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Is my jcm900 an all tube amp??

4/29/1997 1:18 PM
Is my jcm900 an all tube amp??
I've heard its a hybrid and thats why it gets good  
distortion at low volume?? I need an amp with smooth distortion  
and the 900 is really spikey,,, what marshall should i look for?  
I mean which marshall rock amps are still all tube that i can find  
fairly easily.. ?? :)
4/29/1997 4:30 PM
Dan Lang

The JCM900 use solid-state circuitry to do alot of the overdrive and switching.  
The JCM800 and JMP heads are all tube designs. A JCM800 can be a good choice with either a mod or a good overdrive pedal added.
4/29/1997 11:38 PM
The JCM900 Master Vol head uses some diodes as protective circuitry in the preamp. The effects loop is solid state. But the tone you hear is 100% tube. Preamp, inverter tone control and driver.  
The JCM900 Duel Reverb head uses some diode clipping in the preamp circuit and IMHO sounds thin and rhaspy.  
The Marshall amp I would buy if I wanted pure tube tone and a good range of vintage and modern sounds would be the Plexi reissue. I think it is now called the SLP reissue. I own a Plexi reissue and after adding a standard factory type master volume it is my favorite amp. I jumper the normal and bright inputs for most tones and use a Boss GE7 eq pushed up in the mids as an overdrive pedal. I have been frequently accused of perfect tone. I got mine used for $600.00 and added the master vol myself.  
I own two JMPs and I love the tone, but JMPs can sound very different amp to amp so its a "good one" or "bad one" thing. And I like the tone of my Plexi alot better. Good hunting.
5/15/1997 12:48 AM
Mr. Marshall Expert matricciani
Here is the scoop:  
The early Models 1959/1987 which are not the yeras  
but the models are all tube..This is the Plexi  
design..the non-master volume. They also use a  
cathode follower tone circuit..The last tube before the tone circuits is configured as a  
cuurent boost, not a voltage boost.The equiv is  
an emmitter follower in transistors.  
The Master volumes models used a diode for asymetric clipping, because so many players  
were using Tube Screamers and Boss Pedals in front  
of the amp anyway.  
The Silver Anniversary used the same trick with  
also a non-cathode follower tone circuit. It  
also used the diodes in a different design topology.  
The 2210/2205 Channel switching Marshalls used  
diode clippers and yet another non-cathode follower tone circuit...The 2210 is my favorite  
The JCM900s used less tubes as OPAMP were used to  
boost the guitar signal before the tube. The 2210  
did this with all tubes. (5 12AX7s vs. 3 12AX7s in the dual reverb model). The JCM 900 line also  
went back to the cathode follower tone circuit.  
The output transformer was also redesigned on the  
JCM900 models since lots of Marshalls were cashing  
them in. The 2210 transformer actually comes to  
life at medium to high volumes, whereas the JCM900  
output transformers sound the same at any volume.  
The 30TH anniversary Marshall went back to all diode clippers. They also designed  
channel two around the Plexi 1959 model (mode a)  
2204/2205 early master volume non-channel switchers (mode b) and ultra high gain (mode c).  
Channel three also has a totally new topography  
with 7 gain stages, three of which are cathode  
followers. All three channels have cathode follower tone circuits.  
I own a Silver Anniversary, 2210 and 30TH anniversary Marshall. They all sound totally  
different. I also would not worry about diodes  
and op amps. Marshall only puts them in to better  
the sound..My 2210 IMHO smokes any Marshall and it  
has diode clippers. I have also heard some good  
sounding JCM900 amps, all of which use OP amps to  
boost the signal but not to create the distortion.  
By the way....I also own Boogies, Soldanos, Riveras and a 5150. Each amp has its own voicing  
and this gives me many guitar sounds. The problem  
with one guitar tone is that it limits you.  
Guy Matricciani
5/15/1997 12:28 PM
hi Guy. i have a 50w chanel switching 800 combo with rev and 1-12 70w celestion (2210 i think). this amp has the 5 12ax7 that you spoke of. do you know what tubes are for rev, splitter, etc? i run it with 6550's and 4 telefunken 1 sovteck 12ax7's. do you have advice on what spots are best for the better tubes. btw this amp sounds fantastic clean and dist with a les paul. also do you know how to tell the mfg date?
5/15/1997 1:06 PM
Guy Matricciani
The american versions came with 6550 tubes  
up untill about 1990. You can covert those  
to EL-34s if you want. Aspen Pittman has the mod  
in his Book.  
I will refer to the 5 preamp tubes as V1-V5.  
Each 12Ax7 has two triodes.  
V3b- reverb send  
V4b- reverb return and effects send  
V4a- effects return  
V5- phase inverter  
Lead channel V1a, V1b, V3a  
Clean channel V1a, V2a, V2b  
There is a diode clipper between V1b and V3a on  
the lead channel.  
So, you can try tube substitution depending on  
what channel you wish to affect.  
Reguarding 6550 vs. El-34s.....Its up to  
you. I think Marshalls sound better with  
EL-34s, but if you like the sound...don't  
change it.  
Also, the combos are a bit bright. The amp  
would sound much better played into a closed back  
cabinet. A warmer speaker such as a Celestion  
Vintage 30, or Celestion Rivera 85 might tame  
down the highs in the combo configuration.  
As far as 12Ax7 differences, your not going to  
hear much change..I've tried all kinds. What will  
change the sound is converting the amp to EL-34  
power tubes. I've nevr heard a 6550 equipped  
Marshall, so I'm not one to tell you whether the  
EL-34s will sound warmer.  
A cool setting at low volumes is this:  
Lead Channel  
Master -1  
Turn the presense up to add the highs lost from  
the low treble setting. The reverb will also  
add highs.  
Incidentally, on the 2210/2205 preamps, the volume  
for the lead channel acts like a gain..You need  
volume on 10 and gain on 10 to get max gain.  
This is because the volume pot drives V3a.  
Hope this helps Ryan.  
Guy Matricciani
5/15/1997 2:29 PM
hi Guy. i would like to try svetlana el34's but i dont have the book. i hear the 6550 doesnt clip hardly at all. do you know the conversion for 6550 to el34? by the way i run the clean ch at: v=5, t=4 b=7 dist: pre=10, v=6, t=0, m=5, b=10 master~6, rev~4 i have no presence. the tone changed a lot going from the sovtek to the telefunken pre tubes, more head room (no dist on clean) fuller more dynamic everything. i cant run the dist vol at 10 because i switch from clean to dist alot and i need the volumes to match. the dist is more than whats associated with a blues sound, but less than the saturated "cranked marshall" sound where power. do you know the layout of the tubes corresponding to the "v" system? from the power tubes there are 2 tubes to the right and then 3 tubes offset back.
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