Music Man 2275-65 break-up

[lowell]

This amp has new tubes and is biased at the 500mv on cathodes. I do not have a signal generator (need to buy one) so it's difficult to locate the cause of the solid-state'ish distortion this thing has. I would think this amp would be quite clean being 65watts. Anyone know what in the preamp might be causing this distortion? The amp is fairly clean around 2-3 but then this fizzy sorta thing starts to come on if the volume is turned up past that. Both +-16v are good in the power supply for the transistors. I'm going to do a voltage check on all transistors. If there's something specific I should look for let me know... thanks!

[Amp Kat]

How do you know it's in the preamp ? If there is a power problem or the grid bias is off it could sound like a bad preamp but be a power problem. Without a signal source no matter what type you could take that signal before the cathode injection (grid is grounded) and send it to a known good power amp to verify the preamp is indeed the problem. Maybe go down the collector line of the transistors also. You are at a huge disadvantage without a generator in this situation.

[Twist]

Check for any DC voltage on the output pins of the OpAmps. The LM307's is pin 6 and the LM1458's are pins 1 and 7. Otherwise without looking at a sine wave on an O'scope your kinda shooting in the dark. Is this the chassis with the tube phase inverter?