JTM45 makes awful noise when turned up

[keithv]

I've got a JTM45 that sounds fine with the volume at about 2.  But if you strum really hard it makes a really awful sounding distortion.  If you turn it up a little more and strum hard, it goes into self-oscillation.  Not a high pitched self-oscillation.  And not the kind of oscillation when you have the OT primaries mixed up.  If you crank the volume, it seems fine...until you make just the slightest noise on the guitar.  Then it goes into self-oscillation. 

[Ambugaton]

I can guarantee the first step would be to retrace each connection vs. schematic/layout and post pictures/readings.

[Ed_Chambley]

Is it both volumes?  Both inputs used separate?  Did is just start or new build?  Have you tried known good tubes?

[keithv]

Is it both volumes?  Both inputs used separate?  Did is just start or new build?  Have you tried known good tubes?

New build:  yes

Both inputs:  yes

Both inputs separately:  yes

Swap known working tubes:  yes

[keithv]

I can guarantee the first step would be to retrace each connection vs. schematic/layout and post pictures/readings.

I know it's a lot harder to find your own mistakes, but I've been through it with a fine tooth comb and everything looks good. 

[Willabe]

This is a great way to double check a new build;

http://el34world.com/Forum/index.php?topic=17701.0

[sluckey]

And not the kind of oscillation when you have the OT primaries mixed up.

Have you actually swapped the OT primary plate leads to positively rule that out? There are many kinds of symptoms associated with phase reversal of NFB, especially in an amp with a presence control. It's not always a squeal or howl.

[Ed_Chambley]

Is it a new Build?  Has it always had this problem or did it work fine for a while?

[Willabe]

Is it both volumes?  Both inputs used separate?  Did is just start or new build?  Have you tried known good tubes?

New build:  yes

Both inputs:  yes

Both inputs separately:  yes

Swap known working tubes:  yes

[keithv]

And not the kind of oscillation when you have the OT primaries mixed up.

Have you actually swapped the OT primary plate leads to positively rule that out? There are many kinds of symptoms associated with phase reversal of NFB, especially in an amp with a presence control. It's not always a squeal or howl.

Yes.  I just swapped them to see.  It makes the typical low frequency hum you get when you mix up the OT primaries.  Put them back and the problem persists.

[keithv]

Is it a new Build?  Has it always had this problem or did it work fine for a while?

This is a new build.  It's been like this since I first plugged it in.  Voltages all read OK.  Components are all where they should be and oriented correctly.  KT66s bias to 42mV.  Everything seems fine till you turn it up or strum really hard.  It sounds like when you have a bad rectifier tube that has intermittent arcing.  Only it's not arcing and the rectifier is good.

[Willabe]

This is a great way to double check a new build;

http://el34world.com/Forum/index.php?topic=17701.0