Experience with Mesa Studio Caliber noise?

[Randall]

I'm looking at a Studio Caliber with a noisy preamp. The Rhythm channel has a pretty noticeable noise floor, it's worse than the Lead channel with all knobs matched. I tried swapping the preamp tube, no difference there. Most of the other tubes are JJ's, so they must be fairly new, the amp looks like it has seen very little use. It still has the hang tags.

My question is, before I set out to fix something that isn't there, does anyone have any insight as to whether these amps are just inherently hissy? It doesn't sound like power supply to me, but just white noise type sounds.

[g1]

Is this the correct schematic? Did you try swapping both V1 and V4?
Generally I expect a lead channel to be noisier than a rhythm channel.

[Randall]

Yes, that is the correct schematic. Sorry, I should have posted it. Swapping V4 makes no difference. The same wash of noise is in the Lead channel, it's just not as loud for some reason. I can certainly make them match by turning up the gain. It just seems like a noisy amp to me, but I have no other experience with this one.

I wondered about perhaps a preamp tube wasn't biasing correctly, but all cathodes have 1.1v on them, plates look good as well.

[mozwell]

carbon plate resistors ?
Happened to me once, changed them to metal film & noise was gone...

[Randall]

Hmmm. How to justify this with this amp? An hour to try it, and what if it doesn't work? I'd like some more consensus on this one.

[Jazz P Bass]

Beware of the resistors that are smashed down on separate traces.

I have had to lift all of them off a tad.

SGM was right on this one.

[Randall]

Jazz, what does this mean? What is to beware, and what was SGM right about?

[Justin Thomas]

There were some resistors that had been crammed in so tight that the bodies were contacting the PCB and rubbing through traces. Creating all kinds of issues. And Mesa likes to stuff too much stuff into a too-small container on some models; not sure how this particular one is.

So, for all his bluster and bombast and BS, SGM got one problem right - resistors being crammed down onto the PCB in Mesas, making issues. Some just prefer to not give him credit in these parts.

Lifting the resistor bodies off the board seems to fix the problems, replacing them with longer leaded resistors if needed.

Justin

[Enzo]

Jazz just gave him his props. it is so surprising when it happens.

[Randall]

Are we talking raising all the resistors in the preamp section that are touching?

[The Dude]

Sorry, I can't remember, Randall............ Do you have a scope? I agree. Raising all the resistors is not practical. You need to try to isolate the noise to a particular stage, or you could be spinning wheels for a while.

[Randall]

OK, problem solved, partly. There was a 20nF cap tacked in from the rhythm treble pot wiper lug to the volume pot wiper lug. Wrong value, wrong lug. I put in the 120pF and 250pF in series from the treble wiper to ground as per the schematic, and now both channels are about the same. The Lead channel volume pot had it's shield not connected as well. It's still a noisy amp, I get noise at the gain controls, master controls and Master Output control. I think that is just how this amp is.