Buzzing gone at 4 o'clock?

[bcmatt]

So, I've had my BF Vibro Champ for a while now. I really like it's sound. I had some hum problems but, after getting some properly shielded wires in a couple spots, it was helped. There is another bit of buzz which I think can be eliminated.

It buzzes until I turn the volume almost all the way up Past 3/4s. There is a certain spot at about 4 O'clock where the buzzing pretty much stops. Then if you crank it past there, the buzzing comes back. What do you think causes this? How can I fix it?
Any thoughts from the Mighty Guitars Canada Brain Trust?
Thanks

[Michelle]

Gee, mine usually buzzes at 4:20. :smile: There's just a spot that is sensitive to that volume, would be hard to find. If you really want to, you could go over all seams with glue, injecting in the joints, (:smile:), you may never solve it short of an ext cab.

[bcmatt]

Gee, mine usually buzzes at 4:20. :smile: There's just a spot that is sensitive to that volume, would be hard to find. If you really want to, you could go over all seams with glue, injecting in the joints, (:smile:), you may never solve it short of an ext cab.

Sorry, I should mention that it is the amp, and not the cabinet. It happens whether a guitar is plugged in or not. It's a buzzy sounding hum that is pretty consistent and gets louder as the amp is turned up, but does go away at one spot at about 85% volume/power.

So, I will often run the amp at that one good spot which works well for my purposes because it gets a nice overdrive when the humbucker volume is turned up. However, I am just curious what might cause this buzz? Is it the pot itself?

[Budda]

my first guess would be the pot, but i dont know a whole heck of a lot about amps.

cheap part to replace and find out, no?

[Glasstone Amps]

If two separate amps of the same model are both exhibiting the same behavior, I'd say it's a design flaw. Those amps have really crappy grounding schemes, which doesn't help. Does anyone else out there have one of these amps that behaves like this?

[bcmatt]

I don't know of anyone else having this problem, but here is the layout if anyone suspects a design flaw. I thought it is pretty nicely grounded.
http://ceriatone.com/images/layoutPic/fenderLayout/VibroChampCeriatone.jpg

If the pic doesn't show up:
http://ceriatone.com/images/layoutPic/fenderLayout/VibroChampCeriatone.jpg

I'm not lazy but I don't really feel it is the pot. I was kind of just being cheeky when I suggested it. I thought the pot could most often just get scratchy when used; but is it possible for pots to do things like this? I don't really have a 1 Meg Audio-tapered pot on hand to try anyways.

I mostly thought that someone might know exactly what the problem is because it seems like a very peculiar symptom of buzzing until you turn up the power on the amp to between 83%-88%, and then coming back higher than that as well.

[Glasstone Amps]

Is this a Fender amp, or a clone of a Fender amp? When I talked about grounding I was talking about Fender specifically.

One interesting thing to note is that the buzzing seems to be lowest when the pot wiper has equal resistance on both sides. Since it's a log taper pot, the center of resistance would probably be around 4:00 or so. Maybe there's a correlation, not that it helps diagnose the problem.

[bcmatt]

Is this a Fender amp, or a clone of a Fender amp? When I talked about grounding I was talking about Fender specifically.

One interesting thing to note is that the buzzing seems to be lowest when the pot wiper has equal resistance on both sides. Since it's a log taper pot, the center of resistance would probably be around 4:00 or so. Maybe there's a correlation, not that it helps diagnose the problem.

It's a clone of a fender amp.

Brian, I hadn't thought about that spot being the center of resistance, but you might be on to something there. I don't know what that could mean though. I should check to see if maybe there is a grounding problem with that pot. Could a faulty pot cause this sort of thing, or was that crazy talk?

[Glasstone Amps]

No, I don't think the pot's to blame. Maybe try shielding the wire going to the grid of the next stage.

The fact that the buzz is minimum when the grid is "seeing" a maximum resistance suggests that possibly inserting a grid stopper resistor might help. Try putting a 68k resistor in series with the tube grid, right on the tube socket. That's normally used for RF rejection, but it's worth a try.

[bcmatt]

No, I don't think the pot's to blame. Maybe try shielding the wire going to the grid of the next stage.

The fact that the buzz is minimum when the grid is "seeing" a maximum resistance suggests that possibly inserting a grid stopper resistor might help. Try putting a 68k resistor in series with the tube grid, right on the tube socket. That's normally used for RF rejection, but it's worth a try.

Ok, sorry about the lateness of my reply. I was too busy to open up this amp lately. I have FINALLY got a day off. Now I have it open in front of me again.
However, I don't understand what you are saying here. I have shielded wire connected to each grid already (pins 2 and 7).
Also, I am wondering what you mean by putting a 68k resister in series with the grid. Isn't that pretty much what I have because there are 68k resisters coming off of the input jacks, connected to the shielded wire that connects directly to the first grid.
Am I missing something? Do you mean that I should try another 68k resister between the inputs and the grid by putting it right before the pin? So, you are saying I should end up having 136k of resistance before going into the first grid? Or are you saying I should remove the resisters from the inputs and put one right on the grid instead?
Sorry if I'm a bit slow but you might have to spell it out for me. If you look at the layout above; that is exactly how I am wired. The light blue wires are shielded.

[Wild Bill]

Ok, sorry about the lateness of my reply. I was too busy to open up this amp lately. I have FINALLY got a day off. Now I have it open in front of me again.
However, I don't understand what you are saying here. I have shielded wire connected to each grid already (pins 2 and 7).
Also, I am wondering what you mean by putting a 68k resister in series with the grid. Isn't that pretty much what I have because there are 68k resisters coming off of the input jacks, connected to the shielded wire that connects directly to the first grid.
Am I missing something? Do you mean that I should try another 68k resister between the inputs and the grid by putting it right before the pin? So, you are saying I should end up having 136k of resistance before going into the first grid? Or are you saying I should remove the resisters from the inputs and put one right on the grid instead?
Sorry if I'm a bit slow but you might have to spell it out for me. If you look at the layout above; that is exactly how I am wired. The light blue wires are shielded.

I think what GA is getting at is that your amp may have an oscillation problem. Oscillations are caused by improper lead dress, where two wires or part leads run too close to each other. There's a small but real capacitance between them that can act like an unwanted coupling capacitor. If this stray capacitance couples a bit of signal from one stage back in phase to the input of a previous stage it's just like putting a microphone in front of a PA speaker.

Oscillations are not always high pitched squeals. They can be low frequency growls or buzzes or ultrasonic - too high for our ears to hear. These are the scariest 'cuz the amp is flogging itself to death trying to pump out the squeal full power and while the tubes burn themselves out or the speakers cook you don't hear a thing! A 'scope shows these things plain as day and makes fixing the problem much easier.

Anyhow, sure sounds like this amp has a design flaw. One dead giveaway clue is a buzz that changes when you adjust a control.

Grid stopper resistors might help. Might not, too. It's better to stop an oscillation at the source then to try to squelch it out after it's already happening. I'd take a wooden or plastic rod (NOT a pencil! Voltage can travel up the lead and bite you!) and try pushing wires this way and that, looking for any CHANGE in the tone of the buzz! This will identify the location of the problem and then you can try running the wires in different paths farther apart to see if that stops the problem. Sometimes even half an inch extra separation is enough.

You can also try shielded wire on the signal leads. Try first with the blue wires on the diagram that run from the volume pot and the input jacks. Be sure to ground the shield ONLY at the volume pot and the ground point of the jacks! Grounding both ends can set up a ground loop. If the amp is a true Fender clone perhaps you can check to see if it follows the original Fender layout exactly. Ground points in the circuit are also a big factor.

There is an art to wiring that the old guys learned but is often forgotten today. Wires run in parallel couple together a LOT more than wires that cross at right angles! Every time you go through a stage of amplification with the usual common cathode triode you flip the phase. Coupling must be IN phase to start an oscillation!

There's lots more rules. It all boils down to the fact that with a tube amplifier you can't treat your wiring like a car or a boat trailer, where you have DC wires and you tie grounds to any convenient spot of bare metal. Tube amps have high impedance inputs and lots of gain. If you wire up a jungle you WILL have problems!

Never depend on the designer to have done a perfect job! Lots of times his layout may have a few bugs but he either didn't know how to fix it or tried and gave up for lack of patience. If the problem is mild the amp is rushed out the door.

Anyhow, these things can be a real PITA to troubleshoot. Good luck!

:food-smiley-004:

[bcmatt]

Cheers! Thanks Wild Bill. I appreciate the tips.
I poked around with a wooden chopstick and narrowed it down to that first shielded cable going from the input jacks to the first tube. It seemed to be collecting buzz mostly around the input jacks by the 68k resisters. For now, I moved one resister to the tube pin and just disconnected the other input jack (minimizing unshielded areas on that pat as much as possible).
The amp is pretty quiet now, and only starts to hum when the volume is up about 3/4 of full. I would say it is an improvement, and wonder if maybe that is normal on a champ.