Advice on '89 JCM800 4210 channel switcher mods

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Advice on '89 JCM800 4210 channel switcher mods

Postby bobster on Tue Feb 28, 2017 11:20 am

Hi all,
i have this JCM800 50w combo which is pretty nice but have been researching extensively on some light mods which might result in a slightly sweeter tone and response.
It is known as the 'later' version and considered to be the best of the variants by many fwiw.
Sorry i haven't linked a schematic as my understanding of them is poor and there seems to be dubiety as many of the ones on the web may correspond to the earlier version which apparently is very dissimilar to mine.
Before i dive in [ the board is relatively fragile ] i wanted to ask for opinions especially from anyone who has done these mods to see if they represent a major improvement or not...
The amp is odd in that is has a 0.047 cap after input jack before first valve instead of a 68k resistor and im wondering if placing a resistor on the socket would be worthwhile
The proposed mods are changing plate resistors on the front end from 470k and 150k down to 100k for each.

Then changing the PI resistors from 120k / 330k /22k to 1meg /1meg and 10k [ old style 2203 spec ]
May look at changing or removing some of the front end bright caps and also add a presence pot to the board as the holes are there for it.
I already have a G12-80 installed which is much fuller than the M70 that was stock.

any and all thoughts appreciated !
thanks - Bob
Last edited by bobster on Sat Mar 04, 2017 8:12 pm, edited 1 time in total.
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Re: Advice on '89 JCM800 4210 channel switcher mods

Postby Dean Jr. on Sat Mar 04, 2017 5:57 pm

Bob, I am going to bump this with an observation. Maybe someone who reads schematics better than I do will chime in and explain that circuit.
That input situation you see is NOT the input for all of the operations of that amp. That input with only the cap is the Normal operation input through the V1a triode. When in the Boost mode, the input goes through a 68K resistor--as I am guessing you were thinking should be----to V1b. I am not very good at reading that switching, but it appears that in the Boost mode, V1a is part and parcel of that Boost operation. I for one would have to better understand what is going on there before I started changing that input situation.

http://schematicheaven.net/marshallamps ... w_4210.pdf
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Re: Advice on '89 JCM800 4210 channel switcher mods

Postby bobster on Sat Mar 04, 2017 8:22 pm

Thanks Dean , i'll ignore that input cap situation.
I figure the plate resistors and the PI tweaks are about as much as my pay grade merits.....
It's not the worst amp out there by far and is a great grab and go for gigs esp with some pedals thrown at it.
Just looking to make it a bit sweeter and tune out some of the buzzy grittiness......
A lot of the research i've done online from when these amps were topics of modding discussion reveals that guys ended up ripping out the mods and returning to near enough stock.

I suppose i'll just need to try the mods and see.......it's such a flimsy board though not like the older stuff where you could tweak away and assess back and forth
cheers
b
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Re: Advice on '89 JCM800 4210 channel switcher mods

Postby BS78 on Tue Apr 25, 2017 7:53 am

The biggest thing limiting the potential of that 1x12" combo is the speaker. That G12M-70 really is too weak to be work on its own in an open cab. I had great succes with the Celestion Classic Lead 80 in both the early and the later versions of the 4210 amps I had.

A bandmate had his '89 modded beyond recognition and he thought it was the dog's, but after a while, he liked my intact ones better and had a lot of the mods reversed. A classic case of "it's not stock and I paid a lot for that, so it must be better."
One thing he left modified was the tone stack on the clean channel, it had all the values of a blackface Fender. Personally, I liked the clean of my early split channels better than the later ones, but didn't use that channel often enough to be bothered with that.
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Re: Advice on '89 JCM800 4210 channel switcher mods

Postby bobster on Tue Apr 25, 2017 9:00 pm

Yes , i have read of people modding the beejeesus out of them and then regretting it.....
it's a cool little grab and go as is and i have a G12H 80 watt from the early 80's in mine which has helped a great deal
responds well to pedals but i wish the reverb was more full like an old fender.
I have a feeling that if i change to older PI specs and do the plate resistors to 100k i may end up with a noisier amp at idle and id hate to have to de-mod it again as the board could end up with broken traces
will keep researching
b
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Re: Advice on '89 JCM800 4210 channel switcher mods

Postby bobster on Thu May 04, 2017 11:36 pm

Well . to conclude the short saga.......
i dived in and tried some light mods to the '89 [ later circuit ] JCM800 4210 combo
i already liked this little amp despite its [ not drastic ] shortcomings and it gets regular gig use, but now after the mods it is [to me at least] considerably better

i like the vintage fender / jtm45 / ac30 vibe , clean with some grit / clear and sweet but with some balls....

This amp on it's own had some issues with harshness , slightly congested feel and limited touch sensitivity.......[ this was largely smoothed out with pedals etc...]
but after the mods , i tried it tonight and was playing for hours which is not like me as i am extremely lazy , inspiring is the word that comes to mind. [ the 'green' helped i'll admit ] , felt great to play and although not identical to , it sounded not unlike a good old blackface Fender / Marshall nmv on the clean channel and the dirt channel which i already liked is now so much warmer and controllable .

Ok enough yap and down to the MODS which i hope may help others if they are unsure about this amp.....

'89 circuit [ later variant , important / do your comparisons with earlier amps as some of these mods may be unnecessary and the circuit board is fragile ]

[props to Jeff Seal for these]

22uf/25v cap across R16 (1.5k) [ did this under the board , ....neater, NB +side faces towards tubes]
Replace C4 (100p) with 220pf [ i paralleled a 120pf mica under the board ]
..should help the clean channel!
R41 (330k) to 1meg
R44 (120k) to 1meg
R45 (22k) to 10k
50pf across the bias side of C14 and C16 [ i did this under the board with a 50pf and clipped one end of existing 100pf on top of board to take it out of circuit]
...in other words standard 2203 specs for the PI...

next - as i wanted tightness and punch i moved neg feedback wire [ purple ] from existing position over to the grey wire on the impedance selector , fixing neg feedback to its maximum.

had previously sold stock celestion G12m70 and replaced with celestion G12h 80 from early 80's

bias to -39v [pin 5 EL34]

installed 22k pot internally on the board [where holes are already in place for a presence pot e.g. for the wider heads and 2x12 models ] to give a presence control which i set by ear to where it removed some of harsh high end 'hash' before i put chassis back in cabinet. [ also removed presence resistor 3.9k after pot install ]

thats it , pretty light going overall but made a big difference to my enjoyment of the amp , after a lot of research i suspected it had more to offer and it certainly has not disappointed me ,

:rawk: :rawk:
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