Cell phone interfearance in Matchless amp?

[Tdale]

A friend of min has a 94 Matchless Ligthing amp. He has had it for almost 10 years, and has never had any problems with cell phones close to the amp. You know the sound that you can hear when you put a cell phone close to your radio etc?

Then suddenly the amp starts picking up cell phone signals from the audience in front of the stage!

Any idea what can be wrong? He has of course made sure that noone on stage has their cell phone on, but when someone in the audience gets a call, you can hear it through his amp. Removing the effects and guitar (and cable) does not help either...

Tommy

[FYL]

Could be the combination of inadequate/deteriorated wiring - from mains to internal stuff - and a bad solder showing a diode effect thus acting as a detector.

[firechief]

Tdale
Does you friend hear this when he uses the amp in a different location> This is a key bit of information. Mostly cell phones are not powerfull to be picked up by amps, they transmit a tiny signal. Could he hear an actual voice? or just garbled talk like noise? Is it possible that it was some other kind of RF signal. Possibilties include local AM radio, CB, ham transmitter. If it only happens in one location it may not be a problem with the amp as much as a realy lousy transmitter of some sort. Anyway one well known fix is to put a small value cap accross the input choses to short RF to ground while not affecting Audio. A good value to try is in the pico farad range, say 100pf. If the amp is susceptiable to picking up RF there might be a grounding or shieling issue also. This problem is more likely to be in the input stage like the first tube. See if you can make it happen, and report back. Then you will have a tool you can use to solve the proble.
Good luck

[FYL]

No need for an input filter : the Lightning uses a // 12AX7 with 68K and a silly 440K equivalent load per plate for oodles of gain. Bandwith for a perfect source is already limited at 20 KHz or so because of the Miller effect. Just turn the volume pot on the guitar and the first stage acts like a big low pass filter with gain - 100K on the pot leads to a 6KHz or so F3 !

Anyway, the noise is still present with no guitar plugged - the input is then grounded, so it doesn't enter the amp thru it.

[firechief]

OK I should have read the first post more carefully, with no cord plugged in the signal isnt being picked up through the input. This just makes the whole thing more strange. I have almost never found that modern cell phones to cause interfearance to audio gear. I wait with bated breath to hear more.

[FYL]

Bad solder joints can act as rectifiers, with the right lead acting as an antenna the amp becomes a receiver. Seen and heard in studio with some bozos using Bluetooth or Wifi-enabled laptops.

But that's only a guess, not a diag.

[Aurora]

First - and simplest - modern cell phones are digital, so there's no voice to be heard - just the well known horrid tic-tic........

Something obviously happened along the way....
If the power cord is removable -IEC type plug -I'd start to check with a new one - moulded plugs are a real PITA, and often break inside....
A change of grounding or shielding is the most likely culprit - sure no mods have been done ???
Tube shields on the input, broken ground wires , faulty input socket.....??

[firechief]

OK I have not herd the effect, but I believe it can happen. If Tdale can make it happen, then that is a tool to find the failure. (sounds like this is a new thing). Use this as a tool to trouble shoot the problem. Solder connections have been proven to be able to act like a diode, but there are other mechanisms that could be in play here. What does it sound like? Do you have to be close to the amp with the cell phone to make it happen? Try moving to another location to rule out local interfearance.

[M Fowler]

I was sitting in church one day and right smack in the middle of the minister's sermon while using a wireless mike system. My buddy Tom that lives next door to this small town church got a phone call.

It was a very intersting phone call to, we all sat there listening to Tom making plans.

The next week an Air Force pilot (electronics tech too) and I were rewiring the church's system with high quality shielded wire.

Mark

[Aurora]

Don't say.. US cell phones are NOT GSM digital ????????
I have a 3-band Sony-Ericsson that's supposed to work in the US too....?

[M Fowler]

Well now I didn't say which era in my life this event occurred did I cell phone smart guy.

And apparently you haven't visited rural America where some cell phones look like old army radio's these cowboys don't give a flying rip about looking smart like the modern pavement cowboys do.

[Aurora]

No - actually . I haven't visited rural US ! Would like to visit Colorado, Minnesota and the likes, though... Alaska, too.
Have some relatives there.....

And BTW - for the time being, at least, Uncle Work pays for my fancy phones.......

[Tdale]

It's only the tic tic sound, no voices!

I'll have him try a new power cord..

Could bad tubes cause this effect?

Also: I didn't quite get the early post, about 68K resistort, highpass etc. I know the components, but are there other values on the mentioned components, that could be changed, with a better sounding amp as a result?

Tommy

[FYL]

I'll have him try a new power cord..

Check grounding while you're at it.

Could bad tubes cause this effect?

Hmmm, not impossible but improbable.

Also: I didn't quite get the early post, about 68K resistort, highpass etc. I know the components, but are there other values on the mentioned components, that could be changed, with a better sounding amp as a result?

The Lightning uses a very high gain first stage, you can tame it and get IMO much better sound by changing a few passive components as per the attached schemo.

Gain is reduced with the 100K common plate R - seen as 200K per section, bias is adapted, input grid stoppers are lowered.

The cathode bypass cap as well as the coupling caps can now be modified : 4µ7 will give a more dynamic sound w/o blocking disto with the new gain structure, 2n2 (or 4n7 with single coils) will extend bass response - the amp will still be chimey but in a much smoother style.

You may also rewire the input jax in order to have J1 = single section, lower gain, more laidback sound; J2 = standard parallel sections for V1, Lightning std sound.

[guitardude57]

The ATT phones in the house affect any guitar amp I have on at the time, the TV, the stereo.............. pretty funny some harmonic of 1.6 Ghz bleeds all over the analog audio spectrum.....tic tic tic..............