Radio Interference

[zaphod_phil]

I am using the fully-metal Jacks, and the cable shielding connection of the jack is already connected to the chassis ground simply by mounting it to the chassis.

And you're not getting hum problems using those jacks?? How else is the amp grounded to the chassis, apart from at the input jacks?

If your amp is an 18W Lite, adding grid blocking resistors to the two grids of the first 12AX7 should go a long way to fixing the problem. Typically the blocking resistors are 68k, as you'll see in many Fender and Marshall schematics. I usually prefer 10k these days, to allow a bot more guitar signal through, and also to minimise noise.

[Shottky]

I am using the fully-metal Jacks, and the cable shielding connection of the jack is already connected to the chassis ground simply by mounting it to the chassis.

And you're not getting hum problems using those jacks?? How else is the amp grounded to the chassis, apart from at the input jacks?

If your amp is an 18W Lite, adding grid blocking resistors to the two grids of the first 12AX7 should go a long way to fixing the problem. Typically the blocking resistors are 68k, as you'll see in many Fender and Marshall schematics. I usually prefer 10k these days, to allow a bot more guitar signal through, and also to minimise noise.

There is some hum that kicks in when I get to higher volumes.
I'm trying to find some jacks where the sleeve is incapable of being directly connected to the chassis simply by mounting it. Any recommendations on brand/model would be welcomed.


EDIT: Neutrik NYS212/2 seems like it will do the job. I will order some and see how those work out.

[hunterwh]

I had a serious issue with RFI for my 1974x. Marshall wouldn't write me back with any suggestion or advice at all and my local service center just shrugged. I took it in to a real amp repair shop and he shielded all the cables inside. WORKED PERFECTLY. NO MORE RFI. I can plug into either channel and it is as dead silent. The only time it picks up anything now is when I use the trem footswtich. I am having a mod done so I can turn it off/on via a push pull pot for one of the knobs so it still looks stock.

Are your jacks grounded through the chassis? I'm trying to determine if the metal-jacketed jacks I have are incapable of providing a solution.

I am not sure. I will ask my tech when I pick my amp-it is getting a Mercury Magnetic upgrade the push pull pot for the trem. Here is the website for my tech: http://www.leightonaudio.com/. Very, very frindly, trustworthy and without a doubt knows stuff. He even builds 18 watt replicas for musicians in the area.

[zaphod_phil]

I'm trying to find some jacks where the sleeve is incapable of being directly connected to the chassis simply by mounting it. Any recommendations on brand/model would be welcomed.
EDIT: Neutrik NYS212/2 seems like it will do the job. I will order some and see how those work out.

Cliff jacks are normally used in Marshall type amps, but you can use other alternatives as well.

[gambi]

I've got just that problem with an AC-30 build I've completed. In the basement it's pretty quiet. Upstairs it's almost unbearable. So, I'll try this fix. Question: is this more prone to happen in EL-84 amps?

[zaphod_phil]

Having EL84s makes no difference, as this problem happens in the preamp. Apart from that, there isn't much we can say about AC30s here.

[Chappy]

if it doesn't already have a 68k resistor, or two, on the inputs. I've also found that 10k is usually large enough to block radio station interference.

Zaphod

I`m curious as to why you use a 10K resistor vice the more common 68k. Aside from creating the low pass filter to block radio frequencies does the value of this resistor have an effect on something else?

More specificaly, does this resistor somehow contribute to the slight loss of treble when the guitars volume is backed off from 10?

Chappy

[zaphod_phil]

The 68k value comes from the traditional attenuating Hi/Lo jack arrangement which AFAIK originated with Fender amps, and IIRC also has some relationship with the DC resistance of a typical vintage Fender pickup. When you don't have an attenuating Lo input jack, then there's no need to stick to the 68k value.

Apart from reducing interference from nearby radio stations, the input grid block resistor has two other effects. It reduces some of the guitars' high end sparkle, and it can also pick up high frequency buzz, almost like it's a little antenna for buzz. The higher the value, the more pronounced these two effects are. Some amps have no grid block resistor on the input, but run the risk of picking up radio stations. So I usually go for the compromise value of 10k.

[labb]

Don't forget about using ferrite beads. They work too.

[katopan]

Has anyone got a definitive fix for GSM mobile interference? I'm expecting not because it's been raised in other threads, but thought it would be worth asking. I always turn my mobile off while on stage with my 18W. But jamming once at a friend's house I had it turned on. Of course the noise of it touching base came through my amp a few times over the afternoon, but his amp (Ceriatone JCM800 2204 clone) seemed immune to it. After I commented about it we did a test with guitars unplugged to rule out pickups and cable. Still did it on mine but not his. Some time later I was doing a repair on his amp and it was the standard layout, no ferrite beads, no IEC mains filter and no shielded cable anywhere. Now I'd be happy to put any of those into my amp if they worked, but in all the suggestions of this sort of thing no one has ever said they had a tested solution..... Only difference I could pick was his chassis is steel and mine aluminium but I don't know if that would do it. My teacher's Laney L5T also picks it up, but we didn't try his Fender Hotrod Deville.

[stevesuk]

I have just had a cage amp customer report serious radio interference. This is the first time I have ever had that since I played in a band in 1965 and we were rehearsing next to a Police Station.

Anyway, he did the capacitor from input jack ground to chassis trick and he said it worked like a dream.
This is his other comments about the 18 watt Lite IIb cage amp.
Just to let you know the cap to chassis ground trick worked a treat. The amp is now very quiet in operation even when dimmed. The vintage swirl you get from a neck position humbucker on a les paul with vol tone both on 10 is unreal. Also when running the amp clean at vol 2-3 it's one of the best clean tones I've ever heard, and I thought the clean channel was nice on my laney vh100r - no contest as far as I'm concerned, truly awesome amp

I did use lots of mustards in this one and my new design Output Transformer.

Steve UK
www.valvepower.co.uk

[Chappy]

The 68k value comes from the traditional attenuating Hi/Lo jack arrangement which AFAIK originated with Fender amps, and IIRC also has some relationship with the DC resistance of a typical vintage Fender pickup. When you don't have an attenuating Lo input jack, then there's no need to stick to the 68k value.

Thanks Zaphod
Very enlightening about the 68K resistor origin. I think that I`ll be changing out some of mine for the 10k resistor and see if I notice a difference with my humbucker pickups.

Chappy

[katopan]

he did the capacitor from input jack ground to chassis trick and he said it worked like a dream.

The mobile interference still happens with the 18W volume down at zero. To me that would rule out the input and preamp, although ground wire inductance could still mean that high freq digital interference might come through a so called grounded PI input.

[zaphod_phil]

Digital mobile phone noise will get in everywhere if a phone is very close by. If it's further away, it simply won't get picked up. Their power output is pretty low. In North America most of the 'phone networks still seem to be analog, so you don't get interference you can hear from cellphones on those networks. I never hear any buzzing in my amp from my own phone, even when I'm playing and I have the phone clipped on my belt, as its on an analog network.

[katopan]

That's what I figured. The effect does seem to drop of very quickly as the distance is increased.