

 

[schmidlin]

...and get rid of that death cap.

 

[fishinjim]

lol, the death cap is gone.
I snipped that sucker literally seconds after taking the above photos.

Fuse holder and 1amp slo-blo are on the way along with a few lengths of wire.

Rerouted the transformer wires today.
Also slightly rerouted the wire that went from the preamp tube to the volume pot.

Here's how she sits now...


Hum is still there with this configuration. May have lessened a little bit, but not near enough to say its fixed.
The way the transformer wires are now, they don't affect the volume of the hum when moved, jiggled, prodded.
The volume of the hum is now controlled pretty much exclusively by that wire from the volume pot going to the preamp tube. So at least I've localized the issue a little more.
As it is in the above photo yields the quietest results.
As far away as possible from those transformer wires is definitely the trick.

When my extra wire arrives, I'll try using a longer length and routing it out towards the input jacks and around the board.
If that doesn't work, I'll give the shielded wire a shot.

Thanks again!

 

[fishinjim]

Had some wire and time today.
And I think we're done.



That super long lead from the volume pot around the board over to the preamp tube seems to have done the trick.
And as a bonus, after I installed a fuse, it got even a little quieter.
Not sure what's up with that, but I'll take it.

This thing is mic ready.

 

[MIM#1]

Could somebody explain how and where best to ground a shielded wire from the vol to the preamp tube? So you just ground one end of the shield right?
I dont want to create any ground loops by grounding the shield in the wrong location...

 

[Jeff Gehring]

Ground the shield at the volume pot end only. On the end going to the tube grid, strip the outer jacketing and shield away and put a small piece of heat shrink tubing over the place where the shield has been cut back.

 

[MIM#1]

Thanks!
In the amp above, I would remove all grounds from the power/preamp tubes and install a pair of 100ohm artificial center taps to quiet the hum.[google it] My MMB amp was very noisy and this helped considerably, but the main source of my problem was the chassis ground at the PT, and the PT wasnt mounted properly with toothy lock washers and secure mounting hardware. THIS quieted my amp down!

 

[Malcolm Irving]

Increasing the cathode bypass shouldn't decrease hum. ...

It can do for some tubes. Hum current can get from the heater to the cathode inside the tube. The lower impedance of the bigger cathode bypass cap allows this hum current to shunt to ground without developing so much hum voltage on the cathode (which goes into the signal).