1979 JMP 2204 - What's been modded?

[rycack]

Hey All,

I picked up a '79 JMP 2204 a few years ago and just got curious enough to take a look at the guts...

The story I got from the previous owner is that he picked it up from the original owner and it was "mostly stock"

I don't know a whole lot about Marshall mods, but at minimum it looks like it's had the EL34 conversion done (currently has EH 6CA7 power tubes) and it definitely looks like the OT has been replaced

Any additional info that can be provided is MUCH appreciated!

 

[dtier]

It has been messed with but appears to be back to stock. There is some sloppy work but should be working fine. The first two 100k plate resistors and the yellow coupling caps are not original. The input lead and grid stopper to the first stage have been relocated which is an improvement. The mid pot has been changed. You might want to check it's value. It should be a B25k linear taper. Some people try a 50k there.

The mains power supply transformer is the one that looks to be changed. I am including a link to a similar amp that shows what it looked like originally including the original transformer.

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[rycack]

It has been messed with but appears to be back to stock. There is some sloppy work but should be working fine. The first two 100k plate resistors and the yellow coupling caps are not original. The input lead and grid stopper to the first stage have been relocated which is an improvement. The mid pot has been changed. You might want to check it's value. It should be a B25k linear taper. Some people try a 50k there.

The mains power supply transformer is the one that looks to be changed. I am including a link to a similar amp that shows what it looked like originally including the original transformer.

Thanks for taking a look and for the info!

Now I guess it's time for me to go down a rabbit hole about these circuits and try to learn how they work

 

[Spanngitter]

Huh, I went thru quite a lot of Marshalls but never saw a blue PCB....anyway, seems fairly stock (as @rycack already mentioned) but absolutely due for a cap job.
Those Frako are fragile and surely reached their end of life and it seems also to have the black Filter Caps which had been briefly used between DALY and LCRs and in all of those early 2203 and 2204 Vertical Input Version I have worked on, they had been exhausted and also due to be replaced.
The Piher Bias Pot also shows typical crystallization for this vintage, I would recommend to swap it and also remove the piggy backed resistor and install a proper value.
Be aware that these amps have a fairly low B+ (usually only ~380V) and can sound quite nicely brown and it seems like the mods in the preamp are pointing into this direction, so you should be able to get a decent Eddie out of them...

 

[dtier]

Huh, I went thru quite a lot of Marshalls but never saw a blue PCB....anyway, seems fairly stock (as @rycack already mentioned) but absolutely due for a cap job.
Those Frako are fragile and surely reached their end of life and it seems also to have the black Filter Caps which had been briefly used between DALY and LCRs and in all of those early 2203 and 2204 Vertical Input Version I have worked on, they had been exhausted and also due to be replaced.
The Piher Bias Pot also shows typical crystallization for this vintage, I would recommend to swap it and also remove the piggy backed resistor and install a proper value.
Be aware that these amps have a fairly low B+ (usually only ~380V) and can sound quite nicely brown and it seems like the mods in the preamp are pointing into this direction, so you should be able to get a decent Eddie out of them...

I did a double take on the blue board too. I'm not sure what the B+ voltage is with the replacement power transformer, 1202 324/2, is but it sounds like around 400vdc which is pretty close to the original.

Thanks for taking a look and for the info!

Now I guess it's time for me to go down a rabbit hole about these circuits and try to learn how they work

This amp should sound awesome stock, one of my favorites, and not need any mods in my opinion but there are a lot of them out there. Cleaning up the board and replacing the caps as mentioned should be all that's needed. It would be good to replace the base support bracket for the power transformer, it's missing parts. They are available and needed as it is quite heavy.

 

[Spanngitter]

I did a double take on the blue board too. I'm not sure what the B+ voltage is with the replacement power transformer, 1202 324/2, is but it sounds like around 400vdc which is pretty close to the original.

I don't think this is a replacement, the Drake 324 is common for JMP50s and other 50W Amps from this time.
What I miss here is the information of the Output Transformer as the other Drake Nr (T100) denotes the choke normally...

 

[rycack]

I don't think this is a replacement, the Drake 324 is common for JMP50s and other 50W Amps from this time.
What I miss here is the information of the Output Transformer as the other Drake Nr (T100) denotes the choke normally...

This one?

 

[dtier]

This one?

That is what looks to be the original OT. The PT is period correct but I believe was harvested from another amp or off the shelf some time ago. It has fewer mains voltage taps. The original PT taps can be seen clipped from the PC board.

 

[Spanngitter]

This one?

Yep, that's it, Drake 784-139 (Primary: 3,5k Ohm, Secondary: 4, 8, 16 Ohms) used in JMP 50 and early 2204s...so that's correct.

The original PT taps can be seen clipped from the PC board.

Valid point, missed that initially...

 

[Trouble Free]

Huh, I went thru quite a lot of Marshalls but never saw a blue PCB....

they were only around for a brief time starting around late summer of 79 I think they used them for 3 or 4 months, but who knows for sure? I have one too.