Topic: Hum at max gain turns into squealing as gain is reduced on high gain channels. (Read 136 times)

CJE · « **on:** Time Format »

Got a bit of a hum on my MP2. Par for the course with high gain I suppose. The transformer has a hum as sooon as it powers up so I'm wondering if that's the issue? I did the noise mod which fixed a ticking sound (bad caps in the power section I assume?). Now to the present issue... On the high gain channels there is a loud hum, but as I turn down the gain, the hum turns into squeal. Not what I was expecting. What would cause this? Only seems to happen on the dynamic, ultimate and fat high gain channels. Any help is appreciated on brininging this amp back to life.

Harley Hexxe · « **Reply #1 on:** Time Format »

How old are the tubes in it? Have you checked to see if either of them are microphonic?

Even with new tubes these days, you can still get bad ones right out of the box.

To check if they are microphonic, pull the MP-2 out of it's rack, remove the top cover, and power it on with nothing plugged into the input. tap each tube gently with your fingernail and listen if you hear ringing.

rnolan · « **Reply #2 on:** Time Format »

Hey CJE, as Harley says, it could be a tube. The 2nd set of gains (OD2) in the MP-2 are only available on the higher gain voices (i.e. 6 through 10). It may be a bad R913 which is located under the 2nd tube. There a few post about this issue. If you measure from the test point (just above that area) to the chassis (see in attached MP-2-NoiseMod.pdf), the HV should be 190VDC or a little less (mine are around 187VDC). If it's more than 190VDC, that can cause noise issues etc. similar to what you are describing. The fix is replace R913.

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Author Topic: Hum at max gain turns into squealing as gain is reduced on high gain channels. (Read 136 times)

CJE

Hum at max gain turns into squealing as gain is reduced on high gain channels.

Harley Hexxe

Re: Hum at max gain turns into squealing as gain is reduced on high gain channels.

rnolan

Re: Hum at max gain turns into squealing as gain is reduced on high gain channels.