Static noise

[geldo79]

Hi,

I have a question concerning static noise of my ao29 amp. I hear noise and pops regardless whether I activate percussion, vibrato etc.! Can I there conclude that the problem will be inside the "output amplifier" part if the ao29? I already replaced the main filter capacitors as well as C64 and C30. Replacing the main filter caps did not have any effect. After replacing C30 and C64, the noise is very quiet now. But it's still there. Any advice what to check next?

Greetings Christian

[Jaim]

Have you cleaned the tube sockets / pins - especially the 12AX7 preamp?

Are the pops there all the time or only when the vibrato and precussion switches are cycled?

Jim

[geldo79]

It's always there as I noticed in my first message. Does this mean the problem is in the output section of the amp?

I did not clean any tube sockets. How do you clean them?

[David Anderson]

Most tube pin/socket noise problems are on the tube pins because they are made of an alloy that slowly oxidizes over the decades. Just spraying Deoxit on them doesn't always get enough oxidation off.

If the noise gets louder with the expression pedal at high volume, the source is before the expression pedal; if not, the source is after the expression pedal.

Pops and crackles can come from bad connections, including tube pins and bad solder joints, carbon composition plate/screen/cathode resistors, tubes themselves (bad weld inside the tube), or leaky audio signal path capacitors. More rarely, they can come from contaminated tube sockets.

People often cite carbon composition resistors as sources of noise, but they can only produce significant noise if there's a DC current through them. The larger the voltage drop, the more likely they are to be a noise source; thus, plate and screen resistors are more likely culprits. A carbon composition resistor in a feedback loop with no voltage across it will not be a noise source.

[Frankquip]

Try lightly tapping the tubes with a plastic screwdriver handle. You might get lucky!!!!

[geldo79]

"If the noise gets louder with the expression pedal at high volume, the source is before the expression pedal; if not, the source is after the expression pedal"

So....if it is after the expression pedal....would you suspect a distinct component in the output section of the amp (if you could exclude contact problems from tubes an sockets)?

greetings Christian

[muckelroy]

It's merely an isolation step. It could be anything, but if it's say not affected by the expression pedal, that rules out 3/5 or so of the components of the preamp! (It's rudimentary isolation. Pinpointing the problem will require some skill at signal tracing.)

[geldo79]

Ok. Not easy )-: In another thread "bass pedals don't decay" i mentioned that after replacing a cap (c64) there was a broken connection so that c64 was not connected at all. Could this also cause static noise? I dont't know what this cap is for. I only observed that the bass sound obviously can not decay if c64 is not connected.

[csw900]

Hi

When I worked on electronic equipment in telephone exchanges we used to do what we called "bash testing". It involves tapping (quite hard) every component with an eraser (rubber) about 1/2"X1/2"X1/2" fixed on the end of a thick plastic knitting needle. At the same time listening to the output. If the "bashing" affects the noise - either worse or better - then you have found a problem.

This is very effective (but not infallible) and will find noisy components and joints.

csw900