My guitar cable is picking up radio stations: Solution?

[Platypus9]

I have a few average-quality cables, and they all do the same thing.
Strangely, my solid-state fender amp doesn't have this issue, but my Morgan tube amp does, so the circuitry of the amp is somehow involved. When I unplug the instrument cable, no problem.

Is there something wrong with the amp? Will a better-shielded or coiled guitar cable help?
thanks

[resistorman]

Some more information will be helpful. What instruments are you plugging into the amp ? Do all instruments react the same way? Does it happen if the amps are plugged in to the same power source?
Etc.
Many amps have switching input jacks that short the hot terminal to ground when unplugged, so the lack of noise is understandable.

[Platypus9]

I plugged both amps into the same wall socket, to control for that. I get the exact same result regardless of whether the instrument cable is connected to an instrument, or not. I tried a couple of different guitars as well, and there is no difference between the radio signal.

[ef37a]

It is unusual for valve guitar amplifiers to suffer from Radio Frequency Interference, it is usually worse for solid state jobbys.

I suspect the boutique Morgan has, for some cork sniffing, "rocky-rolly" reason, omitted the grid stopper resistor on the input triode*.
This resistor, along with the input capacitance of the valve forms a vital low pass filter and RFI stop.

The Fender is immune I would guess because it has to be built to sell world wide and has to pass EU EMG regulations.

I suspect there is little you can do except get the amp fixed but you could try wrapping the guitar cable around a ferrite ring.

*Just becuase they call it a "tooob" amp does not mean there is not a transistor or IC in it!

Dave.

[Hugh Robjohns]

Often its the guitar itself that acts like an aerial, but your description suggests it happens whether or not there's a guitar plugged to the end of the cable, and so it's the cable itself that is picking up radio signals.

That, in itself is not entirely surprising. But the problem is that the amp is demodulating those RF signals and making them audible, when it shouldn't be... So, as you surmised in your original post, it is a defective amp.

There are lots of possible reasons and solutions. The most obvious and important is that the amp should have a good, solid safety earth connection through its mains plug. So check that first.

If that is good, then the problem is within the circuitry and could be anything from a 'dry' (defective) solder joint in the circuitry to a dodgy valve, to an expired capacitor... It will need the services of a decent tech to sort it out.

If a fault can't be found in the circuitry, then it might be necessary to add some ferrite beads and/or RF shunt capacitors at the input connections to act as an RF filter, stopping the RF from getting into the circuitry in the first place -- again work for a decent tech.

H

[Wonks]

In addition, do you know if you are quite near to any radio mast? The nearer you are, the stronger the signal and the more likelihood of radio signals being picked up (inverse square law). The Morgan amp factory might be quite far away from a mast, so they didn't notice any problems during design. It may well be worth contacting them, as they may be able to suggest a suitable fix.

Unless you know how well your guitar cables are made, the problem may be partly due to a poor braid covering on your cable. When you say 'average' , do you mean a decent quality cable, but nothing esoteric or stupidly priced, such as one made by Whirlwind or Fender etc., or maybe one made up with Neutrik or Switchcraft jacks and Van Damme or Klotz etc. cable?

It's normally only the really cheap cables that have braid shield issues with minimum coppers strands that part easily when the cable's bent, so if you have got a reasonable cable, then it shouldn't really be the cause.

What model of Morgan is it? Is there a schematic somewhere on line?

[Folderol]

The others have pretty much covered it, but have you got a spare plug (with no cable on it)? If so does it still pick up when that is plugged in? It opens the shorting link so may give an idea as to whether its the amp or the cable. Even something like a 3.5mm to 1/4 headphone adapter plug will do! It's not conclusive, but it you get pickup like that then you almost certainly have an amp problem.

[ef37a]

Wonks, I too would love to see a schematic but don't rate my chances of finding one.

The fact is, you don't normally need to do anything special to a valve amplifier to stop RFI. Triodes of course have little gain past 100kHz and the standard grid stopper at the front end, very often 68k, in conjuction with the valve's Miller capacitance is all you need as a rule to stop RF.
Of course, Morgan might have done something bloody daft and used a pentode in the front end! Or possible an IC which makes a lot of sense in terms of noise, microphony, reliablity and cost but you MUST use some serious input filtering.

It was the coming of the Silicon Planar Transistor that exposed gear to radio breakthrough. I know, I was there. Got the grey hair!

Dave.

[ef37a]

The others have pretty much covered it, but have you got a spare plug (with no cable on it)? If so does it still pick up when that is plugged in? It opens the shorting link so may give an idea as to whether its the amp or the cable. Even something like a 3.5mm to 1/4 headphone adapter plug will do! It's not conclusive, but it you get pickup like that then you almost certainly have an amp problem.

Sorry Will my friend but RFI is ALWAYS an amp problem!

No matter how the RF is being coupled into the amp, nor how crap the cable, the RF should not get through to be demodulated and thus audible.

The fact that the Fender is free of the problem tells me that the RF field is not that high because I would expect a solid state amp to show the issue before a properly designed valve device.

Dave.

[Platypus9]

Great responses!
Update: I've discovered that the volume of the radio signal stays constant, even if the amp is set to 0 or 10! I replaced the mains power cable, same result.

My cables include Fender and Planet waves, amongst others, so I'm ruling that out. I believe I'm 5 miles (7km) from the nearest radio tower. I am on the 2nd floor.
The model is Morgan PR12. No schematics are available, as far as I can tell. A brief response from Joe Morgan: "Might just be a bad ground in the house." Not especially helpful, but worth investigating.

I just bought this amp on Reverb, it sounds amazing. I guess I'll see if the seller will refund the repair costs.

[ef37a]

If the RFI level is not influenced by the volume control that suggests it is not getting in via the front end.

The amp is a head right? There is a possiblity RF is getting in via the speaker cable and you could try ferrites on that. Even try a screened guitar cable. No! It won't catch fire even if you bash the beans out of it.

It often helps to identify the station (they might even offer technical help!) Should it be an MF station the last thing you want is a good earth! But of course DO NOT mess with mains safety earths.

Certainly it would be good to get the amp checked and the seller to stump up for any work. My bet is this is a design fault and IMHO not testing for RFI is inexcusable. Could be a dry joint that is acting as a diode detector but in 50+ years of RFI faults I have never seen that and I have fixed a heck of a lot of RFI amp problems. (Northampton is about the same step from Rugby and Daventry both of which towns had some seriously powerful transmitters)

Dave.

[Hugh Robjohns]

If the RFI level is not influenced by the volume control that suggests it is not getting in via the front end.

Er... But it goes away if the input lead is removed...

So if it's not affected by the volume control, it is being demodulated somewhere downstream of that... But it could still be getting in through the front end.

The amp is a head right?

There's a 1x12 combo version...

[Folderol]

If it's not affected by the volume control, yet goes away when the cable is unplugged I'm inclined to wonder if there is a bad earth in the vicinity of that control.

{he said, remembering copper oxide rectifiers}

[ef37a]

Yes Hugh and Will but RFI problems can be buggers to sort out and DON'T play by the rules!

Dave.

[Wonks]

The PR12 is based on a blackface Fender Princeton with a beefed up power supply, if that helps anyone.

[Hugh Robjohns]

If it's not affected by the volume control, yet goes away when the cable is unplugged I'm inclined to wonder if there is a bad earth in the vicinity of that control

Yes, I'd be thinking along similar lines... But Dave is right... Tracking this kind of fault down is tricky.

H

[Platypus9]

This is the Combo 1x12 version. I tried the amp from an outlet on the first floor, and the RFI was barely audible. I then ran a heavy-gauge extension cord to the 2nd floor, from the same outlet on the first floor. Result: intense RFI. Given that I have a half-dozen amps, and this is the only one with the problem, I'll be taking this one to a professional.

[ef37a]

The PR12 is based on a blackface Fender Princeton with a beefed up power supply, if that helps anyone.

Thanks Wonks. If they have followed (ripped?) that circuit accurately I see nothing to suggest poor RFI rejection? The input stage has the common 68k stopper.

I see that quite unusually for a guitar amp they use a concertina phase inverter and that means the triode has almost 100% feedback and thus a very wide bandwidth. There is a 2k7 feeding NFB from the speaker output and if I had it on the bench for this problem my first step would be 100nF across the OPT secondary.

Nothing to do with RFI matters but...fixed biased 6V6s? Unusual again and they will never fail!

Dave.

[ef37a]

This is the Combo 1x12 version. I tried the amp from an outlet on the first floor, and the RFI was barely audible. I then ran a heavy-gauge extension cord to the 2nd floor, from the same outlet on the first floor. Result: intense RFI. Given that I have a half-dozen amps, and this is the only one with the problem, I'll be taking this one to a professional.

That is a big clue, RFI can often get in via a mains cable. I wonder if the amp builder has omitted the 100nF cap across the mains traff primary?

Dave.

[Wonks]

This is the Combo 1x12 version. I tried the amp from an outlet on the first floor, and the RFI was barely audible. I then ran a heavy-gauge extension cord to the 2nd floor, from the same outlet on the first floor. Result: intense RFI. Given that I have a half-dozen amps, and this is the only one with the problem, I'll be taking this one to a professional.

One small possibility is that you mat be simply moving the amp closer to a localised source of RFI. Anything electronic (probably faulty if it is the case) on the 2nd floor (or the attic) that may be retransmitting radio signals? Baby alarm maybe? Something Bluetooth?

Just turning off all possible sources (including computers) and then testing again might give a culprit if the radio signal vanishes.

Or it may stay the same as before, but it costs nothing to check.

[Hugh Robjohns]

I tried the amp from an outlet on the first floor, and the RFI was barely audible. I then ran a heavy-gauge extension cord to the 2nd floor, from the same outlet on the first floor. Result: intense RFI.

I think both of those tests show very clearly that the amp is heavily reliant on extremely good earthing to control RF interference. So in situations where there is little RFI around, or where it has a really solid earth, it will probably perform perfectly.

However, whether its susceptibility to RFI is a fault on this particular unit, or a design flaw I can't say. The attentions of a good tech should sort it out relatively easily though, I'd have thought.

H

[ef37a]

I tried the amp from an outlet on the first floor, and the RFI was barely audible. I then ran a heavy-gauge extension cord to the 2nd floor, from the same outlet on the first floor. Result: intense RFI.

I think both of those tests show very clearly that the amp is heavily reliant on extremely good earthing to control RF interference. So in situations where there is little RFI around, or where it has a really solid earth, it will probably perform perfectly.

However, whether its susceptibility to RFI is a fault on this particular unit, or a design flaw I can't say. The attentions of a good tech should sort it out relatively easily though, I'd have thought.

H

I am sorry Hugh, I don't think this has anything to do with earthing*. IMHO the extended mains lead is acting as a better aerial and possibly a resonant stub and maximising the RF signal inside the chassis.

*Maybe you are just too young to remember but fitting a good earth to a MF radio always improved the signal!

Anyway, we can debate this problem ad n but the ONLY way to fix RFI issues is to get hands on. OP must take the amp to a tech.

Dave.

[Platypus9]

This is the Combo 1x12 version. I tried the amp from an outlet on the first floor, and the RFI was barely audible. I then ran a heavy-gauge extension cord to the 2nd floor, from the same outlet on the first floor. Result: intense RFI. Given that I have a half-dozen amps, and this is the only one with the problem, I'll be taking this one to a professional.

One small possibility is that you mat be simply moving the amp closer to a localised source of RFI. Anything electronic (probably faulty if it is the case) on the 2nd floor (or the attic) that may be retransmitting radio signals? Baby alarm maybe? Something Bluetooth?

Just turning off all possible sources (including computers) and then testing again might give a culprit if the radio signal vanishes.

Or it may stay the same as before, but it costs nothing to check.

Good idea. I turned everything off, but not surprisingly, it didn't help.

[Platypus9]

Discovery: When I turn the reverb knob to zero, the RFI nearly disappears, and the volume of the RFI is directly proportional to the reverb knob position... I thought the reverb tank was acting as an antenna, but unplugging the reverb tank had no effect on the RFI.

[Hugh Robjohns]

Well, at least you now know which circuit stage is faulty... That will help the technician enormously!

H