Radio Frequency in Amp when Guitar Pot Volume is at minimum

[WesPalladini]

Guitar: ESP-MII, 2 Humbuckers SD JB and 59, 1 Volume and Three Way Switch (Wiring Diagram)

Guitar is going straight into a Marshall 410C, and there is a footswitch plugged into the back. No other pedals. I am using some nice cables, 1 monster and 1 high end planet waves. Mind you they are quite long.

When I role the volume all the way down on the guitar I get the radio coming through the amp, however if I role the volume it slightly up (10%) it will go away. I know there are a number fixes to this problem. I don't think I should have to shield the cavities because I think ESP uses a special shielding paint in them. I have heard of some people adding a cap somewhere in the wiring to get rid of the sound, any help?

[Gtr_tech]

If this happens with no pedals in front of the amp input, its likely that the input gnd is isolated from chassis gnd. IIRC, a .01u cap from circuit gnd to chassis gnd takes care of this problem. If its only with the pedals in line, one of the pedals is acting as an AM detector, thus the radio station coming thru.

[WesPalladini]

If this happens with no pedals in front of the amp input, its likely that the input gnd is isolated from chassis gnd. IIRC, a .01u cap from circuit gnd to chassis gnd takes care of this problem. If its only with the pedals in line, one of the pedals is acting as an AM detector, thus the radio station coming thru.

There is no cap I can put in the pickup wiring that can deal with this instead?

[52 Bill]

There is no cap I can put in the pickup wiring that can deal with this instead?

Does the amp do this with any other guitar? How about with the cable end shorted and no guitar?

[Gtr_tech]

Putting a cap in the guitar wiring won't do anything but muffle up the output....

[WesPalladini]

Does the amp do this with any other guitar? How about with the cable end shorted and no guitar?

I tested my buddies LP through it, still made the radio noise so I guess it is the amp.

[52 Bill]

...I guess it is the amp.

Did this just start happening? Have you changed locations? Are any of the input jacks loose? Try swapping around the preamp tubes to see if it might be tube related.

[WesPalladini]

Did this just start happening? Have you changed locations? Are any of the input jacks loose? Try swapping around the preamp tubes to see if it might be tube related.

I have only used the amp at my buddies house. But I will try the amp in different rooms in my house. I don't think any input jacks are loose. I will try swapping preamp tubes

On another note if I have to add a cap I want to do it safely. I know to only have one hand in the amp but I also want to safely drain the high voltage caps. I have been watching "Tube amp repair and overhaul DVD" by Gerald Weber. To do this he says just unplug the amp, Connect an aligator clip to the chasis and to pin 1 of any preamp tube and turn the amp on (still unplugged) and out of standby, wait approx 20-30 seconds. Is this addequite? He did it on a fender, would it be the same for a marshall?

[52 Bill]

The question about location was related more to where the radio transmitter was located. I've seen some cases where the transmitter was so close that there was no way to get rid of the problem.

Most amps will discharge with the pin 1 trick, but in general your amp will discharge to safe levels in less than a minute if you turn it off while leaving the standby switch in the play position. And of course pull the plug out of the wall.

If the amp never did this before, I'd start by looking into what may have changed to cause the problem rather than looking to modify it to hide the problem.

Just my 2 cents.

[WesPalladini]

If the amp never did this before, I'd start by looking into what may have changed to cause the problem rather than looking to modify it to hide the problem.

Just my 2 cents.

Hard to say if it did it before or not because it is my best friends amp. I have been borrowing it for the past few months. He has been busy with school for the past couple years so he doesn't remember if he ever had radio noises.

[J M Fahey]

Just guessing:
maybe the amp isn't broken, but somehow you have a high RF field at home.
A 100pF cap from the first grid to the corresponding cathode, straight at the tube socket , should take care of any RF there and will be unnoticeable.

[WesPalladini]

Just guessing:
maybe the amp isn't broken, but somehow you have a high RF field at home.
A 100pF cap from the first grid to the corresponding cathode, straight at the tube socket , should take care of any RF there and will be unnoticeable.

So the cap goes from Pin 7 to Pin 8 on the first preamp tube? Also what do you think of 01u cap from circuit gnd to chassis gnd as Gtr_tech suggested?

[WesPalladini]

I think I fixed the problem.

When I plug my guitar straight into the amp using my planet waves cable Planet Waves : Custom Series Instrument Cable (Sheilded end going into the amp) I get FM Radio.

When I use my buddies monster cable or the cable I made myself out of mogami cable, 1 Neutrick and 1 Amphenol end it is silent.

The Monster and Planet waves are both around 15FT and the cable I made myself is around 10FT.

Looks like I will be selling the Planet Waves on kijiji

[J M Fahey]

1) yes, from input grid pin to the cathode just by it, straight at the socket.
2) try gtrtech's suggestion, it should help.
3) now you know that good homemade is better than hyped commercial
Good luck.

[g1]

Out of curiosity, did you try with the "shielded" end to the guitar rather than the amp? Any difference?
Could the cable have developed a fault? Sounds like maybe the extra shield is not connected but the ground still is. Maybe you could get it exchanged for a new one, as reselling it may be passing on the problem to someone else.