5E3 Cascading Inputs MAD FEEDBACK! need a fix

[Chodrick1]

I've got a Mojotone 5E3 that I've been fiddeling around with and today I did the "Cascading Inputs Mod" from Rob Robinettes website. I love the gain tone, I love how I can swap back between the standard 5E3 and this drive monster. Unfortunatley I have run into a bit of a problem which seems to limit what I can do with it. The issue is that independent of where the gain volume is, turning the master volume or tone to 9 or higher, I begin go get terrible microphonic feedback, and cuts the guitar signal out completely. I'm reletively new to amps, so I'm not sure what may be causing this. I would love to get this amp working 100%, but this could be a real pain in the butt if this is just how the mod works.

Ive got the standard tube layout, 470 Ohm 5w screen resistors on my power tubes and 300k grid resistor out of the phase inverter. Hopefully this will help narrow down any issues.

Thanks ahead!

[52 Bill]

The issue is that independent of where the gain volume is, turning the master volume or tone to 9 or higher, I begin go get terrible microphonic feedback, and cuts the guitar signal out completely.

By master volume, I assume that you are referring to the bright channel volume control? Is what you are hearing acoustic feedback or oscillation? I'd bet oscillation.

Does it do this only when you are using the cascade channel or does it do this all of the time?

[Chodrick1]

It only happens on the cascaded input. When using the normal bright channel it acts normal, no issues with that. Just when plugged into the gain channel does it start to squeal. And even when the cascaded/gain volume is turned all the way down, I still get feedback when bright volume or tone goes to 9. I believe the only issue is with the first triode used for the mod. When I get the feedback, it kills the guitars signal and starts to squeal.

[52 Bill]

When you did the install, did you use shielded cable for the audio line from the output of the first preamp stage to the input jack of the bright channel? Can you move the wires around and change the oscillation?

[Chodrick1]

I didnt use any shielded cable, I assumed that was only to eliminate noise in the line. I suppose that could be one problem. I tampered with the wires for a little, but the feedback is quite overbearing.

[52 Bill]

I didnt use any shielded cable, I assumed that was only to eliminate noise in the line. I suppose that could be one problem. I tampered with the wires for a little, but the feedback is quite overbearing.

Do you have a dummy load resistor and an oscilloscope? Or maybe a speaker attenuator (power soak)?

[Chodrick1]

I have an attenuator that Im using. A Weber 50w Mini Mass