Bum Switch

theshadowofseattle · Post by **theshadowofseattle** » Fri Mar 09, 2007 9:05 am

The on-off switch on my world famous Bassman has never been quite right and tonight I think it finally shit the bed. I sat there Tom Morello'ing the switch for a full 2 minutes and it just wouldn't turn off. I gave in and unplugged it. Is this something I can fix myself or should I turn it over to a tech? The only thing I know about tube amps is that if you fuck with them it will either kill you or the amp.

Also, I have a grounding switch on the back that says "A - Off - B" and is currently set to off, I get a bit of hum from the amp, should I flip this switch and where to? There's an input jack on the back of the amp that says "HUM BALANCE," if I stick my wilkins in there will it help?

Post by **Mike** » Fri Mar 09, 2007 9:15 am

OK,

1) The Ground switch is a "Ground Lift" which disengages the ground connection from the circuit, it is for removing hum from the sound if you're getting a Ground Loop problem (big hum all the time) in multiple inputs/outputs situation. Guitar -> Amp is not one of these situations so you should have it set to A or B. I don't know what the difference is so I'd go to A. LDC and filtercap may be able to shed more light on Fender ground lifts.

2) Yeah unfortunately you're going to want to take that to a tech, if you can't switch the amp off reliably then there could still be charge on the filter caps (which would probably have been discharged by you unplugging it) but there's a chance not - and that chance could mean death by electrocution. The mains switch has a fuse associated with it which should also be checked and the switch contacts may have to be cleaned or the thing replaced. I'm inclined to think if your amp won't switch off that the switch is fucked and it'll need a new one.

Sorry it's not a simpler fix.

Post by **Mike** » Fri Mar 09, 2007 9:17 am

And keep the wilkins away from the jacks (I read something in the paper today about a guy who FUCKS cars in the exhaust pipe - WTF). The Hum Cancel must be some form of phase inverted input to cancel out 60 cycle hum.

theshadowofseattle · Post by **theshadowofseattle** » Fri Mar 09, 2007 9:21 am

If I can't turn it off tomorrow, I'm taking it to a tech. Thanks, Mike.

Post by **Mike** » Fri Mar 09, 2007 9:25 am

np ngz.

filtercap · Post by **filtercap** » Fri Mar 09, 2007 9:48 pm

I'll have to go read up on those ground switches. It connects the chassis, through a big capacitor, to either the "hot" or "neutral" wire of your house system, or neither when the switch is in the middle position.

I flip mine to either A or B, depending on which way yields less hum. HOWEVER this can lead to uncomfortable surprises if your PA system or a bandmate's amp is grounded the =other= way and you touch a mic or somebody else's strings while in contact with your own guitar strings. People often recommend removing them nowadays and permanently referencing the chassis to the ground of a 3-wire system. I'll do that with mine one-o-these-days.

A new power switch (and permanent grounding if you want) will be a quick job for a tech -- good, safe, and hopefully cheap option.

If you look down into the HUM BALANCE thingy, you should see only a limited amount of potential wilkins room in there, because it isn't a jack at all but a pot with a recessed screwdriver slot.

If that's what you see in there (in other words, if somebody hasn't actually turned it into a jack in the past ???), then you can adjust it with a screwdriver and get either more or less hum, depending. It changes the way the tube heater circuit is referenced to ground. It's safe to mess with.

You may also have a screwdriver adjustment labeled Output Tubes Matching or some such thing. It basically balances the bias of the two halves of your power amp so that they amplify evenly, and as a side benefit cancel out some hum as well. You can try making slight adjustments to that and listen for more or less hum.

JordanD · Post by **JordanD** » Fri Mar 09, 2007 10:22 pm

Mike wrote:And keep the wilkins away from the jacks (I read something in the paper today about a guy who FUCKS cars in the exhaust pipe - WTF). The Hum Cancel must be some form of phase inverted input to cancel out 60 cycle hum.

Yeh, I read that too. WTF indeed.

Post by **Mike** » Fri Mar 09, 2007 11:22 pm

THE IMPORTANT THING IS THAT FILTERCAP IS CLEVER

Bacchus · Post by **Bacchus** » Fri Mar 09, 2007 11:26 pm

Mike wrote:And keep the wilkins away from the jacks (I read something in the paper today about a guy who FUCKS cars in the exhaust pipe - WTF).

Time was, if you searched for 'love' in google images you got a picture of a guy doing that. Not so anymore, such is the way thing's are.

Post by **Mike** » Fri Mar 09, 2007 11:30 pm

BacchusPaul wrote:
Mike wrote:And keep the wilkins away from the jacks (I read something in the paper today about a guy who FUCKS cars in the exhaust pipe - WTF).
Time was, if you searched for 'love' in google images you got a picture of a guy doing that. Not so anymore, such is the way thing's are.

DO NOT FUCK THE JACK

Bacchus · Post by **Bacchus** » Fri Mar 09, 2007 11:40 pm

Mike wrote:
BacchusPaul wrote:
Mike wrote:And keep the wilkins away from the jacks (I read something in the paper today about a guy who FUCKS cars in the exhaust pipe - WTF).
Time was, if you searched for 'love' in google images you got a picture of a guy doing that. Not so anymore, such is the way thing's are.
DO NOT FUCK THE JACK

That's not fair. Who's Jack?

euan · Post by **euan** » Fri Mar 09, 2007 11:42 pm

BacchusPaul wrote:
Mike wrote:
BacchusPaul wrote: Time was, if you searched for 'love' in google images you got a picture of a guy doing that. Not so anymore, such is the way thing's are.
DO NOT FUCK THE JACK
That's not fair. Who's Jack?

My sheep. So do what Mike says.

Post by **Mike** » Fri Mar 09, 2007 11:43 pm

NEVER FUCK THAT JACK NGZ?

Bacchus · Post by **Bacchus** » Fri Mar 09, 2007 11:44 pm

Then what's the "bum switch" for?

Post by **Mike** » Fri Mar 09, 2007 11:51 pm

Turning off the amp.

Sloan · Post by **Sloan** » Sat Mar 10, 2007 12:37 am

1. Pour some beer on it
2. Prod it with a stick/screwdriver (flathead)
3. Curse it to hell (x2-4)

4. GET CRUNK

LimpDickCheney · Post by **LimpDickCheney** » Sat Mar 10, 2007 12:52 am

Take it to a tech / tech savvy buddy. Have him/her pull the ground switch and the power switch. Replace the power with the removed ground switch, place the power switch in the ground switch slot as a dummy, and wire in a new three prong plug. In this age of grounded outlets you have no need for a ground lift that I am aware of.

LimpDickCheney · Post by **LimpDickCheney** » Sat Mar 10, 2007 8:35 am

Come to think of it your amp will have a three way toggle for the ground switch... ehhh... never mind, just order a switch from ampwares and replace it.

theshadowofseattle · Post by **theshadowofseattle** » Sat Mar 10, 2007 8:33 pm

It's in the shop. I also got a single-coil sized Super Distortion for $35 and a pack of strings for my Martin, since I'm playing acoustic for the next few days...

theshadowofseattle · Post by **theshadowofseattle** » Mon Mar 12, 2007 10:54 pm

It's fixed!