5E3 scratchy sound

[mhraposo]

Hello all,

I have been experiencing a strange symptom in my 5E3.
It is a home brew 5E3, based on mojotone kit. It has been running flawlessly for 3 years already.

I start playing and after a long period (40, 50 min) the amp starts with a scratchy sound. Seems like random static.

I've tried replacing all the tubes per phases, to debug any faulty tube, with no success. (In fact the 5Y3 rectifier was not replaced)
The sound remains even if the volume is turned down to zero. another symptom is that as soon as the scratchy sound picks up, the amp sound becomes distorted.

I'm using 6V6 paired JJ's as PT, a mullard 12AX7 as PI and a TAD 12AY7.

I've cleaned the sockets and no change.

Any suggestions or idea will be largely appreciated.

Thank you in advance to all.

[Enzo]

Unplug your cord from the input jack, does the noise continue?

Turn your controls up and down. Do ANY of the controls affect the noise in ANY way? ie, can you turn the noise down with the volume or affect the tone of the noise with the tone controls? Oh wait, you said it does. So the noise is from after the volume control.

A real 5E3 has two volume controls, does yours? If so do BOTH control noise or is only one noisy?


With the volume up so you can hear the noise, remove the first tube. Noise stay or go?


MY initial suspect would be a noisy plate resistor on the first stage tube.

[mhraposo]

enzo, thank you for a prompt reply.

No imput jack and the noise continues.
Neither channel controls affect the noise. Turning down both volumes and the noise is still there, in the same level.

I've removed either the first tube, either the PI and I couldn't get noise.

Would a noisy plate resistor generate noise, even if both volumes are down to zero?

[Enzo]

I am sorry, I distracted myself. I said you determined the volume controls were before the noise then I provided suggestion based on it coming from before them. Duh.

A noisy plate resistor in the active circuits yes, not likey before th zeroed volumes though.

I am assuming pulling the first tube had no effect on the noise? Or pulling it DID kill the noise, it is unclear to me from your last post. I guess even with the volumes down, the first stage COULD put noise on the B+. If pulling the PI tube kills the noise then its circuit becomes suspect. The first triode of that middle tube is a buffer with a 100k plate resistor, then the PI stage itself has a couple 56k. ANy of those could be noisy.

Carefull short across the 1meh grid resistor of the PI stage. That is the same as grounding the grid. Kill the noise?

ANother approach is grounding the signal path. If I ground the grid of the tube - where the two volume controls meet - with a grounded clip wire, I will find out if the noise is coming in that grid. I already suggested shorting the grid resistor of the PI, but another way is to use a grounded clip wire, but at the free end, connect a 600v cap. I use a 0.047uf because I have a bunch next to me, value is not critical. The common 0.02uf is fine. If I now touch that cap to a circuit node with the other end of the cap grounded, now I am shunting any signal - including noise - to ground, the cap allows me to do this where DC is present.

So with these techniques you ought to be able to isolate the noise to one side or th other of the tube, and get closer to the source.

[mhraposo]

A noisy plate resistor in the active circuits yes, not likey before th zeroed volumes though.

I am assuming pulling the first tube had no effect on the noise? Or pulling it DID kill the noise, it is unclear to me from your last post. I guess even with the volumes down, the first stage COULD put noise on the B+. If pulling the PI tube kills the noise then its circuit becomes suspect. The first triode of that middle tube is a buffer with a 100k plate resistor, then the PI stage itself has a couple 56k. ANy of those could be noisy.

I've tested again. So, first tube our and it is confirmed that noise still there.
I've pulled the PI and the noise disappeared (so far).

Carefull short across the 1meh grid resistor of the PI stage. That is the same as grounding the grid. Kill the noise?

Haven't tested this yet. What are the consequences? if the noise dies, the problem is in the grid resistor?

Tomorrow I'm going to verify the connections after the first preamp for each channel, looking for cold solder joints and bad connections.

I will keep you posted. At this point, you still think that is a place resistor?

Thank you for the support

[Enzo]

I don't suspect the resistor itself, though that is possible. If shorting it kills the noise it just tells me the noise was at that grid or before. We are still trying to LOCATE the noise at the moment.

[mhraposo]

Hello Enzo,

today I've reviewed solder joints from the volume pots to the PI. The noise is still there.

As you would expect shortening the 1M kills the noise.
Since removing noise remains if the 12AY7 does not afect the noise, I'm assuming that is after the first pre-amp stage.

Any suggestion?

P.S. I've moved to th 5E3 forum since it would be more adequate.

[mhraposo]

Hello,

Updating the topic with relevant information for future reference.

After isolating the problem between the first preamp stage and the PI, I’ve changed again the PI tube. No noises at all.
At beginning, it sounded that the problem was solved, making me think about a second tube that was also not good.
However, when I inserted the tube shield and start playing, the problem appeared again.

That lead me to think of a bad socket, a bad socket solder joint or event the socket pins.

Took three actions:

I’ve reviewed and re-solded all the solder joints in the tube socket.
I’ve re-tensioned the tube socket
I’ve applied Deoxit in socket.

So far, the amp has been working for already 6 hours without any sound/problem.
It seems solved. I will update the info if there is a change

[mhraposo]

Unfortunately, the noise returned. Now, next phase is to change resistors and finally coupling caps.

Any suggestions?