Yikes! Can this be good for you?

[John Phillips]

Today I discovered that a cranked Marshall 50W amp was putting out massive amounts of RF.

I was bench-testing it after an overhaul, and - unusually, because she was off school feeling ill - my daughter was watching TV in another room. I had the amp fully cranked into a Powerbrake at the minimum volume setting, and shortly after I started testing it she called through that the TV was going funny. I stopped playing and went through to see what was up with it... nothing. I went back to testing the amp again and the same thing happened. So I ran a long cable between the rooms so I could see the TV while I was playing, and sure enough every time I played a chord the picture went erratic and then off... and back on again when the sound decayed back to clean.

Just what frequencies is this thing giving off?!! It must be at a fair level too, since the amp and the TV are about twenty feet apart, through two solid brick walls.

I've never noticed it before, so it might be this particular amp - but I never normally have the TV on when I'm testing amps either, so I can't really tell. It sounds OK - maybe a little more ragged than usual when it's really thrashed - and the tubes don't appear to be under unusual stress.

But if it's normal, is that a good thing if you're standing near one?

[VacuumVoodoo]

Is the TV on the same mains line? Cranked amp can feed high harmonics back into the mains. That's why 2X-Y EMI supression caps are required on amps mains inlet. They work both ways. RS components No 121-5449 http://img-europe.electrocomponents.com/se/img/site/symbols/newSmall.gif will do.
Also, ground routing in these amps is not the best and can make HF radiate through guitar cable. Strange but true.

[Axekisser]

Oh no! Another cellphone and brain cancer scare! LOL!!

I haven't noticed this on any of my amps but then again I don't have a 50W Marshall! I'll have to check this out with my small Fenders.

Interesting....

[fullerplast]

You're probably just saturating the OT, resulting in random square waves, which of course contain considerable high frequency components.

Health hazard? I doubt it. It analogous to an RF signal going to an old fashioned rabbit ear antenna. The levels are low.

I'd worry more about that poor OT getting thrashed.;)

[John Phillips]

You're probably just saturating the OT, resulting in random square waves, which of course contain considerable high frequency components.But into the RF? I wouldn't have expected that. Unless the sheer level even at lower frequencies is enough to disturb the TV - which I wouldn't have expected either, since it's a digital.

Health hazard? I doubt it. It analogous to an RF signal going to an old fashioned rabbit ear antenna. The levels are low.I don't know about that, I thought I had a headache afterwards :). And I'm not at all convinced the levels are that low either...

I'd worry more about that poor OT getting thrashed.;)Nah... I've never blown one yet doing that.

(Apart from a DSL50, but even then I have a suspicion it may not have been perfectly healthy beforehand, even though it 'worked'.)

Is the TV on the same mains line? Cranked amp can feed high harmonics back into the mains. That's why 2X-Y EMI supression caps are required on amps mains inlet. They work both ways. RS components No 121-5449 http://img-europe.electrocomponents.com/se/img/site/symbols/newSmall.gif will do.
Also, ground routing in these amps is not the best and can make HF radiate through guitar cable. Strange but true.Good point about the suppressor though. No, of course a 1971 JMP50 does not have them :). And they were on the same mains circuit, yes (via the distribution box). Maybe I should fit some in my bench power extension and see if it still does it...

[fullerplast]

But into the RF? I wouldn't have expected that. Unless the sheer level even at lower frequencies is enough to disturb the TV - which I wouldn't have expected either, since it's a digital.

I don't know about that, I thought I had a headache afterwards :). And I'm not at all convinced the levels are that low either...

Nah... I've never blown one yet doing that.

(Apart from a DSL50, but even then I have a suspicion it may not have been perfectly healthy beforehand, even though it 'worked'.)

Well digital TV is sent at what, 19.3Mbps? The Fourier transform of a perfect square wave requires infinite bandwidth. Less perfect of course is less bandwidth, but a fast rise time will still generate very high frequency components. It won't be filtered by the amp circuit or the speaker because the source of the square wave and resulting HF is the tranny itself.

From the OT to the attenuator to the speaker you have the makings of a nice big radiating antenna. It may not really take a significant signal to mess up your TV signal. For example, microwave ovens can corrupt wireless networks even with the mesh door closed and a reasonable distance away.....

As for the OT itself, full power square wave saturation is not it's favorite thing to endure as it generates lots of heat.....and we all know there are OT and amp builders that will void warranties with use of an attenuator, especially in the fashion you are using it.:nono But that's certainly your call and you know what you are doing.:AOK

If you want to see if it is RF through the air, or harmonics on the mains, just hook a scope up on your mains while you blast away or hook a scope up to your digital TV signal source....you should see something on one or the other, possibly both.

[hamfist]

That's the last time i'm going to be using a cranked amp in a house with human beings in it (except myself, of course !). I always suspected that playing guitar was seriously bad for my health !!

[rockon1]

I dont know but at this point Im more worried about my hearing! :eek:Bob

[Kiwi]

Well, since you have successfully reproduced, the worst-case scenario is unlikely here ....;)

=K

[GearHeadFred]

Well digital TV is sent at what, 19.3Mbps?

You'd be right if he was located in the US. In the UK, they use DVB-T, which uses lower channel rates (8-ish Mbps I think) and COFDM modulation (rather than 8VSB used in the US)

If it's truly RF interference on the DTV signal, I would expect the digital demod to loose lock and have to re-acquire, resulting in a 1-2 second black screen or still-screen.

My guess would be it's coming in the AC line.

[fullerplast]

If it's truly RF interference on the DTV signal, I would expect the digital demod to loose lock and have to re-acquire, resulting in a 1-2 second black screen or still-screen.


But isn't that what he said?

....sure enough every time I played a chord the picture went erratic and then off... and back on again when the sound decayed back to clean

[John Phillips]

If it's truly RF interference on the DTV signal, I would expect the digital demod to loose lock and have to re-acquire, resulting in a 1-2 second black screen or still-screen.Yes, that's precisely what happened - either jittery interference and then off, or straight off, and back on again once the signal stopped heavily distorting.

[GearHeadFred]

Maybe... I guess we need more detail on what "went erratic" means.. I assumed he meant it in an analog way... but perhaps not?
Digital (MPEG) artifacts are unmistakable.. small and/or large blocks of "garbage" on an otherwise perfect picture..
Analog looks like traditional "TV interference" with loss of sync (rolling), ghosting, snow, etc.

<posted this before I saw John's response> John, does it look like analog or digital interference?

[Tone_Terrific]

If I crank up an amp near (10 ft) my computer monitor, it goes wonky, like a magnet is being dragged in front.

[Tone_Terrific]

I'm speculating from a position of ignorance....(that's Ok on the internet, right?) but maybe it's just low freq magnetic transmission, not RF.

[John Phillips]

Maybe... I guess we need more detail on what "went erratic" means.. I assumed he meant it in an analog way... but perhaps not?
Digital (MPEG) artifacts are unmistakable.. small and/or large blocks of "garbage" on an otherwise perfect picture..
Analog looks like traditional "TV interference" with loss of sync (rolling), ghosting, snow, etc.

<posted this before I saw John's response> John, does it look like analog or digital interference?
Definitely digital. Small or large blocks of garbage followed by a blank screen. I don't know enough about digital TV to be certain, but it looks to me like the incoming signal is being affected, rather than something in the set. I get exactly the same type of breakup in strong winds when the aerial is flapping about.

I'm speculating from a position of ignorance....(that's Ok on the internet, right?) but maybe it's just low freq magnetic transmission, not RF.I wondered that too... but at that range? I know well about magnetic coupling with dynamic mics in front of cranked-up guitar amps (which makes them sound fizzy and crap), but that's only with the mic a few feet from the amp, and it's a highly sensitive device anyway. For the magnetic field to disrupt a TV twenty feet away it would have to be absolutely huge, I would think.

The other possibility is the Powerbrake, which is also highly inductive (unlike any other attenuator I know of). I'm going to try it again with a resistive load and see what happens :).

[GearHeadFred]

Definitely digital. Small or large blocks of garbage followed by a blank screen. I don't know enough about digital TV to be certain, but it looks to me like the incoming signal is being affected, rather than something in the set. I get exactly the same type of breakup in strong winds when the aerial is flapping about.

The digital demodulator takes the incoming signal and tries to make "1"s and "0"s from it. It uses very sophisticated methods (FEC, or forward-error-correction - basically extra 'bits') to validate and fix errors in the video/audio info.. But when the incoming signal crosses below a threshold, the demod cannot recover any useful bits and basically gives up... waiting to re-acquire a lock; hence the delay.

This is very different from analog TV where the TV will always display something - regardless of the quality of the signal.

This is what I do for my 'day job'!

It's nice to return the favor after soaking up knowledge from your posts!

[fullerplast]

when the incoming signal crosses below a threshold, the demod cannot recover any useful bits and basically gives up... waiting to re-acquire a lock; hence the delay.

The loss of lock is precisely why the problem is more likely an RF corruption of the incoming signal.

[Chiba]

Probably a good thing you've already reproduced, because... well...

:)

--chiba

[mpeg2]

It could well be some kind of disruption of the RF signal. To clear up something stated a bit earlier - in the US, the ATSC signal uses 19.39Mb/s (DVB in Europe is something a bit different) - cable is at about 38.8Mb/s - BUT these numbers don't have anything to do with what's being discussed. These are the bitrate of the demodulated signal (i.e. turned into a digital stream of bits). The modulation schemes used (8-VSB for ATSC, COFDM for DVB and QAM for Cable) modulate many bits per Hertz of an analog RF signal. US terrestrial & Cable use 6MHz, Europe uses 8MHz.

If there was disruption of an analog TV signal, you'd see it typically as increased noise (snow). Digital exhibits a cliff effect - it's either good or it's gone. Small bits of "gone" can interrupt the bitstream to cause some of the macroblocking effects. Unfortunately, other things can as well (impairments in the digital stream) - so it's rather hard to pinpoint the source of the problem.

This happens to be my day job also....

Rich

[fullerplast]

The modulation schemes used (8-VSB for ATSC, COFDM for DVB and QAM for Cable) modulate many bits per Hertz of an analog RF signal. US terrestrial & Cable use 6MHz, Europe uses 8MHz.
Rich

This is all academic now and I think we all agree it's RF interference causing John's problem.....but I'm not sure you really meant to say this. You can't modulate a 19Mbps baseband signal with a 6Mhz carrier (think about it...the data is changing more than 3x as fast as the carrier.) The 6MHz you refer to is simply the bandwidth of the channel, IOW the distance between the channel carrier signals.

The 19.3 Mbps signal is actually modulated onto a 44MHz IF carrier, and then again onto anything from 50 to 800MHz or more standard VHF/UHF RF carrier signals. (This is where the 6Mhz bandwidth separation occurs.) The corruption of the signal prior to sync lock could occur at any point in the path including the baseband signal, but I completely agree it would be difficult to pinpoint at which stage the signal was corrupted.

[GearHeadFred]

This is all academic now and I think we all agree it's RF interference causing John's problem.....but I'm not sure you really meant to say this. You can't modulate a 19Mbps baseband signal with a 6Mhz carrier (think about it...the data is changing more than 3x as fast as the carrier.) The 6MHz you refer to is simply the bandwidth of the channel, IOW the distance between the channel carrier signals.


mpeg2 is correct.. The use of higher order modulations (QAM, 8vsb, etc.) enable "multiple bits-per-hertz".. In digital communicaitons, we use the term "symbol rate" rather than bit rate.. A "symbol" in QPSK can be 1 or 4 possible values, and represent 2 bits. A symbol in 8vsb can be one of 8 values, hence represents 3 bits (all at 1 clock time), and so on.... Some cable systems use 64QAM, allowing for 6 concurrent bits per hertz.

[mpeg2]

Actually, cable today is using 256 QAM, allowing them to get about 38.8Mb/s into the 6MHz band.

I did misspeak slightly - the 6 & 8 MHz referred to is the bandwidth of the RF carrier - the actual RF signals are at much higher frequencies.

Rich

[fullerplast]

mpeg2 is correct.. The use of higher order modulations (QAM, 8vsb, etc.) enable "multiple bits-per-hertz".. In digital communicaitons, we use the term "symbol rate" rather than bit rate.. A "symbol" in QPSK can be 1 or 4 possible values, and represent 2 bits. A symbol in 8vsb can be one of 8 values, hence represents 3 bits (all at 1 clock time), and so on.... Some cable systems use 64QAM, allowing for 6 concurrent bits per hertz.

You're talking about two different things. The "hertz" you are talking about is the bit rate of the encoded baseband signal. The "hertz" I am talking about are the carrier frequencies. The "symbol rate" is how you get that much data into the 6Mhz bandwidth of the channel, it's not at all "the hertz of the analog RF signal". It's how you can encode more data into the baseband bit rate.

[GearHeadFred]

I disagree. Symbols are modulated as an RF signal in a 6 MHz carrier. The baseband rate is 19.39 Mbps. The symbol rate is 6 Msps. Symbols don't exist in baseband.

MPEG compression is how you encode gigabits (in 1080p) of video information into 19.2 Mpbs (or less) at baseband.

It's official - we're nerds!

[fullerplast]

I disagree. Symbols are modulated as an RF signal in a 6 MHz carrier. The baseband rate is 19.39 Mbps. The symbol rate is 6 Msps. Symbols don't exist in baseband.

MPEG compression is how you encode gigabits (in 1080p) of video information into 19.2 Mpbs (or less) at baseband.

It's official - we're nerds!

The nerd part I'll agree with.:BEER

To confuse the issue even more, the "symbol rate" for 8-VSB is actually a little under 11 Msps, not 6Msps. That symbol rate is what requires theoretically 5.5 MHz of bandwidth, and the actual practical bandwidth of 6MHz. Again, it's the bandwith of the channel that is 6MHz.

The 8-VSB signal is the baseband signal. That is modulated onto an AM 44MHz IF carrier, then mixed to VHF/UHF RF carriers.

And I bet Jim Marshall could care less what his amps are capable of doing to any of these signals!:cool:

[Old Tele man]

...well, MY guitar amp has VOL and TONE knobs, not TV-Channel selector knobs (wink,wink)!

[Geetarpicker]

My '89 Trainwreck Express use to set the old 6' satelite dish to spinning on the roof whenever I cranked it up and I never used an attenuator with it back then. The "dish" controller had an RF remote, that picked up the RF from the amp just fine. I had to wrap the power tubes in grounded aluminum foil to shield them to keep the TV channels from changing...

[stevehollx]

My '89 Trainwreck Express use to set the old 6' satelite dish to spinning on the roof whenever I cranked it up and I never used an attenuator with it back then. The "dish" controller had an RF remote, that picked up the RF from the amp just fine. I had to wrap the power tubes in grounded aluminum foil to shield them to keep the TV channels from changing...

hahaha!

So THATS what an amp on the verge of oscillation does!

[MikeMcK]

All that said, I'd suspect the power mains first, as someone said. In the '90's I worked for a company that had developed a VDSL system (basically TV, fast Internet, etc. over phone lines).

In our demo lab we had a wall of televisions playing movies through a prototype system. You could cause tiliing/freezing of the pictures by blipping the trigger of a heat gun in the next room.