Marshall 1972 50w Lead back in the studio

soundngin

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Hi new here just looking for some thoughts on servicing this amp I have. I acquired it years ago, more like decades ago, back in the early 90’s. It was handed down to me from a bandmate when we were starting up a recording studio. I remember then that it developed a problem and was unstable. So I put it aside thinking I would repair it one day. One thing led to another and it sat for a very long time. So now the time has come and I would like to put it back in service to use for studio recordings.

Amp Tag.jpg


Looking at the amp tag, I believe it to be a 1972, some type of 1987 50w lead US model. Back when the amp quit working, it was described to me that the amp was “power surging”, whatever that may mean. So recently, I dusted it off and had a look at it. It had a blow 1/2 amp fuse (H.T.). I replaced the fuse, plugged the amp into a speaker cabinet and turned the amp on with the standby engaged. All the tubes looked to heat up nicely. Once I took it out of standby, one of the power tubes glowed red like it was going to catch fire. So I shut it off immediately.



I removed the amp from the head cabinet and found a loose lead on one of the 2 big drop caps. The brown one shown in the photo and the pen pointing to the loose joint. That probably didn’t have anything to do with anything but I re-soldered the joint. I think I read somewhere where people are removing these 2 capacitors for some reason. Which brings me to question #1, should I remove these? But let me finish what happened next.
2 caps.jpg

So I purchased 2 new matching EL34 power tubes. These are rated normal distortion or Medium, claiming to be best for all around tone. I also purchased a bag of 1 ohm, ¼ watt resistors and soldered 1 each between pins 1+8 and ground on each power tubes mounting socket. This was to, as many of you know, facilitate a means to calculate the tubes current draw for biasing the tubes. I learned how to do this by watching a video on YouTube. For biasing, I did the math. For me I wanted to bias at 60%. EL 34’s are 25 watt tubes. So .6 x 25 = 15 watts then divided by 438 volts measured off tube pin 3, gave me .034 or 34 milliamps of bias I should have. The bias setting was already pretty close at 36.6 mV across the 1 ohm resistor but I adjusted the bias pot till I got a reading of 31 mV DC on one power tube biasing resistor and 35 mV DC on the other.


Anyway, so far so good, the amp fires up with no flaming power tube. The amp is loud and bright! The guitar is going straight in to input 1. But there is some sort of interference going on. You hear it when the amp sits idle and every so often, maybe like a couple times a minute. It sounds like if you were listening to a radio station and lightening is causing interference. So the amp sounds normal till the interference occurs. I tapped on the tubes one by one but they all had created the same noise, except for maybe one of the power tubes causing a slight bit more noise but it was difficult to tell for sure.


So this is where I stand at the moment. My goal is to give this amp a little bit of the limelight I think it deserves in the studio. I think this would make my friend happy as this is why he originally left it to me.


Is the amp all original? Mostly, but no it is not. Back in the day my friend picked up on some sort of mod idea from somewhere and I faintly remember him mentioning something to do with diodes as if it was some secret weapon? It was a very long time ago so that could be inaccurate. But I do remember he took it to have send and receive jacks installed. There is a pair of diodes soldered in series shunting across power tube pins 3 & 8. Both power tube sockets have this pair of diodes. They don’t look original. I don’t believe any of the tubes look original. Those 2 capacitors mentioned about don’t look original either. Everything else looks untouched but I feel its beyond the idea of worrying about keeping it original as I want to put it to use. The are certain guitarist that come thru where I would like to offer this amp for recording.


This is where I have a bunch of questions as to how far I should take the maintenance. I know the 2 prong plug has to go as well as the death cap. Then there is the question of the 3 big 50 / 500 filter cans. The don’t look to be bulging but maybe its time anyway? And what about the big brown and dark blue .1 caps that are tied together on one end as talked about above? I’m thinking 3 new preamp tubes might not be a bad idea.


The amp is super bright using channel 1. I heard where you can jumper the inputs in some way but I didn’t try it yet. Is there any harm in this? I think you are supposed to get a tone between the bright channel and the second. I am interested in this. When my friend used to use this amp, he used a Marshall series 9000 preamp plugged in to the return. He could get all sorts of drive and EQ. He would tell me that by using the return on the amp you would be bypassing the preamp section and only use power section.


Anyway, this posting turned out longer than I had intended and if you made it this far thank you for taking the time.


If anyone has any answers they would be kind enough to share, it would be great to hear. I just have basic knowledge of electronics so any ideas on how far I should go to restore this thing would be great. I at least want to eliminate the interference noise and replace the cord.


Ron
 

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Dogs of Doom

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can you take more pictures?

a good shot of the inside of the chassis, all in one shot
a good shot of the front & back panels

does it have an effects loop? that wouldn't be stock either.

but, yeah, take some pics. take them of anything that might be of interest, on the inside...
 

South Park

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Those two caps are a snubber circuit that was put in thar to quiet things down . That interference could a bad solder greetings
 

Matthews Guitars

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That interference sounds like you're describing what happens when you get a cell phone too close to most amps. An intermittent buzzing sound. Easy way to check. Bring your cell phone closer to the amp....and farther away. Make a call while doing this.

It's a shame your amp has been modded. It's a SUPER clean example, with the cleanest, whitest inspection tag I've ever seen for an amp that old, and the finish on the chassis and transformers is close to like new.
 

Chris-in-LA

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Yes, a good chance that the crackling is a bad solder joint. Try tapping on every component and solder joint you see with a wooden chopstick while the amp is running. The loose joint may reveal itself.
 

Spanngitter

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Seems to be a quite clean and pristine '72 1987 LEAD....beside the cold solder joints I would suspect the DALY Filter Caps being EOL after ~50yrs , even when sitting in a closet for a long time, same for Bias Caps.
If you can afford I would give that arp to an knowledgeable tech who checks it out and restores (not repairs) this unit as the chance that a learning by doing might create you a quite large door stopper is high...
 

neikeel

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Given the amount of standing unused and a basic first start up I think that the filter cans are probably going to be past their best. I tend to err on side of caution with bias caps too. Those snubbers next to the rectifier can go in the bin (no need to replace).
Some more pics would be really helpful (sockets/diodes you talk of).
Limited pics so far look like it is in really good condition.
 

George Dickens

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For me I would just give it a good cleaning, and if it needs it, repair it back to original spec and use it.
If it gets too noisey or hummy then look into fixing the filters.
Those things can last and theyll let you know when they need attention.
 
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Spanngitter

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Those things can last and theyll let you know when they need attention.

yep, most likely with a quite massive boom.
Those Dale are better then the LCR had been ever but after +50 years they are way past their expected and designed life cycle. If you want to keep the amp as a collectors item leave them in, if you want a playable unit replace them.
 

black knight

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The amp is super bright using channel 1. I heard where you can jumper the inputs in some way but I didn’t try it yet. Is there any harm in this?

I t should be bright being a lead spec amp, especially if the volume bypass and tone caps are original to the amp, so no worries there. What level did you manage to get the volume knob up to? The rest of the frequencies catch up once you reach 3 or 4 on the knob and that's where the magic starts to happen. And no harm at all jumpering. Easiest way is to just use a short cable going from input of your choice to input of your choice. Some folks use a Y cable. You can even throw an effect in-between. Try and experiment...the fun never ends with these amps.

I would change the filter caps. It won't ever be a museum piece with the loop added. The other mods can easily be reversed.

More pictures will help. And welcome to the forum! :welcome:
 

Matthews Guitars

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Let's clear the air about the Daly filter capacitors..

Yes, they are of phenomenal quality. They are on the grade of quality of CDE computer grade capacitors.

No, they won't last forever.

I was "raised" by my amp guru/friend/mentor to believe that electrolytic capacitors have "Five year shelf life, ten year operating life", and to recommend filter capacitor replacement every ten years.

In the case of cheap run of the mill capacitors like Illinois Capacitors (IC) I stand by that recommendation.

But by virtue of both personal observation and some detailed reading, specifically of a 22 page White Paper published by Cornell-Dubilier (CDE), concerning the design, construction, care, and feeding of aluminum electrolytic capacitors, I have revised my guidance to follow that of the White Paper.

The conditions for LONGEST and SHORTEST useful lifespans of aluminum electrolytic capacitors are as follows:

LONGEST lifespan: Keep at cool temperatures well below that of the temperature rating of the capacitor, but above its lowest rated operating temperature. Keep it charged up at full rated operating voltage.

SHORTEST lifespan conditions: Overheat, overvoltage, operate in freezing conditions. Those are abusive conditions.

SHORTENED lifespan conditions: Infrequent operation at rated voltage. Spends long periods of time without power applied.

USAGE UNDER NORMAL OPERATING CONDITIONS PROMOTES A LONGER LIFESPAN.

Signs of a failing capacitor: High ESR, low measured capacitance (out of tolerance), shrinkage of the shrink wrapped plastic label beyond its "standard' size, higher than normal temperature of the canister under normal operating conditions, swelling, leakage of electrolyte out of the vent seal, bubbled vent seal, loss of the black insulator disc on top of the cap body due to label shrinkage. Electrical hum or crackling or both.

A lot of 50 year old Marshalls still have their original Daly capacitors and they're STILL GOOD because of the initial quality of the Daly capacitors and the usage
pattern, which has them powered up at operating voltage frequently enough for good maintenance of the anodization layers on the plates.

The mechanism of failure of an aluminum electrolytic capacitor:

The capacitor consists of two aluminum foil plates separated by a paper dielectric saturated in a dielectric chemical compound, rolled up tightly, with electrodes connecting it to the capacitor terminals.

After making the capacitor, controlled voltages are applied to the plates which causes the plates to form an anodized layer which is electrically insulative, and gives the capacitor its ability to operate at high voltages without failing.

Normal usage maintains the integrity of the critically important anodized layer as the applied voltage activates the anodization process wherever there is a weakness in the anodized layer.

But, the very same slightly chemistry of the electrolyte that promotes the formation of the anodization layer will over time become slightly caustic, and slowly attacks that same anodized layer when the capacitor is not under power.

Over an extended period of time, this starts to break down the anodized layer to the point that the capacitor's plate-to-plate insulation characteristcs are compromised.

When the electrolyte has lost its chemical stability and can no longer function properly as an electrolyte or to help form and maintain the anodization layer, the capacitor has reached end of life.

So, there is not any reason to AUTOMATICALLY replace those capacitors. Not without a definable reason, a condition that I've mentioned.

Those capacitors in that amp have the outward appearance of ones that have plenty of life left. The shrink wrapped labels are at full size and proper position. The black discs are present and there is no visible sign of a problem.

Leave them in in the absence of an observed problem.
 

soundngin

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Guys, today has been an amazing day for me. Thank you for all the replies as every one of them is taken with great consideration and I am grateful. I know that sounds a bit overly chummy but after having this amp sit for 30 years in need of repair and after 1 day of consulting with the Marshall forum and finding out what is wrong with it, I had a "um you got to be kidding" moment. And to boot, the culprit is not even the cause of the construction of the amp itself but of the mod done to it. It is a testament to how well they built these amps. The amp now, as is, sounds fantastic. It now has a very pleasing "hiss" when the volume is turned up. I'm not getting much in the sense of any hum unless I crank the knobs up to ridiculously loud at which point you get some. I can't believe how simple of a fix this was. No solder required. After finding it, it was an "of course!" and joyous moment.

As to your replies, they helped me get it done. After reading and thinking about what I should do, I learned about the snubber circuit. I contemplated trying to find a local tech and wished I had one. I now feel confident in jumping the inputs, and to be mindful that there are characteristics about how much volume to drive the amp for the best sound. Not to mention the info on filter capacitors and their lifespan. Thank you for this as I don't feel so in the dark. If they fail in the future, well ok, but let's give them a chance and see how far they go for arguments sake. 50 years and counting. I just realized the amp turns 50 at the end of next month. I don't know much about Thelma, Rosemary, Joyce, Anna? and the others, but I'd like to shake their hand.

At first I thought the idea of using a wooden chopstick may possible work as recommended, but I have to admit I was a bit skeptical. But this is what I ended up doing. Everywhere I tapped seemed to cause varying levels of interference. I thought, there has got to be a loose connection. As I went around the entire circuitry, I came to a certain component that had me think "oh wait a second, what about that?" Yep, that's it.

Without the amp in front of you, it's not so obvious. But I bet someone knows what it is, with the clues I have given.

I owe pictures but it is late and I will get them up soon. I will of course get a picture up of the offending component.

As for now, I am in disbelieve how well this vintage Marshall is performing. I think my goal now has shifted towards getting in some recordings to have for reference with the amp as is for now. Although I don't have any projects set up for now, that is now my next mission.

Ron
 

soundngin

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The photos as promised. I have more but they were rejected for being too large. I will try cropping them and reposting. The last photo contains the failed component. You have to look closely.

inside chassis full.jpg

power and switches.jpg

send and return mod.jpg
 

soundngin

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I just had to reduce the image quality to post these images.

Front view.jpg
rear view.jpg

diodes mod.jpg So here are those diodes placed across the power tubes. What could these be for? I have some schematics I found online but I have yet to determine which one matches the amp. I don't see any diodes like this in any of the circuits. I have to look for what I thought was the documentation for when the amp was modified but it was around the year 1987 plus or minus a few years. My friend was heavily influenced by Eric Clapton, Stevie Ray Vaughan and with the Blues. Not sure where he got the idea but I believe it was from a magazine. I'm thinking the mod might have been done by a shop in New Hope Pennsylvania.
 

Matthews Guitars

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lUse Paint to resize the images. This keeps the quality up, but the resolution is still good.

Diodes across power tubes act as snubbers.
 

soundngin

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Matthews Guitars That is interesting! Ok so maybe that is a good thing for the sake of recording. So for some reason these were installed but I'm not exactly sure what the motive was. The one thing on this amp that I have to be careful is the speaker ohm selector knob. It feels tight but it feels like if you pull it out only a small amount like 1/8 inch, you lose contact. It works fine but I'm thinking at some point I might want to see if there is a replacement. I have a couple of different cabs, 1 running at 8 ohms and 1 at 16 that I might swap around with in the studio.
 

Matthews Guitars

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Those impedance selectors are an issue. Failure of them has taken out more than a few output transformers.

I'm not the only person who recommends removing them and replacing them with a high quality 3 position rotary switch that is rated to carry the full power of the speaker circuit. This can be done in a way that does not alter the chassis.

I use 80 dollar mil-spec Cole rotary switches for that mod. Switches I pay 15 dollars each for, as they're surplus. Totally a bargain!
 

soundngin

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Noted on the high quality Cole rotary. I wasn't aware of these Thank you! Wouldn't mind spending for the quality and peace of mind.
 

Matthews Guitars

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Here's a pic of one I installed in my '73 Superlead.
 

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stickyfinger

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Those are flyback diodes and will help save the tube/ output transformer in case of voltage spikes. Don't really know much about them but I build Trainwrecks and that's how Ken made his amps.

The amp is a beauty minus the holes in the back. Don't mod the amp or replace the selector. They really don't just fall out. Amps this clean are uncommon and would recommend leaving it as is.

Installing a grounded power cord for safety is about the only thing I would do.
 

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