Is JCM900 using the same OT as the Plexi?

[soundguruman]

I don't want to hijack the thread as I think its very interesting, but want to ask a question about the 900.

My bass player owns a 4100 head and some times at rehearsal I use it instead of my gear.

The question I have is that his is no where near as loud as I know Marshalls can be, are the JCM900's output normally lower than say an 800 or JMP?

For example, When I use my JMP 2203, rehearsal volume on the dial is around 3.5-4, The 4100 to get the equivalent volume the amp must be at 7-8. This 900 also doesnt have that stereotypical Marshall volume jump. Where it goes from too low to way too loud less than halfway between 1 and 2, its a gradual increase through the travel of the volume pot. Is this normal in these amps?

The only thing that determines this is the type of volume control taper.
The control can be changed so that the transition is equal from 1-10.

The 4100 needs to be modified to make it useable.
In the stock configuration it's not very user friendly. There are mistakes in the design.

For example, the 4100 has no bass. You would find the bass control doesn't work at all...
It's a poor choice for a bass player.

But with some mods, it's just as loud as any other Marshall. And it will have just as good bass too.
So follow the modifications if you want (above)...and I think you will be a lot happier with it.

Once I modified it, there was a huge difference and it was quite acceptable.
I would use it no problem now, it really sounds good...sounds just like a Marshall should.

[J M Fahey]

The actual number shown on the scale is irrelevant.
In fact, the JCM900 Master control behaves properly, you have a smooth control all over the range.
The "Marshall jump" is a mistake.
Both amps on 10 have same power.

[minim]

You can get a half-decent clean sound out of these amps when biased correctly with a set of good 6L6s. However if you want to indulge yourself in a little project just take the pre board out and wire a new preamp up by hand. Don't forget to change the PI tail values, and BOOM! Marshall sound, plus retaining the benefits of the line outs for wet/dry rig capabilities. Stuff a WGS Retro 30 75 watter in there, and let her rip.

Actually, just noticed it's a 100 watter. Only open her up 75%.

[J M Fahey]

Stuff a WGS Retro 30 75 watter in there, and let her rip.

Actually, just noticed it's a 100 watter. Only open her up 75%.

Famous last words

You can't trust your ears or the volume control "number" for that.

Use 2 75W speakers.
4 of them will be even better,and what a Marshall head deserves

[Alan0354]

I don't want to hijack the thread as I think its very interesting, but want to ask a question about the 900.

My bass player owns a 4100 head and some times at rehearsal I use it instead of my gear.

The question I have is that his is no where near as loud as I know Marshalls can be, are the JCM900's output normally lower than say an 800 or JMP?

For example, When I use my JMP 2203, rehearsal volume on the dial is around 3.5-4, The 4100 to get the equivalent volume the amp must be at 7-8. This 900 also doesnt have that stereotypical Marshall volume jump. Where it goes from too low to way too loud less than halfway between 1 and 2, its a gradual increase through the travel of the volume pot. Is this normal in these amps?

You cannot compare like this. I owned a Marshall Plexi 100W. The volume is very sensitive, at 3 it was thundering. But after 5, it just got more distorted but not louder. At 8 it really got ugly, sounded cracking as the coupling cap charged up and cut off the next stage and turn it into like a class C amp.

The 900 is a 100W amp, look at the two transformers and 4 5881 tubes.

I yet to fix my Marshall. I have been trying to finish up my design into the Bassman and I always find things I can improve.

[Alan0354]

I just tested a Marshall DSL40. The distortion sounded better than my JCM900. But the clean still kind of thin and lifeless. I think it's very British sounding. I regret buying this instead of the DSL. 100W is way too loud. Unless changing the few components according to Soundguruman can do miracle, ripping the guts out and redo is in order.

[soundguruman]

You can get a half-decent clean sound out of these amps when biased correctly with a set of good 6L6s. However if you want to indulge yourself in a little project just take the pre board out and wire a new preamp up by hand. Don't forget to change the PI tail values, and BOOM! Marshall sound, plus retaining the benefits of the line outs for wet/dry rig capabilities. Stuff a WGS Retro 30 75 watter in there, and let her rip.

Actually, just noticed it's a 100 watter. Only open her up 75%.

1.You don't need to change the preamp board, it's totally unnecessary.
2. Take out LEDs, and EL34 has perfectly clean sound too. No need for 6L6s.
3. You have to modify the OUTS to get the right levels. Otherwise they will overload your effects units.
4. There is nothing wrong with the phase inverter.You don't need to change the tail of the PI.

This amp will sound just as good as any other Marshall. You don't need to gut it.

[soundguruman]

I just tested a Marshall DSL40. The distortion sounded better than my JCM900. But the clean still kind of thin and lifeless. I think it's very British sounding. I regret buying this instead of the DSL. 100W is way too loud. Unless changing the few components according to Soundguruman can do miracle, ripping the guts out and redo is in order.

It's not a miracle, just basic tube amp design. Yes there are 4-5 basic errors in 900, which can easily be fixed.
100W 900 amp has triode switch. This knocks it down to about 36 watts.
You could even make it less than 36 watts...
depends on speaker mostly. It can be played without the ear splitting loudness, and still retain the Marshall sound.

You should go for a 1X12 cabinet instead of 4X12. (since you are not playing in stadiums)

[minim]

1.You don't need to change the preamp board, it's totally unnecessary.
2. Take out LEDs, and EL34 has perfectly clean sound too. No need for 6L6s.
3. You have to modify the OUTS to get the right levels. Otherwise they will overload your effects units.
4. There is nothing wrong with the phase inverter.You don't need to change the tail of the PI.

This amp will sound just as good as any other Marshall. You don't need to gut it.

Whatever turns you on brother. It's all good

[Alan0354]

Whatever turns you on brother. It's all good

I studied his mod, some do make sense, some I have to try it first and see. I just need to close up my current design and move on to fix the Marshall. He seems to know this amp.

[soundguruman]

I studied his mod, some do make sense, some I have to try it first and see. I just need to close up my current design and move on to fix the Marshall. He seems to know this amp.

I worked on the mod 4-5 times over a years time.
I guess I put about 30 hours into deciding on what changes to make.
I did all the tuning with a Jackson guitar, passive EMG pickups.

So it was not done just with test equipment, a big part of it was guitar, and playing and listening...
by a guitar player, for a guitar player.
And it was played really really loud, to decide which sounded better.

At one point testing this amp, the police showed up. That put a slight delay on the finishing...
(I think the police did not like the really really loud part)

[cyclone]

the big mistake of that amp is all the "pro audio" low level switching, reverb, effects loops blah blah blah, its all on op amps, so kids plug stuff in the wrong holes, wouldn 't hurt a tube amp but that model goes poof and stops working. so, they are cheap. And, the built in bug zapper. if you have to take all that crap out so it sounds ok, isn't that a design flaw?

[minim]

the big mistake of that amp is all the "pro audio" low level switching, reverb, effects loops blah blah blah, its all on op amps, so kids plug stuff in the wrong holes, wouldn 't hurt a tube amp but that model goes poof and stops working. so, they are cheap. And, the built in bug zapper. if you have to take all that crap out so it sounds ok, isn't that a design flaw?

Well I guess whether it sounds ok stock comes down to one's own opinion. I bought a cheap one with the sole purpose of modifying it. I'm glad I did it, I had fun doing it and learned a lot. Some players like them stock, and there's no point in us wasting our breath about it. We may as well be arguing that red's better than purple.

I would still rather buy a 900 and play around with the circuit than buy anything that's come since...at least the valve bases are where they should be - on the freakin' chassis!

[Alan0354]

Well I guess whether it sounds ok stock comes down to one's own opinion. I bought a cheap one with the sole purpose of modifying it. I'm glad I did it, I had fun doing it and learned a lot. Some players like them stock, and there's no point in us wasting our breath about it. We may as well be arguing that red's better than purple.

I would still rather buy a 900 and play around with the circuit than buy anything that's come since...at least the valve bases are where they should be - on the freakin' chassis!

What did you do on the 900? I was planning to gut it and make it a Plexi with a master volume. This is the most unreliable amp I've seen. Blew a plate resistor because it was a 1/4W. All the pots scratches. Now blew the fuse. My Plexi blew the rectifier twice in the 3 years I owned it. Marshall uses 1N4007 and put the cap behind the standby switch. When you switch the standby on, the big surge of current through the small diode proved to be too much. That's the reason old fenders put the reservoir cap before the standby to avoid the surge.

Honestly, I'll give Soundguruman's suggestion a quick try, if it doesn't work out, I'll gut it. It's a lousy amp. I am not going to sell it, I like the strong chassis and the cabinet. Not to mention the two transformer.

Yes, I think the biggest problem with the modern amps are the effects loop. You have to add two extra stages of circuits and that kill the tone. I don't know who in the right mind dream up with this and force other maker to put the effects loop.

[soundguruman]

the big mistake of that amp is all the "pro audio" low level switching, reverb, effects loops blah blah blah, its all on op amps, so kids plug stuff in the wrong holes, wouldn 't hurt a tube amp but that model goes poof and stops working. so, they are cheap. And, the built in bug zapper. if you have to take all that crap out so it sounds ok, isn't that a design flaw?

You don't need to take it ALL out.
Just make a few minor changes.

The tendency here is to over do it.
Just because it has SS switching, does not mean you need to gut the amp and re-design the entire thing.

You don't need to turn a small mod project into a huge overcomplicated rebuild of the entire amp.