Marshall jmp/jcm questions

[mchn13]

For reference, at practice my jmp was at 4 on the master, his jcm was at 2. My amp has been gone through recently. New caps etc...., could the difference in volume fullness just be the 88's?

Are there any mods to the amp and where are you running the preamp gain? Cause if you're running the master at 4, that thing should be ripping your face off. I don't know where you would be able to gig that loud or even hear a drummer for that matter. I have a 2204 w el34's and i can't run the master at even 2 and it's freakin ripping at that setting.

BTW, I have one 2204 with el34's and one with 6550's and the difference isn't as dramatic as one would think. I personally prefer the el34's. But that's just me. Actually you can dial the amps in to sound almost identical.

Thats what i was thinking. My jcm has 34's, and its face melting loud at 3. Thres something else going on here. Just dropped it off at my local fixit tech to check the tubes and bias. Shouldnt be any mods, i know the caps have been replaced with jj caps, but the cicuit is supposed to be stock.

 

[mchn13]

Depending on your amps....You may only have to remove a piggy backed resistor to get it to run KT88/6550 tubes...They most likely came stock with them.
Post a gut shot of your amps. If they are both JMP's...there's a 99.99.99% chance they did

Ok, so after doing a little reading, i see that the majority of the amps that came to the states at that time, came with 6550's, so someone may have changed it over to el34's at some point. Got it biased today. Ill take the chassis out tomorrow and take a gut shot. Maybe someone will be able to tell me what grid resistor is in there.
Thanks

 

[mchn13]

Ok, this is a pic of the board by thr bias pot. If anyone can tell me what resistor is there or what im looking for, awesome!

 

[mchn13]

Wider shot

 

[mchn13]

I also got a couple old tung sol, rca, and sylvania 12ax7's to try in v1

 

[stephen sawall]

All of those are cool preamp tubes ..... The thing to know is every single tube is like a snow flake and has it's own tone shape & gain properties. You can have ten that look identical made by the same factory on the same run, same day & they all sound different.

 

[mchn13]

All of those are cool preamp tubes ..... The thing to know is every single tube is like a snow flake and has it's own tone shape & gain properties. You can have ten that look identical made by the same factory on the same run, same day & they all sound different.

Went to the shed today. The amp is sounding punchier with a good bias on the new tad el34's.
I really liked the old rca's in the pre amp, really had a nice growl with low sizzle even boosted and the gain high. Right now i have the two rca's in v1 and v2, and a jj in v3. Next practice im gonna try the old tung sol in with them.
Still would like to know what resistor values i have if anyone can tell from the pics, so i can tell if i need to mod it to run kt88's??

 

[stephen sawall]

I was just reading about 6550 to EL34 & EL34 to 6550 in Aspen Pittman's The Tube Amp Book. KT88 are in the same range as 6550.

If you get it modified to use KT88/6550..... it needs to be modified again to use EL34 if I understand correct.

I have owned several Marshall"s from that era.... with 6550 & EL34.... I agree with the statement above about EL34 being the best tube for that amp. But this is only taste.