Ceriatone Forum

Hi can somebody help!! My 100w plexi is humming with out a guitar plugged in and it's driving me mad. It's worse with the strat plugged in (obviously) It gets louder when the pre amp gets turned up on the bright channel, but my amp guy says it could be anywhere. Any hints??

What are the specs? Is it my kit? Trannies? Pics of insides?

Hi Nik, Its your 100w superlead plexi. It was rebiased with new power tubes (winged C's) and replaced a pre amp tube. We had dirty/too hot wall power as I'm in Australia. There 's more pics if you need them. The kit was a standard kit apart from a ppmiv.

more pics

Just a thought...... will additional shielding between the head cab and the chasis help??

Are you sure you don't have a noisy tube in there...

Not really. The power tubes have been replaced..... how do I check em??

Well I would start with the preamp tubes you replaced... if you have any tubes lying around and you're sure they are OK, use that and replace one by one to see if your problem goes away.

That would be my first move...

Sorry mental blank. I'll try that. Can you check powertubes the same way?

Well I would not replace one by one... cause you need matched tubes.... and when you replace power tubes, you need to rebias as well.
So when you're done with preamp tubes and if problem still persist...then replace all power tubes (use the old ones if that's the only option).

Hi,

Yes, bias is definitely needed when you change power tubes.

If not sure, read the chupa manual.

The plexi100 doesnt have bias sockets, but the idea is same - read across the 1R resistor on power tubes.

You should also check your voltages, see if it's in line? www.ceriatone.com/voltage

As for the test, it shouldnt have much noise if set low. But if set high, you should simulate the shield (inside head cab) if testing the amp on the bench. Otherwise, you'd have noise.

This is after assuming your solder work are OK, and connections are correct.

Even if Oz can have 250-260V mains, at low volumes it shouldnt matter much, or for a short while.

Thanks!

nik

ok she's back from the amp tech after a having replaced a wire between V1 & V2. All good. I know every amplifier will have a backgroung hum when connected to a box and 1959 will have more hum than a JCM 800..... So how do you remove this from your plexi?? I might put an mp3... Still fkn great amp

Hi

Here's the hum recorded off my phone. The ppmiv is at 12 and the hum obviously gets louder as the master vol gets turned up. This is amp only without a guitar and the pre amp is about 1-2 oclock. Can it be trannies?? Its recorded of my phone. Any help appreciated.

Hi

Did the tech do the voltage check?

Reason i stress on it is, in Oz, sometimes wall can go to 260V.  The amp expects 240V.

2ndly, do you already have the shield inside your head cab?

A marshall thats out of cab, or inside a cab without shield, will tend to be noisy.

3rdly - when the tech biased the amp, are the power tubes still matched?

4th - the clip - whats that sound? is it hiss?

5th - when tech installed PPIMV - do make sure the wires go correctly. ie, each side of PI goes to correct pot lug, and correct power tube. Sounds simple, but we have diagnosed several PPIMV mods that had the mistake (hard to see since same color wire used, and they are twisted together).

6th - eliminate sharing of power socket with any other thing. And, if any lights, computers etc nearby, turn em off for a while. See if that changes things.

7th - Again, do check the voltages, and see if inline with given volt sheet.

Hope this helps, let me know..

Thanks!

nik

From that sound clip what I hear is not hum, but loads of hiss. Just to clarify, was the PPIMV installed after you got the amp from Nik? When did this noise problem start? I would check the wiring on the PPIMV for sure.

Once again, what I hear is hiss. Did the tech you had it to difinitively rule out any bad tube?

Hey guys,

The wall voltage checked at 248volts peak from that socket and the tech I used allowed for that and said he has biased it allowing for the extra wall voltage in Australia.

The head has shielding under the chassis and is in the cabinet when this was recorded. I have some stick on copper shielding...... will this assist in reducing the problem if I ad this to the base etc??

All power tubes are matched and were biased on replacement and the sound: the hiss: is the one I'm trying to get rid of.

The PPMIV was installed at the factory and was like that from the start. Which wires is to go to which tube from the PPMIV??

The tubes all passed the "ping" test ie no rattle when you give them a tap when in operation.
I've tried the amp in other areas with out lighting etc, fridges off....

Hey guys

Happy to report that Ross Giles has fixed her. Buzzing pre amp valve and capacitors issue. The new valve and capacitors has the amp sounding great. It has a massive amount of eq settings.... I am know running it bass, middle and treble around the 9-10 oclock positions, pre amp 3 oclock is good and rolling of the volume is cleans the volume up nicely. Top job Ross and thanks also to Nik for his help too (and others also).

Hi

Talked to Ross.

I think it was 2 68K resistors, not caps, that he replaced.

It was before he changed the valve, I think, so it could have been the 12AX7 all along?

Thanks!

nik