'77 jmp 2204 gut shots *update w/vid*

[mchn13]

Somebody help a brother out. I dont know half as much as most of you about amps and how they work. I like as few knobs and as much volume as i can get.
I picked this jmp up in a trade locally, and i like the tone(first off), but am obcessed with trying to find out whats been done to it, if anything. If anyone has a referrnce to stock schematics or stock gutshots, pls let me know. Im really stoked about learning more about the workings of this amp, and fully mean to get this thing in its best working order.
Would also like to know if by the pics, anyone could tell me what tubes this model came with in the power amp? Now i have a set of TAD el34's.

Close up of the bias circuit:

Wider shot:

Whole board:

( in this pic you can see the green wires at the top of the board where something looks to be done. Might just be a repair.?) note the yellow resistor? Cap?, on the first pre amp knob.
And a close up of that area by the input and first knob:

Thanks for any interest. Im really stoked to try and learn about my amp. Mind you i will still be taking this to my tech to work on, but maybe ill learn.

Steve

my local guy did me a solid and totally went through this guy and got it back up to speed. switched the bias resistors and went to kt88's. this thing rips now and keeps up with the other marshalls in the playroom lol. been home sick with the damn flu all week, but mad a short video. playings shit but i havent been playing much due to being ill. and its too loud for the iphone mic for sure, and its only on one. i need to get a setup soon for making decent clips..

 

[Anonymous]

good luck with that question.

not a lot of people around here are going to give you pointers out in the open...you're more likely to get the electronics engineer/this is sooooo dangerous, you shouldn't even be looking at it/take it to a tech/if you're asking, you shouldn't even be thinking about it/you need years of training under controlled environments/you're going to die...type responses.

that being said, there ARE other forums out there you can dig around in and find more answers than here with less resistance.

 

[mchn13]

good luck with that question.

not a lot of people around here are going to give you pointers out in the open...you're more likely to get the electronics engineer/this is sooooo dangerous, you shouldn't even be looking at it/take it to a tech/if you're asking, you shouldn't even be thinking about it/you need years of training under controlled environments/you're going to die...type responses.

that being said, there ARE other forums out there you can dig around in and find more answers than here with less resistance.

Honestly i just need the info from people that have seen these circuits before. I dont mind paying my amp guy to fix it for me. But yeah, i get alot of that responce, and not alot of help.
I have started looking other places for info as well.

 

[Anonymous]

Looking at it my advice would be take it to a tech..Some shoddy work has been done in there and needs to be addressed..

 

[glip22]

Looking at it my advice would be take it to a tech..Some shoddy work has been done in there and needs to be addressed..

Some crappy work but it can be reversed and made to look good. Looks like they were attempting to add shielded wire to the grids. They didn't know what they wee doing hence the green wiring on the board.

 

[mchn13]

Thanks for taking a look kurt and glip. Kinda what i was thinking about all those wires on this side of the board. How about that yellow resistor/capacitor thingie on the first knob from the input?
Im gonna take itto someone this week and get this thing straight. This is the info i need so i can tell my local guy what i need done.

 

[londaxe]

From what I can see, it looks like a fair bit has been done to that amp. it's still basically a 2204 circuit it seems

-all coupling caps replaced, mostly with xicons (great sounding caps actually)
-a few resistors replaced with modern carbon films, but it has most of the stock pihers
-mixer bypass cap replaced with what looks like a brown silver mica. it would have probably had a ceramic like the one for the treble cap (probably 470pf)

I can't quite tell what else has been done to it...some rewiring around V1, maybe to decrease noise? a bit sloppy.

there's that sozo capacitor on the preamp gain pot, and I don't think they originally had any cap there

Also, I'm pretty sure it would've had 6550s for the power tubes originally.

for reference:

http://www.amparchives.com/album/Marsha ... index.html

I'd definitely have it gone through either way, doesn't look like the neatest work

 

[mdc1mdc11]

Here a few shots of a 2204 if it helps.

 

[hthomas]

That cap on the gain pot is added. It's a Mojo Dijon. looks like a 1nF. Take it off and see how ya like on/off. Later on 800s used a .0047 there stock I believe.

Do take it to a tech. need to fix some of the sloppy wiring and what not

 

[mchn13]

Bmp

 

[jerrydyer]

he placed an output coupling cap. he alse remove the cap over the 470k res between the first two gain stages and made the bright cap a higher value for more gain.

I disagree with the "you shouldnt even be looking around in there" bs. just be carefull around the filter caps and main switches. dont go poking around in there with your hands.

looks pretty close to stock. I dont remember those yellow box caps like that but ive never owned a 77. they look kinda shiney and new. sort of .

 

[mchn13]

Well those shots were in an earlier post, before he switched everything back to stock, and i wasnt cool with it. The video is after he fixed it with kt88's and its alot better imo.
Im boosting it with my fulldrive II.