marshall g50r cd resistor value

[Enzo]

Hi, welcome to the forum.

Shorting the input to prevent oscillation is pathetic.

That isn't why they do it, you are just guessing. Most amps have shorting input jacks. It is done to prevent noise. If your input jack didn't short to ground, it would sit there humming when no guitar was plugged in. So yes, the jack shorts to ground. Go ahead and be 100% sure. On many Marshalls, not only is there the shorting input, but extra contacts on that jack also mute the preamp. Same reason.

The resistor and cap across the output is not a crossover, it is called a zobel network. Go google that. Amps can go into oscillation for a variety of reasons. Amps are designed to be stable, and most are. Now and then some failure in the circuit or failure in the load causes trouble.

[Mrcloc]

Hi Enzo, and thanks!

Noise measurements of amplifiers are when the input is shorted. But, a well-designed amplifier won't need this. Mine don't, that's for sure. Yes, they have better noise specs when the input is shorted, but they're as stable when the input is shorted vs not, and they're quiet enough without. I have been building amplifiers for some time now, and I have plenty experience with amplifiers oscillating when the input is different - different cables, input connected vs none connected. The point is if that is how the amplifier was designed to be stable, it isn't a good design. I am guessing, but it's a pretty good guess, I would think.

In addition, you can plug and unplug the input on my amplifiers without the popping. Very useful in the setting the amplifier was primarily designed for.

The Zobel network is a crossover. I couldn't think of the correct name at the time. Thanks!

Yes, amplifiers are designed to be stable, but the amplifier they put into these Marshall's is an LM3886 - known for noise and oscillations (at least to me they are). It's not that they don't work, it's just corner-cutting to increase profits. It's exactly what I say about SS amplifiers - usually the electronics are cheap, and most often SS are the cheaper amplifiers, and the speakers inside them are cheap as well. This gives SS a bad name. I'll happily put my SS up against any valve amp. Sure, there is something special about valves, but it will be difficult to choose the best in a blind test with my amps.

Anyway, I've rambled far too much. Regardless, that R69 that's burnt should have been a 3 W at least.

[Enzo]

I am not worried about noise figures and specs. A guitar amp with an unterminated input is an antenna, and will pick up noise. On the occasional amp built that way, you can bring your hand up to the jack and hear the hum increase as you come near. Grounding the input is not done to meet noise specs, it is done to reduce racket on stage or sitting in a studio. Same with the channel mute controlled by input jacks as an additional measure. It has nothing to do with amp stability.

By definition a crossover takes a wide range signal and divides it into two or more separate frequency bands.

We are glad you have joined, but just my personal opinion, until we get to know you, broad blanket statements are likely to get in the way more than help. Like calling amps pathetic, and so on.

[Mrcloc]

Sure, noise specs aren't as important in guitar amplification.

A crossover is a filter. A Zobel network is also essentially a filter, I just couldn't think of the correct name at the time. It's still the thing I'm talking about - it's not the other resistor - the 39k on the schematic.

Ok, don't get me wrong - grounding the input is fine to quieten the amp. The evidence above shows that there is an oscillation when there is no input. The input is the thing that was replaced due to a broken PCB. Thus, the oscillation appears when the input is not shorted to ground, or if there is no input present. I don't have the electronics in front of me to test this, so I can only make a guess based on the information I have.

I have been through a massive amount of building and testing, investigating stability of the preamplifier specifically (power amplifier bit is easy to stabilize). It's not even my job - I'm not paid to do it, but I can do it myself. Marshall is a big company with, supposedly, engineers whose job it is to make these electronics. If my guess above is correct, then using a mechanical thing to prevent oscillation is bound to fail sooner than later. It's not what I would want if I spent the money on a Marshall. Describe that any way you would like, I used the word pathetic. And this is in the event that my guess is correct as well. I didn't say "these amps are pathetic" I said, "Shorting the input to prevent oscillation is pathetic." That's just my opinion, and I stick with it.

So I hope you see I'm not trying to make blanket statements here. I'm in engineering, and precision is very important. I will call out bad engineering when I see it. I was very disappointed to see how the Marshall I opened up was put together, the 0.25 W Zobel resistor, for example. The LM3886 was also not what I expected - that's what you find in the local electronics shop's guitar amplifier kits. These things aren't necessarily bad, but they point to corner-cutting, which is too prevalent in modern electronics.

Anyway, my original reply here is only meant to help. I hope it does.

[Jazz P Bass]

Jumping right in.

Quote:" The evidence above shows that there is an oscillation when there is no input. ".

This is poor detective work to say the least.

The 'cause' of the oscillation was never determined.

It very well could have been the IC itself in a failed state.
(It's a high powered op-amp)

[52 Bill]

And looking at the photo with the replaced input jack, there appear to be a couple of traces that are broken and not connected to anything. Maybe this is the cause of the oscillation.

[nosaj]

And looking at the photo with the replaced input jack, there appear to be a couple of traces that are broken and not connected to anything. Maybe this is the cause of the oscillation.

Either way . It was deemed it was not feasible to mess with and It's no longer in my possession.
So it's all just postulation here on out.

nosaj

[Mrcloc]

Jumping right in.

Quote:" The evidence above shows that there is an oscillation when there is no input. ".

This is poor detective work to say the least.

The 'cause' of the oscillation was never determined.

It very well could have been the IC itself in a failed state.
(It's a high powered op-amp)

Ye - I am jumping in the deep here it seems.

In my original post I did say that the amplifier could have oscillated. The LM3886's aren't the most stable things in the world.

I didn't give a numbered list of suspected areas of origin of the oscillations. I put out some speculation. Regardless, I did give information that is most likely helpful:
1. Use a higher power resistor
2. Replace the capacitor in the network
3. Consider replacing the input connector with one that shorts the input with nothing connected
4. It may be useful to use an LR in series with the output, just to try to dampen those oscillations - even though this is not what it's usually for

Honestly, if you want me to find the cause of the oscillations, ship me the amplifier and I'll get to work measuring it. All I have to go on is the info provided.

nosaj - even if you get rid of it, many of these or similar amplifiers exist, and helpful information on solving a similar problem is always good.

It seems I'm getting a lot of flak here without people actually reading correctly what I've written. No thanks for the probable help.

[Mrcloc]

Quote:" The evidence above shows that there is an oscillation when there is no input. ".

When nothing is plugged into the input jack this thing will self oscillate and then burn that resistor. Keep a guitar plugged in and never a problem turning it on.

Just thought I'd point out where I collected that one from.

[Enzo]

It seems I'm getting a lot of flak here without people actually reading correctly what I've written. No thanks for the probable help.

You just got here, get a feel for the place, the people in it, their skill sets and experience.

[J M Fahey]

I see that the burnt resistor looks like the output crossover. It's part of the LM3886 (I know Marshall like to use these) basic schematic. On the datasheet they show a 2.7 ohm resistor.

FWIW Marshall favors TDA729x , a very different beast.
Not sure they ever used LM3886 .
And thatīs not a crossover but a stabilising Zobel Network.

Ok, so what now? That can only blow with the amplifier oscillating tremendously

Yes.

Crap sound is likely because that resistor was responsible for eating the oscillations, and now they're causing the amplifier to clip at low power.

Not exactly.
"Eating" is the wrong word, try "avoiding" instead.

How to fix the oscillations? That's a mystery. Perhaps replace the burnt resistor and C70. Use a 100V polyester capacitor (100nF) and a 5 W wirewound resistor (10 ohm).

Try finding the cause and solving it first.
Over rating the Zobel parts means you have actually NOT solved the problem.

That little 0.25 W thing was never going to cut the mustard.

The mustard here is avoiding oscillations caused by inductive loads, such as speakers and long wiring ... and it does that well.

Pitiful that these big name manufacturers create stuff that's going to blow sooner or later, and use low quality amplifiers and sell millions, where a guy like me, who makes ridiculously high quality stuff can't sell one.

I smell a Conspiracy here.

You could connect an inductor and resistor to the speaker at the speaker like so:


Use 1mm magnet wire and a 2.7 ohm 5 W wirewound resistor. Wind the magnet wire around the resistor, there and back, which makes about 18 windings. That might help.

Good idea and often used.

How to fix the oscillations? That's a mystery.

Well, you might start by building your amplifiers on a proper metallic chassis, grounded of course, that alone works wonders cutting hum, noise, oscillations and RF interference.
If you just mount jacks and pots straight on the wooden cabinet wall, just a couple inches away from speaker and with long leads, I will *expect* instability and noise.

[Mrcloc]

Mr Fahey - a familiar name.

You are being a little nitpicky. We don't all come from the same part of the world, and our language differs a little, I use different ways of speaking sometimes when it really isn't important. I say the Zobel is eating the oscillations because it isn't going to stop the preamplifier from oscillating (or de-stabilizing the power amp), so it's taking the full brunt of the high frequencies - at those frequencies, the speaker's impedance is too high, and the only power-path to ground is through the Zobel.

I have one of these open at the moment, and it is an LM3886. Look at the schematic posted further up as well.

I have said time and again, I couldn't think of the correct name - Zobel network (I have heard referred to as a Zobel crossover), but it's neither here nor there - I'm still talking about the 10 ohm resistor, not the 39k connected to the output as well.

I have said again and again that the cause could be the amplifier itself, but I don't think it is, because the Zobel should stabilize the amplifier by adding an extra pole. I suspect the oscillations to come from the preamplifier. The resistor wouldn't have burnt if it had a better rating, but the "crappy sound" would still have been heard. Based on the available information (as I have said previously), it's likely to be because of the botch of the input connector, but I cannot diagnose the source of oscillations from here. You can't either.

The mystery in fixing the oscillations is finding the cause. This is important, but I have said a good place to start is by replacing the input connector with a proper one. Replacing the burnt resistor and related capacitor is recommended.

Thank you for linking my guitar amp. You will hear no hum in the video, and it was recorded at low volume. It's just construction. It hardly picks up noise from surrounding electronic equipment - much less than almost every single other guitar amplifier I have played on. And it does not oscillate under any circumstance I have tested (I have tested MANY scenarios). The pots and input jack are not mounted on the wood, but on a plate, and everything that should be earthed is. The effects loop jacks are isolated chassis mount connectors (box type). There isn't one op amp inside that amplifier. It uses very robust components as well. I have thrashed it for a few years now, and it's singing like a bird still. In addition, you can plug in or unplug while it's on (and turned up) without the pop.

No conspiracy. I live in a part of the world where the market just isn't there. I have bad business sense, and I don't have any money for marketing. The point is that you would expect a big-name brand like Marshall to have a more stable input, and better quality components.

And (ETA) don't put too much faith in that 0.5 mm sheet metal for low frequency shielding...