Blue Angel Inquiry, Would Really Appreciate Help!

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Hey, I have a Blue Angel that I'm having some issues with, I've tried a few external preamps into the FX return, and something really weird happens. The sound with the external preamp is extremely muffled, very badly dark, and the controls on the preamp don't seem to do what they're supposed to; then very weird abrupt jumps in volume and brightness can occur, it almost sounds like feedback, sometimes it doesn't stop once it's started until I put it into standby. I know the preamps I've tried work well into the FX returns of other amps, so it's not their fault. I know the Blue Angel's loop is parallel, could that be causing the issue? When I'm going straight in the Blue Angel's front inputs all is well. Everything is fine with a cable straight from the send into the return also. I contacted Mesa and they recommended to see a tech, is there anything I can try myself? Maybe converting the FX loop from parallel to serial?
 
Are you running the loop 100% wet? Could be that the blend pot for the effects loop is oxidized and needs cleaning. Also, did you make sure that the effects loop jacks are clean? That can also cause some strange problems.
 
I tried anywhere between 0 - 100% on the mix knob, and with my external preamps it didn't seem to do much, really as if nothing was working properly. With a cable straight from the send to the return it just seemed to adjust the level like it should.

I can check the jacks and see if it still does this after a re-tube today, but until then, I'd really like some insight if anyone has any!

I looked at the Blue Angel schematic a bit and it doesn't seem like the Mojo Module would be the culprit, but I've heard of the FX loop being parallel causing problems, even at 100% wet.
 
I know that one of the popular mods for the DC-5 I have is the effects loop to serial. My amp has had that done. For many purposes having a parallel loop just doesn't work well, while it works better for others.

Most of my amps by other makers are serial loops, though many are switchable.
 
You would have to make sure that the impedance of the preamp is matched reasonably well to the FX return. Likely the combination of preamp/poweramp is unstable. It could be oscillating well out of human hearing.

You might try a buffer between the preamp out and the power amp in.

Also, make sure that the Blue Angel's volume control is set to zero so that you are not driving into a preamp signal.

I've had more than one guitar amp that did not take an external preamp signal well.
 

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