Musicians Roadhouse

I have an '87 Marshall SJ 2550 that is emitting static noises.
- I cannot hear it when the amp is turned up.
- The static noises do not increase with the volume, nor is it affected by the tone controls.
- It IS affected by the presence control! Gets brighter as I increase the P control.
- It stops when I switch the amp into SB mode, and begins as soon as I remove it from SB.
- If I let the amp cool and then turn the amp on, it doesn't do it right away. I have to leave it on for about 15-20 min for the static noises to resume.

Here's what I've done so far...
1. It's not in the preamp. The volume and tone controls have no affect on the noise. I did change the tubes, just in case... No affect.
2. It's not the power tubes. I'm using a proven pair of =C= EL34's that have less than 30 hours on them. Was doing the same thing with the previous set of tubes as well.
3. I've done the "poke around with a chopstick" thing, but found nothing.

Any ideas on what could cause this?

Does it do it if you unplug your guiar and cable from the amp?

Have you tried working backwards from the power amp. Remove the PI and fire it up, do you hear the noise? If not then replace the PI and remove the next tube in the line, do you hear the noise? Keep doing this until the noise appears, this will give you an indication of where to look in the circuit for the noisy part(s).

Also i just looked at the schematic and the FX loop is between V2A abd V2B. Place a jumper into the loop, is it still making the noise?

surfnorthwest wrote:Does it do it if you unplug your guiar and cable from the amp?

Yes.

Anitoli wrote:Have you tried working backwards from the power amp. Remove the PI and fire it up, do you hear the noise? If not then replace the PI and remove the next tube in the line, do you hear the noise? Keep doing this until the noise appears, this will give you an indication of where to look in the circuit for the noisy part(s).

Will do. I got busy today, so couldn't do anything with it.

Anitoli wrote:Also i just looked at the schematic and the FX loop is between V2A abd V2B. Place a jumper into the loop, is it still making the noise?

I'll try this. Don't know if this matter or not, but I didn't have anything in the effects loop.

Here's something... I've been running an old triple mica 5751 tube in the PI. A 5751 seems to get noticeably hotter than a 12AX7, so maybe it's this tube? I know preamp tubes aren't suppose to have an affect on the gain structure of a SJ, but they do seem to affect "feel" and attack. JJ preamp tubes seem to create what sounds like blocking distortion, so I don't use them. Sorry... Just thought I'd mention that.

If you suspect the 5751 swap it with a different tube and you will know if that is it. But a 5751 is a lower gain tube than a 12ax7. My testers only need a gain of 70 out of 120 to be in the good range while a 12ax7 needs 92 or more out of 120.

Anitoli wrote:If you suspect the 5751 swap it with a different tube and you will know if that is it. But a 5751 is a lower gain tube than a 12ax7. My testers only need a gain of 70 out of 120 to be in the good range while a 12ax7 needs 92 or more out of 120.

Thank you, I'll swap it out today.

Just curious... The gain ratings differ, but will a 5751 draw more amperes (compared to a 12AX7) under demand? It's definitely noticeably hotter to the touch, and burns brighter than a 12AX7.
The 5751 tests okay on my tester, but could it be failing under demand?
Would a failing PI tbe not emit the same sounds as a failing power tube?

MacFangus wrote:...could it be failing under demand? Would a failing PI tube not emit the same sounds as a failing power tube?

I don't know why I would have asked such a thing?