1975 4-hole 50w Lead USA - schematic?

[mattoreg]

I've got a four hole 50w lead, can someone point me to the correct wiring schematic? Year 1975 "G" serial. There seems to be a ton of schematics out there, I can't figure out which one is accurate for my head.

This amp is gainy as hell, I think something maybe modded...

Thank you,
Matt

 

[john l]

Hi,
That should be a model 1987 and if its gainy then yes somethings probably been tinkered with. Pics of the board?

Does it say MKII on the back panel?
If so then here

http://www.drtube.com/schematics/marshall/1987mk2u.gif

and here

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=5&cad=rja&sqi=2&ved=0CDcQFjAE&url=http%3A%2F%2Fmetroamp.com%2Fwiki%2Findex.php%2F1987_chassis_layout&ei=fdeOUr3GHLHwiQLKyoHoCA&usg=AFQjCNFQ_JDEppxFf-VvWTwKDFqi1T6FPA&sig2=Bav4FuCfVFqwH1PAK5bfBA&bvm=bv.56988011,d.cGE

 

[mickeydg5]

Hi Mattoreg
Are you versed in reading schematics, opening amplifiers and dealing with high voltages?

Other schematics for that amplifier can be found at drtube as well.

 

[mattoreg]

John
Yep, it says MKII on the back. It doesn't say model 1987 anywhere. Thanks for your help!

Mickey,
No I don't know how to work on amps really, I was hoping to track down the correct schematic so I can bring it to my amp tech. I figure that would be best.

cheers!

 

[mattoreg]

Hi,
That should be a model 1987 and if its gainy then yes somethings probably been tinkered with. Pics of the board?

Does it say MKII on the back panel?
If so then here

http://www.drtube.com/schematics/marshall/1987mk2u.gif

and here

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=5&cad=rja&sqi=2&ved=0CDcQFjAE&url=http%3A%2F%2Fmetroamp.com%2Fwiki%2Findex.php%2F1987_chassis_layout&ei=fdeOUr3GHLHwiQLKyoHoCA&usg=AFQjCNFQ_JDEppxFf-VvWTwKDFqi1T6FPA&sig2=Bav4FuCfVFqwH1PAK5bfBA&bvm=bv.56988011,d.cGE

The metro amp link says its cerca 69. The drtube says 70. Will the values and such be the same on mine? (seeing that mine was built ~5 years later?)

Thank you guys.

 

[john l]

If it says MKII on the back and its a 50w head then you have a metal face PCB version of the plexi in the layout unless Im mistaken (someone please correct me if Im wrong so I don't give out bunk info).

One difference that does come to mind is the mains filtering Ive seen on some of these.Some have a dual 50uf and some (mine and a few others Ive seen) have a single section 250uf.

Great amps however I absolutely despise their PCBs. Their pads and traces are totally exposed on one side so are always dirty and have some corrosion (dirty joints abound if not cleaned well). Theyre very easy to over heat, lift and break so make sure to take it to a guy that knows his stuff and is ready to address this since you think its been modded.

Theyre very solid amps theyre just a bitch to work on and fix (for me anyway) although the few I have been inside were pretty butchard so take this with a grain if salt. The best tone Ive ever heard to date came from a bone stock one of these so enjoy it!

 

[mickeydg5]

The MetroAmp layout is based on a 1969 year amplifier.
The schematic date is just general and used for 70's JMP amplifiers.

The MKII schematic reference the metal face JMP 1987 model which came with 6550 power tubes.

Mattoreg
Your amplifier should have 6550 tubes. If it were swapped to EL34 tubes you will have to reference the other (non MKII) 1987 model schematic.

 

[damienbeale]

Mattoreg
Your amplifier should have 6550 tubes. If it were swapped to EL34 tubes you will have to reference the other (non MKII) 1987 model schematic.

This is of course assuming that the OP is in the US, or has a US spec amp at least. European specs of course have a mains selector, and use EL34 tubes.


A '75 amp will vary slightly from a '70. There were subtle variations that came in. Differences across the eras such as vol bypass caps, negative feedback, and various differences in bypass caps through the gain stages add up to quite different sounding and feeling amps especially given the '72 onward amplifiers having PT's with lower voltage, adding more compression and earlier power tube breakup, not to mention that the pre-amp usually has slightly lower voltages too.

It's not unusual for these later amps to be gainy aggressive beasts.

 

[mickeydg5]

I thought MKII was an American/US thing designating the difference since only we had the 6550 tubes.

 

[damienbeale]

I thought MKII was an American/US thing designating the difference since only we had the 6550 tubes.

No. They're all MkII's.

 

[mickeydg5]

Ah my idiocy.
It must be that I always see MKII on the 6550 schematics and associated with that.

 

[john l]

This is of course assuming that the OP is in the US, or has a US spec amp at least. European specs of course have a mains selector, and use EL34 tubes.


A '75 amp will vary slightly from a '70. There were subtle variations that came in. Differences across the eras such as vol bypass caps, negative feedback, and various differences in bypass caps through the gain stages add up to quite different sounding and feeling amps especially given the '72 onward amplifiers having PT's with lower voltage, adding more compression and earlier power tube breakup, not to mention that the pre-amp usually has slightly lower voltages too.

It's not unusual for these later amps to be gainy aggressive beasts.

Great points. The devil is in the details with old marshals. I referenced the metro layout because my 74' was just like it to the letter (with the exception of the odd ball mains filter I mentioned above and now that im remembering a bit better than this morning mine also had the old 5k/.68uf presence circuit as well).

As far as the gain thing goes I guess its all in what you consider gainy. Ive played quite a few of these and have always had to boost the hell out of them to get to what I consider aggressive.

 

[john l]

Heres a site with some general info and some good pics.

Marshall JMP 50 Lead (1987 model) | It-11 Audio / Tonegeek

 

[mattoreg]

Hey thanks guys, when i bought it the amp was set up with el34s even thought the transformer stated "only use 6550s". Had it altered slightly to accept the 6550s and be biased correctly, added a diode? To the bias circuit. Forgive my ignorance i dont know what the small little tubular thing was he soldered in is truly called. With all that said, I'm assuming the amp was built for the states... and thus the USA on the paper chassis label. And yes, metal face.

To explain what i say is gainy. Channel II is perfect, 0 is 0, 10 is full on tube saturation. Everything in between i as you would expect. Channel I is 0 to 3 clean to aggressive clean, beyond 3 it's full on saturation beast!

Please steer me in the right direction, I'm gathering that this isn't normal... I was hoping to take a clear diagram/schematic to my tech and have him pour over it... what is the right schematic that my amp should follow?

Thx
Matt

 

[damienbeale]

That IS normal. That's exactly what the bypass capacitor on the volume pot allows. Try a 1000pf instead of the 4700pf if it is too much for you.
All your tech would have needed to add would be one resistor, or possibly three if it had the proper conversion to EL34's beforehand.

 

[mattoreg]

Thanks for all the good info.
Matt

 

[mattoreg]

To catch anyone up who may be watching. Unsoldered one leg off the bright cap on the channel A volume pot. Made all the difference in the world. Sounds amazing. Thanks!

 

[hotpockitrockit]

I've got a four hole 50w lead, can someone point me to the correct wiring schematic? Year 1975 "G" serial. There seems to be a ton of schematics out there, I can't figure out which one is accurate for my head.

This amp is gainy as hell, I think something maybe modded...

Thank you,
Matt

I have a 76 MKII and I agree, its gainy as hell. if I want to play AC/DC type riffs, my ch1 is on 4.5 or so and still seems like too much gain, and that is with no bright cap