Mesa MK II excessive noise.

[fredcapo]

After doing a badly needed de-ox of pots and sockets, the formerly intermittent and scratchy Mesa MK II seemingly has plenty of power. However there seems to be a lot of what i'll call "white noise" or similar, even when the input jack is unplugged. The noise appears to be cumulative, getting louder as I turn the pots CW. Similarly, with all pots maxed CW, there is more noise when in the "lead" mode, than in the "rhythm" mode. Also switching V1 between the 12AX7 and the FET device makes only a nominal difference.

This is the first Mesa or similar high gain amp I have dug into. I did not swap any tubes or split the preamp and power amp...yet.

I am wondering how much noise is normal with the high gain type of amps?

How do you typically identify and quantify the noise with a scope? Thanks.

[Mick Bailey]

I'd expect the lead channel to have more noise than the rhythm. If you can get rid of the noise by turning the controls down, it's coming from the preamp. Without hearing how loud it is all I can say is in a high gain amp a certain amount of noise is inevitable. If it's excessive it could point to a noisy plate resistor. You need to balance the perception of noise level with how loud the amp actually plays in relation to that noise. Mesa amps can get out of hand at higher gain levels and the handbooks stress keeping controls to a sensible level.

Sometimes it's useful to evaluate the noise levels firstly with a clean setting and take it from there. Set the master volume to max and the channel gain right down. Other controls at mid-position and reverb at zero. Now play the amp and turn the gain up to get a reasonably loud clean signal. Any white noise? Should be fairly minimal and in proportion to the volume level.

The thing with high gain is that it amplifies all the inherent electrical noise, and master volume amps allow you to push up the preamp noise at the expense of maintaining a good S/N ratio.

If I'm scoping for noise I look at what noise is going into the grid and what's coming off the plate for each preamp stage. You can also ground the grid if you have a suspect stage. If there's still noise coming off the plate resistor, then it's generally a faulty resistor or bad tube, though sometimes a cathode resistor can play up.

[fredcapo]

Mick, Thanks. I 'm going to consider it normal for now, and let the owner play it and decide if it needs more tweaking, though there is another minor problem that may be related...

When switching amp down to standby mode, I get a hf squeal through the speaker that lasts about one second.

I don't know where to start with that one...power supply bypass?

[Alex R]

if you turn up all the controls on the gain channel it's gonna be REALLY noisy that's normal. Irregularities in the noise might point to a fault.

[pdf64]

'When switching amp down to standby mode, I get a hf squeal through the speaker that lasts about one second.'
Try tacking an additional (smallish eg 10uF) reservoir cap on the cold side of the standby switch, to see if that stops it.
Pete

[fredcapo]

That did it. Thanks.