Radio Interference

selznick · Post by **selznick** » Sun 02/24/08 4:11 pm

I just bought a used 36W Ceriatone. The tone is great of course, but I'm picking up a radio station fairly clearly.

I've tried 2 different power outlets, and three guitar cables. I notice it has the problem as long as a guitar cable is plugged in, whether or not the guitar is.

There was shielding on the bottom of the guitar cab, though not wide enough to contact the mounting screws, so I extended it with Aluminum tape, and used folded over tape to connect all the shields.

Anywhere else obvious I should check?

I should say that the last owner believed it was built by Nik, but didn't know for sure. There has been two mods, one moving the half-power switch to the rear, and one adding an effects loop. He believed these mods were professionally done, and reversible.

Thanks for any advise.

Sandy...

unchained · Post by **unchained** » Sun 02/24/08 10:21 pm

Check your input connections. Make sure it is grounded properly and is using shielded wire only grounded at one end by the input, not at the tube.

Phil_S · Post by **Phil_S** » Mon 02/25/08 8:51 am

Put a very small value cap between the input jack and the ground connection. It goes on the sleeve (ground), not the tip. Try a 1000v ceramic disk. How small? Not sure, but probably in the pf range.

ajkimba · Post by **ajkimba** » Mon 02/25/08 9:23 am

Phil_S wrote:Put a very small value cap between the input jack and the ground connection. It goes on the sleeve (ground), not the tip. Try a 1000v ceramic disk. How small? Not sure, but probably in the pf range.

Aiken says to use a .01 cap from input ground to chassis, shortest lead as possible, Here's what he says:

Important note: While the star ground is excellent for eliminating ground loop hum, it is not always the best scheme for preventing radio-frequency interference (RFI). Fortunately, there is a simple addition to the star ground scheme that will make for a very quiet amplifier with no RFI. Simply add a 0.01uF capacitor from the chassis to the ground lug of the isolated input jack using very short leads. This will shunt all of the RF "riding" on the shield of the cord straight to the chassis ground before it can get into the amplifier and cause problems. Any kind of ground lug can be used for the chassis connection of the capacitor. You may be able to find a solder lug that slips over the shaft of the isolated input jack for a convenient ground lug. Use of an internal-toothed lockwasher is recommended for these types of connections to insure a good "bite" into the chassis for a good ground.

veets · Post by **veets** » Mon 02/25/08 10:03 am

Or connect a 68K resistor between the input jack and the grid of the preamp tube, like on a Fender.

Post by **zaphod_phil** » Mon 02/25/08 10:05 am

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.

selznick · Post by **selznick** » Tue 02/26/08 4:34 pm

zaphod_phil wrote: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.

I haven't had a chance to take all your great suggestions yet (thanks, though). However, I notice that it did already have 2 68k resistors going to the tube.

Is it possible these went bad? (Actually as I write this, I realize that can't be right, because it didn't matter which inputs I used).

As for the capacitor, from input ground to chassis, should I connect one to each input? Or can I find a common ground point and just put one cap there. If I connect the input ground to the chassis, doesn't that create a ground loop with the star lug? (Sorry, for the questions, I have a very rudimentary understanding of this stuff.)

Post by **rjgtr** » Tue 02/26/08 9:45 pm

selznick wrote:As for the capacitor, from input ground to chassis, should I connect one to each input? Or can I find a common ground point and just put one cap there.

You'd need one from each input jack, but you can probably get away with just doing the hi input jacks.

If I connect the input ground to the chassis, doesn't that create a ground loop with the star lug? (Sorry, for the questions, I have a very rudimentary understanding of this stuff.)

It could, but not through the caps. If you do ground the inputs directly to the chassis, just don't connect their grounds anywhere else.

etch · Post by **etch** » Wed 08/13/08 4:40 am

Phil_S wrote:Put a very small value cap between the input jack and the ground connection. It goes on the sleeve (ground), not the tip. Try a 1000v ceramic disk. How small? Not sure, but probably in the pf range.

Bringing this thread back to life because I tried this last night (on an 18 watt) and it worked great; no more radio! Very cool tweak...

sub · Post by **sub** » Wed 08/13/08 8:53 am

etch wrote:
Phil_S wrote:Put a very small value cap between the input jack and the ground connection. It goes on the sleeve (ground), not the tip. Try a 1000v ceramic disk. How small? Not sure, but probably in the pf range.
Bringing this thread back to life because I tried this last night (on an 18 watt) and it worked great; no more radio! Very cool tweak...

Etch,
What value cap you used?
If i understand correctly you need replace the ground wire with a cap?

Post by **rjgtr** » Wed 08/13/08 9:55 am

sub wrote:
etch wrote:
Phil_S wrote:Put a very small value cap between the input jack and the ground connection. It goes on the sleeve (ground), not the tip. Try a 1000v ceramic disk. How small? Not sure, but probably in the pf range.
Bringing this thread back to life because I tried this last night (on an 18 watt) and it worked great; no more radio! Very cool tweak...
Etch,
What value cap you used?
If i understand correctly you need replace the ground wire with a cap?

I have used .001uf caps. The exact value probably doesn't matter too much, just something smallish, probably between 47pf and 1000pf. Ceramic are cheap and work well; I haven't tried other types.

To wire it you take the ground from the input jack to the ground point of the input tube's filter cap. You add the small cap from the input jack ground lug to a point on the chassis as close to the input jack as possible. I put a ground lug right underneath the jacks and wire it there. The idea is for the cap's leads to be as short as practical. I also find it better to run the input's ground wire separate from any pot's grounds.

sub · Post by **sub** » Wed 08/13/08 11:21 am

Thanks Rjgtr!

etch · Post by **etch** » Thu 08/21/08 1:50 am

Sub,

Sorry didn't see your question.

I used 250pf (I believe).

All I did, based on a post here, was to connect one end to input ground and the other to the chassis ground.

I kept the leads as short as possible and got the caps for super cheap at radio shack as I happened to be there and remembered this thread

I was surprised at how well this worked, I had really loud radio at my house starting around six everyday coming through the amp. The caps zapped it straight away.

sub · Post by **sub** » Thu 08/21/08 3:10 am

Thank you Etch!

JamesHealey · Post by **JamesHealey** » Tue 08/26/08 3:10 am

my RFI starts at about 6 in the evening also strange, wonder if they up the juice at around that time or something? I live in the UK way way away from you so thats strange.

nice tip though guys I'll be doing this to my super bass aswell as my 36w

Post by **rjgtr** » Tue 08/26/08 7:45 pm

It's not so strange, as radio travels more easily at night. Here in the States some radio stations have to reduce power at night. Maybe less interference from the sun?

Post by **zaphod_phil** » Wed 08/27/08 7:44 am

You're very close. This effect comes from the lowest part of the ionosphere, the D-layer, which has a negative effect on radio waves because it absorbs radio wave energy, particularly with frequencies below 7MHz. It develops shortly after sunrise and disappears shortly after sunset. This layer reaches maximum ionization when the sun is at its highest point in the sky and it is also responsible for the the complete absorption of sky waves in the AM broadcast band during daytime hours, as well as the lower frequency short wave bands.

So now we all know!

Shottky · Post by **Shottky** » Mon 01/26/09 10:25 pm

I've been picking up radio waves while my amp is plugged in to my guitar. It seems that I should be putting a cap between the shielding ground on the jack and the chassis. However, 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. There is a lug where I can solder a capacitor, but it doesn't seem like this would do much if there is already a ground connection. Does this mean I need to use plastic jacks where the ground connection isn't made directly by contact with the chassis?

hunterwh · Post by **hunterwh** » Mon 01/26/09 11:28 pm

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.

Shottky · Post by **Shottky** » Mon 01/26/09 11:58 pm

hunterwh wrote: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.

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