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.

 

[M Gregg]

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.


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

Cheers

Something you could try is to gently tap the tubes with a pen when the amp is on. It could be the vibration when the amp is turned up. Start with the Output tubes and work back to the input! Also try cleaning the volume control.

Regards
M. Gregg

 

[M Gregg]

You should hear the tap sound through the speaker. If the tube is faulty or you have a dry joint it will be folowed by a scratchy sound that slowly dies. A faulty coupling cap will also create a problem. Also look for any anodes glowing red "Bias Problem".

 

[wdcw]

Gday Guys,

firechief,
That was one of the first things I did was to check for visible signs of arcing, couldn't detect any.


M Gregg,
I tried tapping the valves etc but could not replicate the sound, your comment on the scratchy sound that slowly dies away is exaclty what it does with the volume up & playing the low notes.

After your comment also on the Anodes maybe glowing red, after having another look there is one valve that the Anode is certainly glowing red without doubt.

Time to have a closer look.

Thanks everyone, i'll post some results when I know more.

Cheers

 

[Wavebourn]

I thought it sounds like a hum...
If it is scratchy, most probably some carbon comp resistor in PS filter or in screen grid supply went bad.

 

[wdcw]

Thanks Wavebourn,

No not a hum, it's like a scratchy sound that dies away slowly after the note is played--very annoying. Very similar to the sound on the you tube clip I linked to but not as bad as the problem they have on the video.

I have taken the amp out of the head & am now checking things out, I will post what I find.

I want to buy a new pair of EL34/6CA7 valves, what are descent quality these days.

I can buy Mesa Boogie, Electro Harmonics, JJ Tesla, Tungsol, SED, or the Mullard reproduction Valves at the moment on Ebay.

What to buy?

Cheers

 

[M Gregg]

For anyone interested,

Here is the GA40 Schematic.

Moody

wdcw remember HT bites!

Regards
M. Gregg

 

[wdcw]

I removed the amp as mentioned previously & after a quick check to see if any of the components looked faulty, burnt etc which they didn't.

I thought I would clean all the pots & try the amp again before touching anything, so I cleaned them & then tried it again with the noise still there.
I then swapped the two output valves over & the noise disappeared, even with full volume playing all the strings very hard--no noise.

There is one valve that is running very hot so I swapped it back over to see if it was the particular socket but it seems to be the valve as it runs hot in either socket & the other valve is a lot cooler.

I have attached a picture, you will see what I mean.

Why would it produce the noise when in one socket & not the other?

Thanks M Gregg

Yes, my amp is the one about half way down that page, black head & speaker cab.

Yes I am careful around any power.

I see they have a 12AX7 valve on the schematic, mine has been fitted with a 12AU7 & may explain why this amp won't readily go into distortion. Maybe someone has fitted the 12AU7 to keep the amp clean?

What do you think?

Cheers

 

[M Gregg]

You have a fault!

I would change the coupling caps on the EL34's they show on the schematic as 100nF thats a 0.1 uF if you use polyprop you will reduce the leakage! Also change the cap on the EL34 cathode resistor!


Regards
M. Gregg

 

[Arnulf]

Red plating

As I said the other day, measure the relevant voltages to see where your tubes are opeerating (inside or outside maximum suggested limits). As you mention you amplifier has obviously been a victim of a "modder" so chances are bias conditions were changed beyond suggested maximum which some tubes handle better than others.

 

[Wavebourn]

There is one valve that is running very hot so I swapped it back over to see if it was the particular socket but it seems to be the valve as it runs hot in either socket & the other valve is a lot cooler.

Do they have some resistors soldered from pin 8 to the ground?
If yes, what are their values?
Do you have a DMM to measure voltages on them?
Also, voltages on 1'st grids need to be measured: if a tube is gassy negative bias voltage on it's grid would go down to zero volt.