crackling when playing with bass guitar

[Leevi]

I have recently built an SE bass amp which is based on Fender champ
but where 6L6 is used as a power tube and there is SS rectifier. I'm wondering a crackling noise which comes
randomly when playing with bass guitar. The amp is very simple and I have checked
all the possible contact problems. Also tubes have been changed.
/Leevi

[sluckey]

Separate the head from the speaker by 6 feet or more. Still crackle?

[Leevi]

The amp is a head and I'm using longer than 6 ft cable. What is your point
regarding the distance?
/Leevi

[sluckey]

vibration maybe shaking a component.

[LooseChange]

That was my first thought. You can tap the chassis with a rubber mallet and see if you get any noises.
Do you get the noises playing a guitar on the low strings played hard?
I'm wondering if you are hitting the grid too hard. You can add a grid resistor to the preamp gain stages and see if that helps.

[punkykatt]

Im sure you checked the guitar cable and the bass guitar too?

[mrm0to]

Can I nominate the speaker? This week I saw a Gibson bass amp that had coil rub which was only apparent if you played a bass, sounded fine w/ a guitar.

[Leevi]

I'm wondering if you are hitting the grid too hard.

That was my first thought. I'm using 1.5k grid resistor on power tube which probably not enough.
I forgot mention that I have an EQ bypass in the amp and the problem exists more often when
the EQ is bypassed.
/Leevi

[LooseChange]

Put a grid stop on the second preamp stage... 10k-33k. Go big to see if it does anything, then try something smaller to find the smallest, workable value.

I'll assume you tested speaker, jacks and cable.

[Leevi]

Put a grid stop on the second preamp stage... 10k-33k

I'll try this next, cable, speaker, tubes, outputs/inputs have been tested.
/Leevi

[Leevi]

Actually there is already a grid stop through the vol pot:

http://ampwares.com/schematics/champ_aa764.pdf

The problem exist on a lower volume as well.

The NFB is not used in my amp.

Should I remove the 47 Ohm resistor from the cathode of the 2nd stage?

/Leevi

[LooseChange]

Thats not a grid stop and the power tube could use one too.
The grid stop goees diectly on the tube socket soldered directly on the pin.
I don't think removing the 47r will make much difference but you should try the NFB.

[Leevi]

Unfortunately the grid stopper didn't help but I made a new observation:

When I repair the amp it's up side down and the problem never comes up.
If the amp is in its normal position it starts to crackle. This happen
with a Tung Sol's 12AX7. With a new JJ I don't get it crackle any more.
However the guy who is using the amp mentioned that it has also happened with the JJ.

I believe that the Tung Sol is now faulty but could it possible that the amp has damaged the tube
with wrong settings especially when playing with a bass guitar?
That could be the clarification that the same has happened with a JJ as well.

/Leevi

[Leevi]

I think the problem is solved now. It had nothing to do with the tubes. The
reason was the push/pull volume pot (made by Alpha). I'm not sure which one caused the problem
the switch or the pot part but anyway I succeeded to localize the problem with a chopstick.
/Leevi

[bluesbear]

Good for you! That's the trickiest kind of problem to track down.
Dave