RM50c is making squeeling very hi pitch feedback noise

[SuperV8]

Hello

I just bought a RM-50 Combo and took it out to an open mic night-- when I turned it up to stage volume which was Master volume at about 11 O'clock and the masters of the modules at about 2 o'clock, the amp was picking up radio broadcasts in the background and there was a very hi pitched feedback type squeel happening both with a Strat and a Prs?

tubes were seated and seemed to be working fine and there was no visible damage on the amp anywhere?

Anybody out there know of what might be the cause of this or any type of testing I can do to trace the cause?

Thank You

S8V

 

[drewiv]

I would change out the pre amp tubes first and see what happens.

 

[Mike361]

Not the first time I have heard of amps picking up radio, but I don't know what the cure is. Tubes are always the first to check though.

 

[kc2eeb]

Are the preamp tube shields in place and/or missing? Most important one is the one by the input jack. You can solder a 5-10 pf cap across the end of the guitar cable that plugs into the amp. Rotate the amp position and/or move the amp to a different spot on the stage. Coil the guitar cable 3-5 turns about
3 inches in diameter at the input to the amp. Do the same to the ac line cord.
Hold in place with quick ties.
These are all common RF blocking techniques.

 

[SuperV8]

Yes the preamp all 3 shields are there, I pulled the preamp tubes out today all were 12ax7 Ruby tubes-- they all looked fine, I tapped them with a chopstick and they did make any noise through the amp?

I tried a different cable and that seemed to help a little bit?

I have tried with several guitars and they were all making the same noise?

I don't think it's the modules because they don't make noise in the Rm4?

I'm thinking the club I was in has poor wiring and the amp wasn't getting along with that situation, although the other guitarist rig was not having any problems?

It's probably a combination of a few problems--I'll replace the tubes and cables and try the Cap on the input?

Thanks for all the advice please keep it coming!

Rock On

 

[Jaded Faith]

This was discussed a few times in the last six years here:

http://mtsforum.grailtone.com/viewtopic.php?t=10117&highlight=radio

http://mtsforum.grailtone.com/viewtopic.php?t=5102&highlight=radio

It's not an MTS thing, I see it occasionally in many a customer's amp. Any of the advice given here is good.

 

[kc2eeb]

2 more thoughts.....
If your RM50 has the on board reverb, the reverb can feedback a high pitch
ringing. You can fit the reverb pan assembly into a Fender type bag and/or pad it to keep it from feeding back.
If you were using a boost or OD pedal, etc. any pedal that uses diode clipping
is also a great demodulator for RF signals. Try taking the pedal out of the signal chain and see what happens.