How similar does a JCM900 Mk III 2500 sound to a JCM800 2204?

[MP+]

No need to get abusive, but I stand by my findings on the different sounds between the vintage 900 cabs and the vintage 800 cabs. This had a bit to do with this much talked about "800 sound" comment that gets thrown around more than a little.

 

[MP+]

Getting back to whether a 900 can sound like an 800, I've played through both and have pulled both apart more than a few times. The power amp configuration in both is really close, the only visible difference I spotted is in the way the transformers are mounted to the chassis, but they were still Drake, so I would get back to saying that with enough experimentation and tweaking of the preamp at breakup master levels, you would be more than a good chance at pulling a top 800 tone, possibly a better tone than an 800 that has been poorly set up. Ditch the effect loop if you are looking for raw crunch, even plug a through connector between send and return to kill off any loss across the crappy spring contacts that should stay normally closed but sometimes don't..... and take it from there. Don't worry about cathode voltages or any of that..

 

[MP+]

I've heard people claim that the Mk III JCM900, with the diode distortion knob ("sensitivity") turned all the way down, sounds pretty close to a 2-hole JCM800. I've never been able to get my hands on a Mk III to try it out.

I have the 1x12 combo version of the JCM800 (4104) and I love it, but at a recent gig (I don't play out much, so I'm only just now encountering this problem) I was going to use the 4104, but ended up using a TSL so I could get a solo boost in the FX loop. The 4104 has no loop.

I know there's several solutions to this problem for the 4104, like a Boss TAE, Ultimate Attenuator with 2 volume settings, mod the 4104 (it already has holes in the chassis), get a JVM, get a modeler and move to the 21st century, etc. I'm looking into some of these.

However, here I'm specifically wondering how much of a different tone I would have by using a Mk III head or combo, which will already have a loop, compared to my JCM800 combo.

If you already have holes in the chassis why not hook up a parallel resistor set up across the master volume knob via a jack socket in the back? All the right things happen with a simple bump up of the master volume for a solo. More gain, more presence, more cut through the band mix. I've done it with my JCM 800 4 hole via the DI socket so that no holes needed to be drilled and the mod.can be fully reversed, I use a mini dpdt relay at the back of the amp that just changes the master volume setting with the tap of a footswitch. The line out to the footswitch operates voltage to the relay only which can be done either by battery or power supply. No hum or noise issues, no change to the tone. It's a simple thing that Marshall included as a standard feature on the SL-X and is the only thing missing from the 800 series as far as real level boost goes.

 

[MP+]

For anyone who might be interested in getting their lead breaks heard on an 800, this mod. is simple, but I will start by saying that I can't see the sense in sticking a muffler in the effects loop for the sake of getting some sort of makeshift level boost. The rhythm tracks are typically anything up to 90% of a song, so why dull down your rhythm sound? On the other side, everyone knows that the more you can crank your 800, the better it sounds. Here is an opportunity to crank up to one of the best lead sounds of all time, it's just sitting there waiting to be used. This whole "attenuation" approach is going in the wrong direction. It sounds a bit far out, but it's simple and easily reversed, I've used this system at dozens of gigs and ten times as many full level rehearsals without fail or issues. To hook it up via a DI socket, taking all proper electrical precautions, and don't try this if you are not well experienced at hands on technical work, remove the preamp board, de solder the master pot remembering to turn the slider either fully clockwise or anti clockwise first. solder in 3 wires where the pins were on the board and run them out to the DI socket with a little extra slack for terminating. Replace the DI jack with an insulated "Cliff" 3 terminal normally closed socket and store the removed original DI socket along with the DI dropping resistor. On that, I'm not a fan of these normally closed contact set ups as far as signals go, but this is just resistance alteration, so if they do vary slightly in resistance, it's not a major drama. Run 3 wires back from the closed socket contacts and solder to the potentiometer pins. The mod. is now invisible as far as sound and operation goes. The amp functions as if nothing at all has been done. The master pot. ends up rotated 90 degrees so that the pins clear the board with re-assembly. Following re-assembly, reposition the front knob 90 degrees back so that it points to zero when the pot is fully turned down. The master volume can now be accessed by using a plugin 1/4" stereo plug. Now if you had holes in the back of your amp, this approach wouldn't be necessary, a second master volume knob could be mounted using additional holes, but using the DI non hole method, simply hook up a small plastic case with two log potentiometers, I think I used 2 meg from memory. This case sits behind the amp secured with velcro or any other effective method. When plugged in, the front panel master knob is disconnected by the opened "Cliff" contacts and your two volume controls now operate from the instrument case. Mount a mini 9 volt DPDT relay in the case to switch between the two new master volume knobs, pretty simple to work out the wiring hook up between pots and relay, you could probably do it with a single throw. From here. all you need is any solid two core wire like speaker cable out to a footswitch in an enclosure, doesn't need to be shielded since it just carries DC voltage to the relay. I've added Led's to the footswitch and the new instrument case to indicate when switched on. Hook up a 9V dc battery in the footswitch case or feed in 9 volts from a pedal supply outlet to power the set up. The current draw is very minimal since it's just powering a couple of Leds and a relay during lead solos. As far as circuitry goes, this is about as basic and simple as it gets, as far as effective boost goes, is there really any better way to do it than just cranking up the master for a solo? The other option would be to just get a roadie to do it for you.

 

[Tatzmann]

Quite off-topic, innit?

 

[MP+]

Quite off-topic, innit?

View attachment 156390

Yeah, fair enough, I could see Nig. signalling to his roadie to crank up for every solo. I would say that the thread originated with the mention of an issue related to actual level boost, not just distortion boost for solos and if the originator is happy with what he already has but for a minor problem, why trash the set up to potentially take on more problems. I did the mod. to my 800 for the very same reasons that this thread originated from.