Opening up a 1987x without killing myself

[KeithColiseum]

I have a 1993 1987x reissue head that sounds great but has the annoying hum or white noise issue that apparently is pretty common. A tech recently looked everything over and found no major issues. My research tells me it's probably something to do with preamp wiring, tubes, etc. I'd like to open it up and just look at this area and make sure the wires are braided and against the back plate, but I won't be messing with any soldering or anything like that. I know there's serious voltage inside and I don't want to kill myself. I'm thinking I should be alright without really messing with anything like a transformer or capacitor, but I don't want to accidentally bump into something that could cause a problem.

Any advice or tips (about opening it up or fixing the hum in general) would be appreciated. Thanks.

 

[chuckharmonjr]

Have the filter capacitors ever been replaced?

 

[MM54]

If you don't touch anything, you'll be okay. Otherwise, drain the caps before working inside... in this case, working is defined as touching anything.

 

[KeithColiseum]

At most I'd only be touching the preamp wires. I've heard about draining the caps, but have no idea how to do it. I'm sure if I don't even know how to do it then I shouldn't even be touching preamp wires.

 

[MM54]

At most I'd only be touching the preamp wires. I've heard about draining the caps, but have no idea how to do it. I'm sure if I don't even know how to do it then I shouldn't even be touching preamp wires.

 

[Marshallmaniac]

If you're really worried, leave the amp for a few days then they should be drained.

Move the green wires between preamp tubes around. Use a plastic thing if you're really scared.

I'm guessing your noise bias pot may have moved though? It should be turned either all the way right, or left.
There are two of these inside though, (they are blue square things with a white centre, variable pots with a small flatblade bit in the middle) one is for your output tube bias so mark where they're set before you move any of them. It's no real big deal and you can't break anything by changing their variance, just as long as the bias is set to the middle or where it is currently (as long as it is set correctly) Bias is quite a misunderstood concept and I remember talking to a master tech John Phillips who agreed that bias really isn't that important most of the time. But thats a whole 'nother story

Also, wriggle your input jack in the sockets, see if you get more, and less noise. If there is any variance, suspect these.

Also, I highly, highly doubt your caps would be failing. Heard about it many times but never ever experienced it myself. Well, once in a 1960' Fender amp that had original caps. It was motorboating. Caps today are pretty bloody good and most don't have the stuff they used to have inside

How loud is this hum anyways? You're plugging straight in, right? Not through a pedalboard>?
Guitar is o.k? Lead is definitely ok?

 

[jcmjmp]

Caps can hold a charge for days. Beware!

 

[Marshallmaniac]

I like to leave the amp turned on, and pull the power chord/plug to drain off most of the charge. Be sure to leave the cab lead plugged in for this to work effictively. .

Caps can hold a charge for days. Beware!

 

[TradAmpGuy]

Yeah, bias isnt important. Really? All the way one way or the other???!!!??!

Dont listen to this. Dont go changing bias pots willy nilly trying to get rid of hum. These Marshalls dont have hum balance pots.

Learn how to drain the caps before you do anything. The easiest way is to, USING ONE HAND, clip a jumper to ground (chassis) and then to pin 1 of V1. This will drain the caps through the plate resistor and in a few minutes the caps will be safe. Verify this with a meter. If this doesnt make sense, and you dont know why you are using one hand, then you are not ready to go poking around in a tube amp.

 

[Les Moore]

Hey Trad, when I changed my caps I did with an electrician frien of mine. He showed that you could just hook up the meter on the cap, + and -, and that drained the cap. He showed me the reading on the meter and you could see the voltage drop all the way down to zero. Have never seen anyone here mention it. What´s your opinon?

 

[Marshallmaniac]

Ahh Trad, you sure you know what I'm talking about here?
I said the HUM BALANCE pot. Not the bias pot..

Don't have hum balance pots..?!!!
Please open up a 1987x someday.. Really beginning to question your ability to read posts accurately mate. Only advise if you're sure you comprehend what's being said.

 

[Marshallmaniac]

Please see John's response below.

In purely technical terms, the correct bias point for a Class AB push-pull amp is that at which crossover distortion just disappears for a full-power clean signal, or very slightly above this. The most accurate way to do this is with a scope and a signal generator (and a dummy load, unless you like listening to sinewave test signals at full output volume ). Usually (depending on the amp design, and whether it uses the full capability of the tube) this will occur at an idle current of around 50% of the tube's maximum rating. You can do it by ear, but it's not very accurate... though it also doesn't need to be.

In musical terms, they don't always sound best like this, and need to be run a bit hotter. But not too hot - you always need to keep the maximum dissipation at any output power below the maximum rated power for the tube, or you will shorten the tube life (possibly a lot, depending how much and for how long you exceed it). In a typical Class AB circuit, this means keeping the idle dissipation below about 70% of the tube's rated value. In some amps, which push the limits of the tube rating, it needs to be less than this. In others it can be hotter, because the voltages in the amp may be low enough to allow that... the 70% figure is not an accurate, or 'magic' target. It just works pretty well for a typical Class AB amp designed to use most or all of the tube's capability.

Basically, the correct bias for a guitar amp is where it sounds best to you, and doesn't exceed the tube's maximum ratings - or if it does, you can accept the shorter tube life.

It's also my opinion that tone changes from biasing hotter (although not non-existent) are often less to do with the actual tone, and more to do with the perception of better tone caused by the slightly increased gain - this is a well-known psychoacoustic effect. I always bias as cool as reasonably possible when repairing amps, because there's absolutely no doubt that this gives better reliability and longer tube life... and I don't get too many complaints about tone.

Plate glow is not usually a good indicator of when you're too hot BTW... it's an indicator of when you're WAY too hot, and the tube is at risk.

 

[jcmjmp]

Hey Trad, when I changed my caps I did with an electrician frien of mine. He showed that you could just hook up the meter on the cap, + and -, and that drained the cap. He showed me the reading on the meter and you could see the voltage drop all the way down to zero. Have never seen anyone here mention it. What´s your opinon?

If you do that, you have to keep your meter of the amp for the duration of the time you're working on it. Not very practical if you need the meter for other things, no?

If you remove the meter when the voltage has reached zero, voltage will start to creep up again. The BEST way is what Tradamp stated a few posts up.

 

[MM54]

Hey Trad, when I changed my caps I did with an electrician friend of mine. He showed that you could just hook up the meter on the cap, + and -, and that drained the cap. He showed me the reading on the meter and you could see the voltage drop all the way down to zero. Have never seen anyone here mention it. What´s your opinon?

In sticky situations I've done this for lack of any other means to drain individual caps, it does work well but takes a long time, and holding the probe on the charged cap for a long time is less than enjoyable.

 

[jcmjmp]

Please see John's response below.

In purely technical terms, the correct bias point for a Class AB push-pull amp is that at which crossover distortion just disappears for a full-power clean signal, or very slightly above this. The most accurate way to do this is with a scope and a signal generator (and a dummy load, unless you like listening to sinewave test signals at full output volume ). Usually (depending on the amp design, and whether it uses the full capability of the tube) this will occur at an idle current of around 50% of the tube's maximum rating. You can do it by ear, but it's not very accurate... though it also doesn't need to be.

Its amazing what people say on the internet.

In a class AB amp, there's always a some amount of x-over distortion. You can never completely get rid of it.

Where did you get the 50% idle current figure? Have you ever tried this with a scope and signal genearator? What kind of scope was it and what was its bandwidth?

 

[Les Moore]

Sorry for borrowing the thread here, but it is relevant to the OP I believe.
Thanks guys for the reply about cap-draining. The voltage was already low when he started. But he started working on them after and touching them without anything happening. I wouldn´t do that cause since I´m not knowledgeable in the area there could be something more he knew that made him do what he did, then I try the same thing and sparks starts flying cause of some other thing.
Anyway, thanks for clearing it up.
While I`m at it. The "pulling the power cord" method, always felt like "it has to be bad for the amp somehow". Is it?

 

[Marshallmaniac]

Yes there is, and thats an (almost secret) part of why tube amps sounds so great 'clean' and distorted. They're at least 10% distorted when set 'clean'

Part of this post by JP (who by the way is my favourite amp tech, in the WORLD) is to admit that biasing isn't as bigger deal as some are led to believe it is. FWIW I haven't been bothered biasing my amp for years and just leave the pot at 50% and I can't notice any difference (to my ears) in sound sitting there and moving the entire width of the bias pot. Maybe a little at full tilt, but really not much. As long as my tubes aren't running too hot I'm happy. I've sat there and watched the osci while doing this and the difference was much smaller than I initially thought it would be.

Sorry to hijack the thread. Please carry on I'll leave it be now.
Apologies about getting a little frustrated earlier too Trad.

Its amazing what people say on the internet.

In a class AB amp, there's always a some amount of x-over distortion. You can never completely get rid of it.

Where did you get the 50% idle current figure? Have you ever tried this with a scope and signal genearator? What kind of scope was it and what was its bandwidth?

 

[TradAmpGuy]

Aight, didnt see the hum balance in the 1987x. Its not shown unless you look at the complete circuit diagram... my apologies as I was looking at the preamp and power amp schematics. FWIW, that pot should be adjusted to the position that gives the lease hum... nothing more complicated than that. Im used to older Marshall Circuits that dont have the hum balance.

As far a scope biasing, not a big fan. You stated that you should bias to get rid of the crossover notch. Then you proceed to say that this doesnt sound the best musically. So you advise to bias hotter but not too hot. How should that be done if you are not taking some sort of measurement some where? You also state to keep the dissipation below 70%... again, without taking some sort of measurement, there is no way to know if have done that. Lastly, you state that there isnt much difference when you dial your bias full up or down. So then why wouldnt it sound musical after biasing with the scope method and then being done with it instead of needing to bias hotter?

I'll stick with actually measuring the idle current either with a 1 ohm resistor or if that isnt an option, the transformer shunt method. It's consistent, repeatable and you are sure you are staying out of the red zone.

Not trying to be contentious here, just trying to wrap my head around a few inconsistencies here. I think Im comprehending what you are saying... it just doesnt seem to add up.

 

[Marshallmaniac]

OK Trad. I know you think logically, and not musically. Which is good for the proffession you're in!

When I say I leave the bias pot at 50% and then play my amp for the next 6 months before changing tubes, I mean I listen, make sure I'm happy with the sound. Play loud and look at the power tubes. Are they redplating? No. Good. Off to the gig.

I placed my amp on a sig gen and osci with an amp tech a while back and moved the bias pot from its lowest, to fullest poinjt and I said to the tech while watching the osci "whats the point?" He replied, well.. it's just there to keep the operating voltage correct, but it matters little" The tube is in trouble if something else goes and starts redplating but the bias can pretty well be set anywhere and it will work ok on your amp". Could barely hear a difference, and all were safe operating settings. Case closed for me. I didn't say anything about not sounding good that I remember?

As for the technical stuff. JP is the one who should be answering that. He wrote it and I trust him 100%

 

[RickyLee]

. . . . And then throw in the fact that in this age of higher MAINS Service/wall voltages in many parts of the World, quite a few Marshall amps - and other brands as well - run a typical power valve plate voltage well over 480VDC. Doesn't this factor in to the O'scope biasing method and actually cause issues?

I just set all my rigs to 60% dissipation of valve rating in relation to the running plate voltage. Sometimes by cathode bias resistor method. Sometimes by transformer shunt method.

All is good.