Vibro Champ problemos

[runrunrun]

Hi,

My -76 vibro champ just started to make a pretty loud, constant hum. The volume goes up and down by itself, sound crackles away totally if i hit the strings hard, especially when hitting the lower strings. Loud crackles.

And of course i don´t have replacement tubes for testing Have to order them by mail... I did take away the tremolo tube and tried that instead of the first preamp tube... no effect on the problem so i guess the preamp tube is not the problem. But like said i cannot test-swap the power tube or rectifier tube. I did tap the tubes gently with a pencil but it did not affect the sound.

I´m pretty new to tube amps and i´m just worried about the constant hum. The hum is not affected by the volume knob,  it stays the same all the time.  Does this mean the problem cannot be in the tubes?

Hope i don´t have to send the amp for repair to a bigger city.  I would not touch the insides of the amp myself because i have no experience...

[Jay]

Well, I'm no tube amp expert by any means but your probably on the right track with regards to checking the tubes.  Luckily, the vibro-champ only uses 4 so it's not too costly to pick up a set and start swapping.  Maybe you even have a friend with a champ (or similar amp) that you could swap tubes with as well.  Another thing to check is whether the hum changes or goes away when turning the knobs.  A bad or dirty pot can be the cause of a lot of noise as well.

[runrunrun]

Yup i guess i have to ask around for some tubes. Of course it´s a good idea to order a full set anyway for backup... but the problem is i´m currently really low on money. 

The hum stays the same all the time despite the knobs... also the crackles seem to have nothing to do with turning the knobs.

[GUITARmole]

If it was the rectifier your amp likely wouldn't work at all.  It's either the pre, power, or something else alltogether. 

As mentioned, check those pots...run them up and down quickly 50 times in a row on each one and see if it goes away.

[runrunrun]

Ok, I took your advice and fiddled with the knobs like mad...  I also removed (again) all the tubes and put them back. Either of these seemed to help, the problem went away! Let´s see when it comes back though.. I´m gonna order a set of backup tubes anyways when i get the money.

Thanks for all the help! 

The Vibro Champ sure is a sweet amp when it works! Gonna get a 2x12 cabinet when i get the money, i bet that will sound sweet! 

[GUITARmole]

Ok, I took your advice and fiddled with the knobs like mad...  I also removed (again) all the tubes and put them back. Either of these seemed to help, the problem went away! Let´s see when it comes back though.. I´m gonna order a set of backup tubes anyways when i get the money.

Thanks for all the help! 

The Vibro Champ sure is a sweet amp when it works! Gonna get a 2x12 cabinet when i get the money, i bet that will sound sweet! 

So...from what you described...I bet it was the fiddlin' w/ the knobs like mad that fixed it.  You can get a product by CAIG that's called "moving contact lubricant" which is made to clean pots and switches.  If you spray some of that into the back of the pots (there should be a hole there) and work them a few times it should clean them out.  It also leaves a lubricant behind that prevents oxidation so it helps stop the problem from returning.

[blacktiger]

When, if ever, did it last have a cap job?  If you don't know, then there's a good chance that it needs one.  30 year old electrolytics  are a time bomb that could take out your output transformer when they go.

[runrunrun]

Roger that sir, gotta get me some pot lube! Also need that for my guitars actually... cheap pots.

I bought the Champ last summer from a dude who did a cap job on it. So they should be ok...

[øøøøøøø]

Here's some advice:

#1, the vibro champ is an EXTREMELY simple circuit.  Extremely so.  If the amp is making any sound at all, whatever it is is not an expensive problem. 

#2, here are the possible things that could be causing your symptoms: Tube(s), tube socket(s), filter capacitors.

You can rule out the preamp tube thusly: If turning the volume control causes the level of the noise to go up or down (i.e. can you mute the amp by turning volume to zero) then the problem is in the preamp stage or earlier.  Otherwise, it's not.

Many people are quick to replace tubes when the tube socket is at fault.  I suspect this is likely in your case.  Tube sockets occasionally lose their tension and need to be re-tensioned.  Get a dental pick or some other sharp implement, discharge the capacitors and bend the little metal contacts inward so they 'grab' the tube better.  You may also squirt some contact cleaner like Caig De-Oxit in there to clean them.  I suspect your contacts were dirty if plugging and unplugging the tube fixed it.

The filter caps on a champ are in the form of a metal multi-section can.  You can get a reproduction can and replace it or you can replace the caps with discrete capacitors mounted on top of the can (leaving the can in place for looks).