Marshall JCM900 SL-X 50Watt high gain master volume 2500 Muffled sound

[diydidi]

Hi
Got this amp for repair. Complaint is a muffled sound. I checked the frequency response and found that the Power amp section , feeding a signal into the FX return, gives a -3db drop at 6khz already. Isn't that a bit early?
I tried swapping the phase splitter tube, no change. I can see the same response when probing the grid's of the 5881's with my scope on x10.
Is this normal?.
I'm sure the preamp is fine as the fx send doesn't show this problem and only rolls off at about 18khz.
Any ideas anyone?

[Jazz P Bass]

It may help to know 'where' you are seeing the 6khz rolloff.

Is it happening at the output tube grids?
If so, try temporarily lifting the feedback circuit to see if it goes away.

If then the grids are good but the OT output is rolling off, you may be looking at a new OT.

[g1]

I wouldn't get to involved with that 6khz roll off until you verify it is the source of the complaint.
Have you tried another preamp connected to this power amp?
I understand it looks like the culprit on the scope, but you should verify with your ears using another preamp.