The JCM900 is a Marshall I'm clueless about - there seems to be a lot of wrong information and pre-judging about this amp. I assume in JCM900 DR, the DR stands for dual reverb. Without removing the chassis, how can you tell if possible if the amp has a reverb tank or a digital reverb ? By build date or something else ?
The 90s DR's schematics show, that they are more of a SS/tube hybrid preamp paired with a classic Marshall poweramp (almost classic...). Most of both channels' gain (in the technical sense of 'gain') is achieved by opamps, most of the distortion and overdrive of both channels is achieved by diode circuitsin conjunction with the opamps. In some stages tube triodes bring back voltages to tube-like level, and there is also a classic-ish third stage + cathode follower stage somewhere. I think the tube stages add their little bit of harmonics, and the overall picture of the sound reflects that. I would say, the reissues are the same circuit-wise. I still can't understand, whay Marshall did such a rather complicated circuit. A classic 3-stages + CF a la 2203 / 2204 with some variations ((switchable) gain boost, clean channel) could have done the trick way more easily. However, that was what they wanted to do then, and if it sounds right, why not. I had a 50W DR in the mid 90s, but I really can't remember, if I would like it today or not. But most of the clips I find, sound really promising. It's a rather classic MArshall, in the end. I would like to test one rather sooner than later.
I've done this easy mod to my 4500 a few years ago and the results were stunning! One of the best amp I ever had but I've made the mistake to bring it to a tech after for something else and he didn't know what he was doing and the amp lost his Mojo ,..here is the description below from Gravenmade site , he is a member here aka @tschrama http://www.sgravenmade.nl/onderhoud/marshall-jcm900-dual-reverb-mod/ This is a describtion of the mod in a nutshell: 1] C27, (4n7)snipe one leg off 2] C35, (220pF) to 100pF 3] R42, (10k) piggy back another 10K over it. But i Like it as is. 4] D10, (diode) snipe one leg off 5] R60, (470k) Pickybag 47K over it 6] R13, (220k) snipe one leg off What this mod does: This mod effectively takes out the pre tube diode clipping. The tube is running open-loop, with full gain, no more “handcuffs”. There you have it, no more diode-clipping, genuine tube stage with cathodefollower connected traditional Marshall Tonestack. Here are some quotes about the mod: “First and most importantly, the mud is gone! You can tell what I’m playing even in the high gain clips, which before I always struggled with. ……. The tone stack is now very different ……. No matter the channel or gain setting you can dial in exactly what your ears are wanting…… The amp now FEELS very different. It feels alive like when you have the stock amp cranked…. The lows are super tight and yet there is more of it now, the low mids now dropkick you in the chest, and you can dial the treble knob up to 7 or 8 before you ever think it’s ice picky, but if you need more, its all there. “ “At lower gain settings (6/20) on ChB, it sounds very plexi-like to me (or at least compared to all the clips I can find).. At Pres 10, Bass10, Mid 8, treb4 it’s got the KeRRRANG and extremely high note definition. Bring the gain up to half way and it is more like the 800 clips I hear. Throughout both of those settings, the sustain is ridiculous.. strike a chord and it just keeps going and turns into harmonics” “Now ChB with the gain cranked is just bone crushing yet crystal clear.”
As others have said the JCM900 had no digital reverb, however, you just need to look in the chassis to see the reverb tank (mounted on the front panel above the amp, behind the logo on the front panel, it looks like a blank metallic rectangle with two wires running to it, the wires are rca connections).
I had an original JCM900 4100 and it had all the bottom end I could ever want. Is there a difference in the cabs you use vs what your buddy is using? If you have treble focused speakers and he has bass heavy ones, that would make the same amp sound very different - I know this cause I had 2 Carvin 412s, one with V30s and the other with bassy JBLs. The tone was noticeably different if I only ran one cab - higher pitched with the V30s, bigger bottom'ed with the JBLs.
I hear you but I have a feeling the 30 years old caps versus the newer ones probably made a difference in having a rounder tone ,......maybe the Tung-Sol 6L6GC had something to do with it also. Tried it through his 4x12 loaded with G12t-75's that came with the head .
