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Anyone Try a Marshall JMP with Bass?

#21
chaosMK said:
2 inputs Low/High and 4 6550's in the back.
Click to expand...

Ok, that's a model 2203 - 100W Master Volume amp. You should be able to get a roaring. thick, guitar tone from that amp. A couple of things I would check: check the bias on the power tubes. A too cold bias will make the amp sound thin and harsh. Power supply caps (the big cans on the top of the chassis). If they've been sitting for years without playing, they should be replaced. If you can get your hands on another known good set of 6550s, try swapping them in (bias them correctly, too), and see if that helps. If the tubes are GE branded, they're most likely the originals that came with the amp, so they will be at least 34 years old (1980 was the last year of the JMPs), and while they WILL last a very long time (a LOT longer than CP tubes), if they were used a lot/at high volume for long periods of time, they could very well be worn out. Also, take a look at the preamp tubes. If they're cheapos, or worn out, that will affect the tone of the amp in a big way.

Cheers!
 
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#22
All the the techs I've talked to here and elsewhere say you really want el34's in that amp.

Yes, they sold them in the US with 6550's because of some import or warranty thing at the time and made them work "well enough" but they're missing out on some tone and reliability running 6550's.

Some of our resident techs here could explain it much better than me.

I believe the one I played did have el34's. Didn't look close at all, just felt back there, but they wern't big enough to be 6550's, certainly not coke-bottle ones at least, and the thing certainly wasn't lacking any tone.
 
#23
Yes I ran one for about a year in 1972, It was a great amp, but twice the power and 4x KT88's seduced me to the Ultralinear Major beast of the tribe.:-)
image.jpg

I was young and power hungry.:-)
 
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#26
Well My Parents Had me imprisoned in Kings Canterbury choir school for a while, so I had to play piano whatever instrument I majored which was Oboe, Contra Bassoon and Double Bass.
Did you have the Slade Shiney Jacket and huge Platform Boots too.:)
This version was a pre Photoshop painted negative done by the amazing New York photographer
Stan Shaffer
image.jpg
 
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#29
Hi.

chaosMK said:
After more experiences with the JMP for guitar, neither of our guitarists like it! It's just a little harsh and thin (except extremely loud) for our taste I guess.
Click to expand...

Exactly how the un-modded Marshall sounds with 6550's.

OTOH, by now the tubes (or some of them) are probably not in their best condition, and neither are the filter caps mentioned earlier.
A trip to a tech is recommended before any drastic measures like letting the amp go are taken.
If the tubes test good, only the bias mod shouldn't be more than 1 hours of work, the caps perhaps another, but both should be doable in an hour.

waltdogg said:
Marshall don't run on 6550s, EL34s just weren't surviving the import to the US so American distributors had Marshall change the bias circuit for American tubes and when the amps arrived they were loaded with 6550s.
Click to expand...

Actually, the circuit (when modded) takes 6550's or KT88's just fine.
The tone is different, and some actually prefer 6550's in a Marshall.

AFAIK at first they didn't even change the bias circuit, but rather just slap 6550's in there, and I have a hunch that the JMP in question is one of those ones.

Regards
Sam
 
#30
T-Bird said:
Hi.



Exactly how the un-modded Marshall sounds with 6550's.

OTOH, by now the tubes (or some of them) are probably not in their best condition, and neither are the filter caps mentioned earlier.
A trip to a tech is recommended before any drastic measures like letting the amp go are taken.
If the tubes test good, only the bias mod shouldn't be more than 1 hours of work, the caps perhaps another, but both should be doable in an hour.



Actually, the circuit (when modded) takes 6550's or KT88's just fine.
The tone is different, and some actually prefer 6550's in a Marshall.

AFAIK at first they didn't even change the bias circuit, but rather just slap 6550's in there, and I have a hunch that the JMP in question is one of those ones.

Regards
Sam
Click to expand...

AFAIK, the only change they made when fitting the amps with 6550s was to the bias resistor in series with the bias trim pot, to allow for the range needed to bias the 6550s correctly. Otherwise the amps were exactly the same as their EL-34 counterparts. If anything the 6550s should give a thicker, more "bottomy" sound with more clean headroom. Whenever I hear a thin, harsh sounding, vintage Marshall, the first thing I check for is cold bias.

Cheers!
 
#31
will33 said:
All the the techs I've talked to here and elsewhere say you really want el34's in that amp.

Yes, they sold them in the US with 6550's because of some import or warranty thing at the time and made them work "well enough" but they're missing out on some tone and reliability running 6550's.

Some of our resident techs here could explain it much better than me.

I believe the one I played did have el34's. Didn't look close at all, just felt back there, but they wern't big enough to be 6550's, certainly not coke-bottle ones at least, and the thing certainly wasn't lacking any tone.
Click to expand...

Actually, the Marshall [Invalid or Expired Link Removed] indicate EL34, KT77, or 6550 as output tubes in the JCMs.

"Much of the debate about the difference in tone between the plexi- and aluminium-panel Marshall amps originates from 1974 when Marshall's US distributor had them change all the amps sold in the US and Japan to the much more rugged Link Removed Link Removed instead of the EL34 output tube, with a different sound."
Wikipedia, no citation
 
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#33
The Partridge iron has something to do with it, in the old ones.

Lots of amps get output tube changes over their lifetimes. Some sound better, some not. Different isn't always worse. You can easily use 4 or 5 different output tubes in Marshalls, just depends on taste. EL34/6CA7/KT77/6550/KT88
 
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#34
okcrum said:
The Partridge iron has something to do with it, in the old ones.

Lots of amps get output tube changes over their lifetimes. Some sound better, some not. Different isn't always worse. You can easily use 4 or 5 different output tubes in Marshalls, just depends on taste. EL34/6CA7/KT77/6550/KT88
Click to expand...

Marshalls never came with Partridge transformers from the factory. It was Radio Spares, Drake or Dagnall since day 1. Any Marshall with a Partridge transformer has had it installed after the fact (there may be the odd prototype floating around with different brand transformers, but the production stuff was always the 3 mentioned earlier).

Cheers!
 
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