Buzz In Guitar Amp

[wdcw]

I have a Moody BA40 Bassman tube amp around a 1970 model that has a buzz through the speakers after playing the low notes on a guitar.
There is no buzz at lower volume levels but when turned up you get a buzz after a note is played.
It's not the sound you get from bad speakers but it sort of sounds like there is a faint arc being produced somewhere after the lower notes are played.

I have changed the guitar lead & used a different guitar but the results are the same with each.

I found a clip on you tube that has an amp with a very simillar buzz but it is much worse than in my case.
YouTube - Strange buzz on peavey amp...

Any idea's where to start to fix the problem.

Cheers

[chris661]

There were various comments about the problem on the youtube vid - seems a cable was the problem.

I get a buzz on mine when the downstairs lights are on - they're on a dimmer, making them electrically very noisey. I can't play unless I move the cable off the floor... Any lights with dimmers in the vicinity?

[wdcw]

Thanks for the reply chris661,

No, no dimmers etc, i'll move a couple of things around & see how I go, I tried several leads of different brands as well just incase it was a lead problem but with no luck so far.

I have another solid state amp & don't get the same problem with that one only the tube amp which made me think it may be something wrong in the amp itself?

EDIT:

I just tried with things moved around but with no luck.

On the you tube clip they had a lot of buzzing, with mine it is just like a small arcing sound, sort of like when you jump an arc across an airgap sound & not a mulititude of interference like on the video clip.

Cheers

[SemperFi]

Any way you can try another speaker cabinet? Perhaps hook up the amp to the speaker in your other amp.

This problem only just occured? Has the tubeamp been stored a while, or is it a while since you played it?

[Wavebourn]

What can be the difference between high and low notes on high volume?
Output transformer impedance.
Low notes may cause some sag of power supply.
Just thoughts... Now, what can cause buzz as the result of a PS sag?

[wdcw]

Hi SemperFi,

Yes, the Amp has been stored for a while, I have only recently aquired it.

I have just noticed that the output valves are different brands, I'm not sure if that is ok or not & have asked the question in another post.

I will have to check to see if the speakers in the other amp are the same resistance I guess before I try anything.


Hi Wavebourn

The low notes will be higher amplitude as the strings vibrate a lot more, do you think striking a single low note would cause PS Sag, I'm not sure without testing it?

Cheers

[Wavebourn]

Output transformers, of course, have finite inductance, so current that flows through them is higher on lower notes.
Check if your amp has a voltage doubler in B+ PS, and probably it's the time to refresh filter capacitors.

Also, different transconductances of output tubes causes DC bias of the output transformer, that makes what I pointed before even worse: the tranny starts saturate on lowest notes so the amp consumes even more of current from power supply.

[Arnulf]

Could it be that one of the output valves is either defective or doesn't have good contact in socket ? The noise at high input levels could come from rectification of the signal once output stage leaves class A and enters class B, if one of the valves in PP isn't working properly. It could also be a result of preceding voltage amplification stages working with incorrect bias, again chopping off one halfperiod of the signal.

Anyway, this guessing game is pretty much pointless until you provide some reference data (measurements taken from the relevant points of your amplifier).

[wdcw]

Thanks Wavebourn, I see what your saying, I will remove the amp & change the Capacitors & order a pair of new valves for it.

Hi Arnuf, Yes it has turned into a guessing game, I thought it may be something simple to start with but it's time to have a closer look & change a few things. I want to add a reverb circuit etc to it so now is the time to do it properly. I'll draw the circuit & then test a few things while I wait for the components.

I'll post the test results.

Thanks to all that replied.

Cheers

[firechief]

You have used the term "arching" a couple of times. Before you take it apart, try turning the lights off (in the room) and making the problem happen. Look for arcing,. I know it sounds a little weird, but if it sounds like arcing, perhaps it is. Good luck.