Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TubeGeek on March 20, 2011, 01:21:51 am
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I have a JMP 2204 on the bench right now. It's been a while since I've worked on a Marshall.
The amp came to me from a guy that scored this amp in a purple tolex cabinet at a pawn shop for $150. Lucky guy.
Looks like someone has worked on the amp before. It has JJ filter caps. I noticed the input section/jacks are not wired correctly. The owner said the low input never has worked.
What I am thinking to do is replace all the caps on the board, the electrolytics on the board, install new tubes and set it up.
Any mods that are recommended? I have a feeling this amp is going to rock. :headbang:
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That amp will rock! The only problems I've run into with Marshalls of that era are blown output transformers and broken Cliffs Jacks. If neither of those are an issue then it's great amp!
I actually don't recommend any major mods to it, leave the caps on the board except the electrolytics (even those are probably OK still) Return it to stock. The 2204 originally would have had one less gain stage in "Low" than "High" that was switched by the jacks.
It is what it is and it's a phenomenal hard rock amp! If more headroom is desired, try 12AU7 in V1 they sound good (12AY7 is a good choice too).
Clean it up, match it up to the original specs and let it rip!
j.
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+1 with bender. Why on earth would you change parts that are not prone to failure? Clean her up a little and get her as close to stock as possible.
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+2 with you guys... Fix what's broken.
Yup, replaced my shaare of OT's too.
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The 2204 originally would have had one less gain stage in "Low" than "High" that was switched by the jacks.
I had a look at the schematic because I wanted to see how they did it. I don't see it. Am I looking at the wrong schematic?
/jmp_mastervol_50w_2204u.pdf on schematic heaven is what I am looking at.
Dave
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Look on page 6...
http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf
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Steve, Great scrapbook.... I think I'm going to have to add some of this to my binder!
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Ditto
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Steve, those are nice tutorials. Pictures truly are worth a thousand + words. That needs to be in everyone's library. I'm inspired.
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Got it. Thanks.
Dave
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Y'all take any of that stuff you want. That's why I put it out there.
Most all that stuff comes up occasionally and has been posted here before. I had a bunch of one page Visio drawings. Last year while cleaning up some old HD clutter, I came up with the idea to just put all those little sniplets into a single 'scrapbook' file. I need to update it if I can ever remember when I'm at home.
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Oh yeah, those are great sounding amps. I fooled around with mine a few times and always returned it to bone stock values (minus the bright cap).
Thanks for the scrapbook 'summary' Steve. I read pictures pretty well :icon_biggrin:--printed and in my amp binder notes. Regards
dennis
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They are indeed amazing sounding amps.
I have a 2204 that purports to be from 1977, but looks somewhat different inside and out. It has rocker switches rather than toggles, and the rectifier board is not separate. Anyone know when that changed?
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I have a quick question about the schematic diagram for the Marshall JCM-800 2204 Channel Inputs on page 6. http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf
I understand the arrow after the first coupling cap, pointing to the low jack, indicates it makes a connection when nothing is plugged into the low jack, but what does the other arrow connected to the same point, pointing to the hi jack's, sleeve indicate?
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but what does the arrow connected to the hi jack indicate?
There are two arrows on the hi jack. They are both switch contacts, one for the tip and one for the sleeve. The switches are both closed when nothing is plugged in.
If you have a 4 lug Cliff jack you'll easily see the operation of the 2 switches. You may even be able to tell just from a good picture.
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Thanks.
I get it now. I can picture it and know exactally what you mean.
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I have the 100w version JMP Lead 1976 model I think. Agree with all comments fix only what needs to be fixed. If the OT needs replacing I had to replace mine about 4 months ago and I went round and round with which to put in because I didn't want to alter the way it sounded and went with a Mercury Tone Clone. If I knew the sound would have been this good I would have done it years ago. I have heard pros and cons about them but in my JMP it was like night and day for the better. Just my $.02
And Slucky great info thanks for sharing it has already helped me a few times already.
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Finally got time to work on this amp. I have re-wired the input jacks, the previous tech had some funky wiring going on.
I replaced only the board electrolytic 10uF caps.
I installed the correct value grid resistors, 5k6.
Can someone verify that the impedance selector switch is wired correctly. I have a feeling it is not...
Orange is common
Yellow is 4 ohm
Green is 8ohm
Gray is 16 ohm (taped off in this amp)
What I don't seem to fully comprehend is how these stinking switches are wired. :help:
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Check this:
http://www.el34world.com/Transformers/marshall.htm (http://www.el34world.com/Transformers/marshall.htm)
Seems itīs ok in your amp.
Those Rotary selectors are funny. I saw a power transformer blow up. I friend of mine connected a 1973 plexi wrong.... 120v primary in a 220 wall wart....... :sad2: :cussing:
Iīll use the DM to check continuity and identify who connects who.
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Yeah I did check the transformer page before posting. Thanks.
I did check continuity also but there is continuity on multiple lugs. The wiring is probably ok, I just want to confirm and figure out how these switches work.
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The low resistance of the secondary windings makes this very difficult. You'll have to disconnect the yellow and green OT wires from the selector. (I'd snip them but leave enough colored wire on the selector terminals to identify them.) Now you can use your ohm meter to verify the selector and jack connections.
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The low resistance of the secondary windings makes this very difficult. You'll have to disconnect the yellow and green OT wires from the selector. (I'd snip them but leave enough colored wire on the selector terminals to identify them.) Now you can use your ohm meter to verify the selector and jack connections.
Exactly what I did do. Makes sense.
Busy days ahead of me...had a Magnatone M10, two old bogen pa's, fender hot rod deville, hughes and kettner amp, a SF princeton, fender blues deville and an orange rockerverb 50 all come in for repair today. :help: :laugh:
BTW this Marshall sounds awesome for rock. I'll be using the amp on a recording tonight. :m11
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had a Magnatone M10, two old bogen pa's, fender hot rod deville, hughes and kettner amp, a SF princeton, fender blues deville and an orange rockerverb 50 all come in for repair today.
Glad I have an easy job! :wink: Good luck.
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simonallaway, I have an early '77 2204 which was modded to a 2203 preamp before I got it. It looks like this, see pics. I think yours is a '77 too, most likely.
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simonallaway, I have an early '77 2204 which was modded to a 2203 preamp before I got it. It looks like this, see pics. I think yours is a '77 too, most likely.
Indeed mine is a '77. Yours has a useful strip of turrets for the transformer wiring, which mine doesn't. Otherwise it looks identical.
I'd say these 2204s are just perfect for 'classic rock'. I got mine so very cheap back in the late 80's when nobody wanted them. I dread to think how much I'd pay to replace it retail (obviously I could build one for perhaps $800).
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late 70's are awesome amps. and to make them higher gain really only requires a cap over the gain pot. people spend all this money modding a great amp and all you have to do try a few caps like 0027 0033 0068 etc. Makes em rock like mad.
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Probably my favorite amp. Nothing but RAWK! I concur with everyone, and am stoked you left all the goods and just replaced the electrolytics on the board. That is a cool amp. I built my main gig amp based on the 50wt version. But nothing beats the 100watter.
Regards,
Megachunk
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late 70's are awesome amps. and to make them higher gain really only requires a cap over the gain pot. people spend all this money modding a great amp and all you have to do try a few caps like 0027 0033 0068 etc. Makes em rock like mad.
When you say "over the gain pot" do you mean from wiper to output like a bright cap?
Thanks
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late 70's are awesome amps. and to make them higher gain really only requires a cap over the gain pot. people spend all this money modding a great amp and all you have to do try a few caps like 0027 0033 0068 etc. Makes em rock like mad.
When you say "over the gain pot" do you mean from wiper to output like a bright cap?
Thanks
I think he means the cap from the wiper of the MV going to the PI. BTW PPIMV can be a nice addition to these. Its cool to have both MV's.
My favorite amp for rock ever built - I like building them without the terminal board and do it up like Carr does with Terminal strips, then right to the pins. Makes ground easier and cleaner too andeasy to do local nodes, then run them to the star making sure to isolate inputs. Here's a good link showing input wiring if you need it:
http://www.dreamtone.org/JCM.htm (http://www.dreamtone.org/JCM.htm)