DragonSarc

I checked the Trim pot switch on the board and theres only 1 trim pot - does that adjust all the tubes? cuz on the Video on the euro tube shows 2 trim pot for the JCM 800 i know its 2 different amps im just making sure. thanks

PaoloJM

You sure the vid showed two trim pots on a JCM 800? My JCM 2203 has only one and most of the schematics I've seen have shown only one.
With one pot it basically raises/lowers the amount of voltage to the resitor network connected to the grid of the tubes.
This is why using matched tubes are recommended as tubes can vary widley in internal resistance and because the same bias voltage is applied to each, mismatched tubes will show different DC plate currents. Personnaly I like 'em slightly mismatched and you'll rarely get a set that's perfectly matched anyway.

DragonSarc

sweet just the answer i need thanks paolo, would you happen to know the position of the preamp tubes? which one is the V1 2 and 3 is it from left to right looking from the back?

PaoloJM

V1 will the one closest to the input jack, V3 will be closest to the power tubes and V2 obviously is in the middle.

DragonSarc

k i did what euro tube vid on biasing my amp checked the volt plate which is 502(first time using a multimeter) and did the calculation(30 watts dissipation / 502 vp x.7 =.042 so it should be 42mA bias right? on the multimeter dial it should be set on the 200ma and shows 4.2?) how come my amp is not as loud? my gain is all the way up and my master is on 7 or 8 and not loud at all what did i do wrong? i put in new sovteks 5881 and 12ax7s. need help

DragonSarc

never mind i got my amp to work 1 of the new preamp tube that i bought is busted so thats why it was weak, now my amp is RaWkInG!!!! thanks for all the help - I might try different tubes to see more option and sound!!!!!! i got the sovtek 5581wxt and sovtek 12ax7wc - the sound is more grungie and heavy metally. Ill try to switch see if i can get a little bit warmer sound and not as bright - any suggestions?

stryker59

congrats ds!!! glad you got it all straight!

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coldsteal2

oh yea, those tops can pop off real easy on the
preamp tubes, just happend to me putting in a new
one, caught it in time.

Yea get an EQ and run it through the loop to boost
the low end and cut the high end fiz, its what alot
of 900 owners do

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Adwex

I have a 900 too, and like coldsteal says, it absolutely needs an EQ in the effects loop. First I had a Boss GE-7 pedal, then a rack processor later on. Boost some lows, cut some highs, play around with the mids.

IMHO, a 900 is useless without it.

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DragonSarc

will my amp sound different if i have a marsall cab like the 1960a? will it have better lows on my JCM900?

PaoloJM

Sorry dude, been away for a few days. Glas to hear your amps back in working order.
Is yours the JCM900 combo? If so 1X12 or 2X12? Can't find what speaker is loaded in them from the web, do you know?
I have a 1960A with GT75s loaded. They're like the "greenback on drugs" speaker.
A 4X12 will sound "bigger" (not louder) than either a 1X12 or 2X12. You'd notice it particularily against a 1X12. It feels like the lows are hitting you harder and you'll more "punch" if that makes sense? It's kinda hard to describe a sound!! The best way to find out is to try one out in your local store.

coldsteal2

the cab will help but really you got to put
an eq in the loop. They dont cost too much
you can get an MXR, DOD or Marshall Gov'nr II Plus
it also has a great EQ in it.
Boost the sub lows and lows, dont need alot
and it sounds just as good as any marshall

I use the Marshall Gov'nr II Plus just for its
EQ

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DragonSarc

sweet thanks cold and paolo ill try the eq i got a dod i just need to find it hehe - my JCM 900 is a head and the cab is an ampeg 412 it kinda sound ok but i want to hear an actual marshall cab see the difference

Brett Blackmore

Hi,

the cab is really important with the JCM900 as its EQ frequences are rather narrow (but still fine - I do not use an EQ at all) - I have my JCM900 in a rehearsal room with 4 different 4x12 cabs (not mine) and I have tried them all out with (permission of course). ONLY my Marshall 1960 4x12 300w cab truly brings the real hi's and low's out into the open.

1) The Crate GT140 (120w) box buzzes and sounds crap - I am worried about blowing the speakers.
2) The Behringer 4x12 (300w) box is surprisingly good yet slightly dull/deeper in comparison with the Marshall (this is a border case for calling it a question of taste). Stiil for the money a good choice.
3) The Kitty Hawk 4x12 (approx 300w) sounds like cardboard - it lacks both hi's and low's and was a big dissapointment.
4) A Peavey 5150 box which was very good but very expensive and it did not sound better than the cheap Behringer box.

Of course I do not know how good the speakers are - this was just my comparison on 4x12 boxes which were available at the time. Conclusion - I am still waiting for a box to come along which clearly is better than my Marshall boxes.
Hope it helps. Cheers Brett

DragonSarc

thanks brett