kanders1

Hello, I'm new around here, so forgive me if I haven't found the right info yet (yes, I have used the Search feature many times ).

I bought a brand new 1977 Marshall 50W head in 1977 in the US, but I can't seem to find much info on this specific model. I think it's basically a 1987, but unlike the descriptions I've read and the photos I've seen, mine has NO "JMP" written on it anywhere; the back panel just has "50 W" & "MKII" on the right end and "Jim Marshall Amplification..." or something similar in small block print on the left end under the fuses/selectors/speaker jacks. Sorry, I'm at work and don't have it in front of me now. It has the 4 inputs, no master volume (until I had one added later), and the power and standby switches are the small black plastic toggles, not the big illuminated rocker switches I've seen on similar models made in 1977. Can anyone tell me exactly what I have? (Sorry, no pictures available at this time)

Also, I changed the tubes a long time ago, before I know what I know now about biasing, but I don't remember if I bought it with 6550s or EL34s in it (it currently has EL34s), so I'm a bit confused about whether I have the correct resistors for the pair near the bias adjuster (150K vs 220k ?). The photos I've seen of other similar 50-watters' printed circuit boards look pretty different from mine, at least most of the capacitors and some of the resistors. I'm not too bad at reading schematics, and I have a DMM but I can't even FIND two resistors that both read around 150K or 220K

Of course, the master volume mod complicates things too; there appear to be an extra pair of rather large metal-film-type capacitors added to the board that connect to the master volume pots. What would really help me get my head around this would be a photo of the board from the exact same model, without any mods.

Thanks very much for any info on the model and/or PCB, and I'll try to post some photos of it when I get a chance.

Kevin

Marshall Amps

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Mullard Man

I also think it's a 1987.

Only other thing is that it might be from Canada? I've seen Canadian models have toggle switches for power/standby during that time period.

custom53

Sounds like a 1987... Could be a European or Candian model with those switches... The Canadian version usually had the small metal toggles...
Here is my '80 JMP 2203 Canadian version...

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core

From pictures I've seen European and US models had the metal toggles for the non master volume series and Canadian version had black plastic. But this is a transition period and Marshall is not known for consistency. But it does appear to be a 1987 model from what you've described.

I will quote Jon Wilder on the EL34 / 6550 conversion so you can see which yours is. Once you post some photos it will help out the tech guys a lot.

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kanders1

Thanks for the input.

I don't think it would be from Canada; I'm in Houston, TX, about as far away from Canada as you can get in the contiguous states. It has the standard label on one of the transformers that shows the valves to use in the UK (EL34) and the US (6550).

I looked at the back plate again and I misspoke about the writing on it. Only the left 4 or 5" or so of the panel is free from cable/selectors/fuses, and has the following text:


Note that the front panel has no "JMP" or any other text on the left of the power switch.

There are several issues that prompted me to find out more about it:

1. I'm planning on replacing the tubes and caps, haven't decided what to use for power tubes yet. I read (several times) the thread on EL34 vs. 6550 that describes changing or piggybacking the bias resistor. It also discusses whether those other two resistors (I forget what they're called) should be 150K or 220K, but I still can't figure out which resistors those are on my PCB. I want to be sure I set everything up for the type of tubes I buy. They'll probably be EL34s, but I'm considering something with later breakup and better clean tone, since I currently play only at home with a distortion pedal for the high-gain sound (I know, I know, it's like not using the best part of a Marshall, but I don't have a multi-amp setup and I like to switch from clean to wailing overdrive and back - think Eric Johnson).

2. I want to understand what the tech did to add the "master volume" to my amp. It's not that it sounds bad, but I can't tell if it was inserted pre- or post-phase inverter, and I'd like to know if he did it correctly. I may even want to remove the MV.


This weekend, I'll take some pictures and post them. I know you tell much from my description.

Thanks again,
Kevin

core

After some further research I found an example of a 76 model, 2 hole that have the little black switches, which is their transition period for the MKII and it was slated for England as the sticker on the inside stated. Could be the 4 holer model 1959/87 had the black switches as well for the period. I found a 1974 year example like this that was slated for USA. Sorry for the confusion.

On the pedal thing, if you use something like a Boss SD-1 with 0 gain and mostly volume you keep the great Marshall tones that we are all after but you give it a boost that really does add to the amp. These amps take OD pedals really well but distortion pedals is to each person's tastes.

Once you get some pictures hopefully Jon or the other techs can pinpoint where on the board the resistors are and figure out what you have to work with.

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kanders1

hmm... trying to figure out how to insert pictures....

kanders1

OK, let's try attachments:

IMG_0002.jpgIMG_0003.jpg
IMG_0001.jpgIMG_0013.JPG
IMG_0012.JPG

I'll post the guts with my questions separately.

core

Looks like you have a '77 1987 made for England as it says "HOME" on the sticker. It was probably shipped over right after. Seems it was tested Oct 15 1977 from the date on the sticker. Also I don't think US models had a voltage selector and a three prong plugin like this one does. Meaning it should be set up for EL34 like you have.

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charveldan

November is the 11th month, tested 15 nov, 1977'.

core

Oh yeah doh! I had oct in my head cause it my dads birthday lol

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kanders1

Thanks for taking a look, guys. I had a feeling it came with EL34s, because I probably would have been too afraid of changing tube types back when I got new ones.

So the next question is: Can anyone tell if the master volume mod was done correctly from these photos?
IMG_0004.jpgIMG_0018.jpgIMG_0020.jpg

What's the deal with the big dark blue capacitors attached to the two resistors, and shouldn't these resistors be the same value? I'm really having trouble identifying the board's components and have no idea what it should look like without the MV mod.

Also, referring to this thread EL 34's vs. 6550

it looks like the bias range resistor is a 56K, not a 47K, which adds to the confusion. And could someone point out for me the two grid load resistors that are supposed to be 220K for an EL34 amp or 150K for a 6550?

Thanks again,
Kevin

kanders1

OK, I found a resistor color code and I'm guessing that the two resistors connected to the blue caps ARE the grid voltage resistors in question (left is 220K w/5% tol., reads 232, right is 220K w/10% tol., reads 206). If this is correct and the two resistance values are close enough, I'd like to check the bias with my current set of tubes and have some idea if I'll have any problems when I install and bias new EL34s.

So to anyone who may read this, please give me a hint if I'm way off base here, since I'd not like to destroy my amp.

Thanks,
Kevin

Wilder Amplification

@Core...I have no idea why you think "home" meant the amp was sold in England. Moreover, I have no idea how you see it was set up for EL34s as the range resistor in the bias supply is a 56K, which is usually the value used in older Marshalls that are set up for 6550s.

Yes, that master is a PPIMV type (post phase inverter master volume). It's what they call an AC coupled type that uses coupling caps to isolate it from the DC bias and DC plate voltage of the PI circuit so that the pot itself only sees signal. From what I can tell in the pics it appears to have been done correctly.

Now if you really wanna know what valves its set up for, measure your voltage on pin 5 of one of the output valve sockets. If it's more than -50V, it's a 6550 amp. If the range falls somewhere between -38 and -50, it's an EL34 amp.

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core

Glad you cleared that up! I'm mearly going off of how it was set up from the back panel as to where it was going. Of all the US models I've researched I have not seen one with a multi selector for wall voltage and a three prong plugin. But as you say "the only thing consistent about Marshall is their inconsistency." And from those factors and England being where it was made I assumed HOME as in the home country. Never seen that before

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Wilder Amplification

I've got a 2210 model here on the bench for a Dual Classic mod that's got a mains selector on the back. It's a US model set up for 6550s as well.

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kanders1

Thank you so much for looking at this and the explanation for checking the voltage setting.

My amp sat around for years without getting turned on much, and I want to replace the filter caps and tubes and bias it correctly. It must be sorta rare, because I have never found a picture of a PCB that looks like mine (I know they use different brands of caps and resistors, etc.), nor any pictures that look like exactly like it on the outside.

I'm not an amp tech obviously, but I've rewired and modified my strat, built a few electronic gadgets, and can read schematics somewhat, so I feel confident about re-capping, tubing and biasing. So I take it that I need to replace that 56K resistor with a 47K, or piggyback another resistor on it? I've read the details of this in other threads.

Appreciate it.
Kevin

Wilder Amplification

Depends on what your pin 5 voltage adjustment range is. If it's currently between -50 and -60ish, piggyback a 330K resistor onto the 56K and that will give you an equivalent resistance of 47K there.

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kanders1

Thanks Jon, but shouldn't the piggyback be a 280K to get more precisely 47K? (Not a huge difference, I know)

Wilder Amplification

A 330K gets you a value of about 47.9K, which is within 2% of 47K.

Most resistors don't measure EXACTLY their marked value. They measure within a certain percentage of the marked value give or take. That is why most resistors have a tolerance rating, and the more common tolerance rating is +/- 5%, which states that the resistor's MEASURED value will be within +/- 5% of its MARKED value.

To even suggest a 280K to get a value of 46.6K would be splitting hairs at this point.

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kanders1

Yep, like I said - insignificant. I just wanted to be sure my math was correct:

1/R1 + 1/R2 = 1/Rtotal

kanders1

Hey, Jon, what would you say about a pin 5 voltage reading of -34.1, with the bias control "minned" out, i.e. I can adjust it from about -26 to -34 and it won't go any lower? Doesn't that seem contradictory to the expected voltage range with a 56K resistor?

I think I'm doing this correctly, but maybe not. I took the pin 5 readings with the power tubes removed and the amp on standby only. Correct?

kbhe

It might be a good idea to try a different voltmeter,just for fun!
Good luck.

MartyStrat54

Yeah, don't be using a cheap multimeter. I threw my cheap one's in the thrash. One was brand new and it was reading the bias voltage all wrong. Junk. I have two real nice one's, but I got lazy and used the cheap one. Big mistake.

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kbhe

This schematic from 1981 jcm service manual.Bias for el34 (-30volts) with 365 volts at plate.What's up with that!

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jcm 50w 001.jpg (93.1 KB, 14 views)

kbhe

Or use bias probe and set current with new tubes.
Have fun!

kanders1

Thanks for the input, guys. I think my Sperry DMM is accurate enough at measuring voltage and resistance; it also has a fused amperage mode which I've never used.

However, there is more to this story than this thread. Please see Biasing - pin 5 voltage range question also.

That schematic for the JCM 800 series is interesting. I wonder if my amp was built in some transitionary phase that your schematic more closely represents; I may have to do a crosscheck.

fourdogslong

Hey I think I have pretty much the same amp.
No JMP written on the front panel, 2 toggle switches, mine are metal though, and on the back panel it's written MKII Made In England...
On the inside of the chassis it's written Bass instead of lead.

Is it a JMP?
When I bought it I compared it to a JMP SuperLead if I remember well and mine as so much more balls than the Superlead had, it doesn't crunch as smootly though, it sounds more dirty.